ENCODER AND METHOD FOR GENERATING A STREAM OF DATA
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
Embodiments of the invention relate to the field of gene-
rating streams of data including a plurality of encoded da-
ta blocks, wherein such kind of streams are transmitted to
a receiver and decoded for presentation of the data within
the stream. More particularly, and not by way of any limi-

tation, the invention is related to the field of media
transmission, reception and playback, and embodiments of
the invention concern a fast tune-in into a stream trans-
mission over IP networks using a second stream for a tune-
in.
.2. Description of the Related Art
In the field of media transmission over an IP network (for
example IPTV systems) video data is transmitted in encoded
and compressed form, and popular video compression stan-
dards, such as MPEG-2 and JVT/H.264/MPEG AVC, -use intra-
coding and inter-coding. For proper decoding a decoder de-
codes a compressed video sequence beginning with an intra-
doded picture (e.g. an I-picture or I-frame). and then con-
tinues to decode the subsequent inter-coded pictures (e.g.
the P-pictures or P-frames and/or the B-pictures or B-
frames) . A group of pictures (GOP) may include an I-picture
and several subsequent P-pictures and/or B-pictures, where-
in I-pictures require more bits to code than P-pictures or
B-pictures for the same quality of the video. Upon receipt
of the. video stream on a particular channel, for example
after changing to this channel or after turning on the re-
ceiver, decoding has to wait until the first I-picture is
received. To minimize the delay in the coding of the video
stream I-pictures are sent frequently, i.e. are included
within the video stream at a fixed distance, for example
every 0.5 seconds.

One problem of current IPTV systems is the so-called tune-
in time into streams that are distributed over multicast
IP. The delay between the initialization of tuning into a
channel and rendering the content of this channel is due to
several effects of which client pre-buffering time and ac-
quisition time for random access points within the stream
to be switched to are the dominant ones. Both effects are
direct implications of the design of modern video codec
schemes, in differential video coding schemes, like MPEG-2
video or-MPEG-4 AVC/H.264, only a few pictures of a stream
are self-contained, e.g. the above-mentioned I-pictures.
These pictures include all information that is necessary to
decode the complete picture. Most other pictures are diffe-
rentially coded and depend on one or more previously trans-
mitted and decoded pictures, e.g. the above-mentioned P-
pictures or B-pictures. In other words, the P-pictures or
B-pictures do not include all information that is necessary
to decode a complete picture, rather additional information
from preceding or following pictures is required.
To obtain the best coding efficiency at a given bit rate,
the number of I-pictures should be low. On the other hand,
the I-pictures serve as random access points (RAP) to the
stream where decoding can be started. Hence, there is a de-
lay when' tuning into a new stream, since the client (re-
ceiver) has to wait for a random access point within the
stream to arrive, before it can start decoding and display-
ing video.
In differential coding schemes, the encoded video bit rate
is not necessarily constant but rather depends on the com-
plexity of the video scene.Within a video stream, the var-
iation in coded picture size can be large, for example the
I-pictures can be many times as large as differentially en-
coded pictures, the P-pictures and B-pictures. Upon trans-
mitting such a bit stream over a channel with constant
channel bit rate, the client needs to pre-buffer incoming

picture data so that the video can be played with the same
rate as it was sampled. This buffer needs to be large
enough to avoid buffer overflow and shall only be emptied
on reaching a certain buffer fullness for avoiding buffer
underrun during playout.
Whenever the buffer cannot be filled instantly to the point
where the client can start emptying it, delay occurs before
rendering can be started.
This functionality is disadvantageous as the receiver which
begins receiving a program on a specific channel, for exam-
ple following a channel change or turning on the receiver
must wait until the random access point, for example an I-
picture is received, so that decoding can start. Thus, the
distance of random access points within the main stream is
one of the main causes for the tune-in delay.
One approach to reduce such delay in a multicast linear TV
scenario is to send a second stream in parallel to the main
stream, wherein the second stream has a higher frequency of
random access points. This second stream is for example
called the "tune-in stream" or the "side stream".
Fig.. 7 illustrates tuning into a main stream using a sec-
ondary or tune-in stream. Fig. 7 illustrates along, the X-
axis the time and along the Y-axisthe quality level of the
respective streams. In Fig. 7, the full quality Qs of the
main stream 100 is 100% and the side stream or tune-in
stream 102 has a lower quality Qi, which is an intermediate
quality level, which is lower than the quality level of the
main stream 100. When the user initiates a channel change
at time t0, tuning into the side stream 102 occurs. The
side stream comprises more frequent random access points so
that the initial start-up delay (tr-t0) for decoding is re-
duced by using the tune-in stream 102 having more frequent
I-pictures. A decoder within a receiver will obtain a first
I-picture from the tune-in stream 102 for the new channel

earlier than the first I-picture of the main stream 100.
However, as mentioned above, the quality of the tune-in
stream 102 is lower than the quality of the main stream,
e.g. the pictures are encoded at different quality levels,
which is necessary to limit the additional bit rate that is
necessary for the tune-in stream as same comprises more- I-
pictures which are many times larger than the other pic-
tures. Therefore, the tune-in stream 102 is encoded at a
lower intermediate quality level Qi, for example, using a
lower image resolution, for example, only a quarter, resolu-
tion when compared to the full resolution of the main
stream.
During, the transition period (tT-tR) starting at tR, the re-
ceiver or client decodes the pictures derived from the
tune-in stream 102 until a full resolution I-picture ar-
rives on the main stream at time tT. Once this I-picture
arrives, the low resolution stream is stopped and the full
quality pictures of the main stream are decoded and ren-
dered.
The main stream and the side stream may be received at the
client or receiver using different scenarios, one being the
simultaneous transmission of the main stream and the side
stream to the receiver. Such an approach is, for example,
described in US 2007/0098079 Al the disclosure of which is
incorporated herewith by reference. Alternatively, the re-
ceiver may obtain only a single stream for decoding from a
server which provides both, the main stream and the -side
stream. Upon initiating a channel change- or upon turning on
the receiver, a respective request for tuning into a spe-
cific channel is forwarded to the server which then pro-
vides information on the basis of the side stream or tune-
in stream until high-quality information, namely the first
I-picture, of the main stream becomes available. Such an
approach is for example described in US 2007/0248165 Al the
disclosure of which is incorporated herewith by reference.

As mentioned above, the tune-in stream 102 is encoded with
a substantially lower bit rate which results in lower video
quality than the main stream. This may be accomplished by a
reduced image resolution or more aggressive lossy coding
parameters, for example a higher quantization is used dur-
ing encoding of the side stream.
While this approach of providing the lower quality side
stream is advantageous for reducing the tune-in delay as
discussed above with regard to Fig. 7, the information pre-
sented to the viewer of a tune-in stream is of low-quality
during a short time period, namely the transition period.
In conventional examples, this transition period may range
between 1 and 5 seconds. However, at the end of the transi-
tion period, namely at point tT the presentation is switch-
ed to the full quality main stream and a. visible difference
between the tune-in stream and the main stream may be quite
severe. For example, when looking at a static scene sudden-
ly appearing details due to the switching from the low-
quality tune-in stream to the high-quality main stream will
be easily. noticeable. This effect may lead to a bad user
experience that will potentially reduce the subjective pos-
itive effect of the faster tune-in.
Therefore, a need exists to provide an approach avoiding
visible artifacts when switching into a main stream.
SUMMARY OF THE INVENTION
One embodiment of the invention provides a method for gene-
rating a stream of data comprising a plurality of encoded
data blocks, the plurality of encoded data blocks comprises
a plurality of self-contained blocks including all informa-
tion for decoding the block and a plurality of blocks in-
cluding only partial information for decoding, wherein a
cistance of self-contained blocks in the stream of data is
dependent on the content encoded in the stream, wherein the

stream is a main stream. Tuning into the main stream is ef-
fected via a secondary stream comprising at least a subset
of the data blocks of" the main stream encoded at a. quality
different from a quality of the data blocks of the main
stream, wherein the self-contained blocks of the main
stream are . inserted at positions . within the main stream
where differences in the quality of.the data encoded in the
main and secondary streams are less detectable.
Another embodiment of the invention provides an encoder for
generating a stream of data comprising a plurality of en-
coded data blocks, the plurality of encoded data blocks
comprising a plurality of self-contained blocks, including
all information for decoding the block and a plurality of
blocks including only partial information -for decoding,
wherein the- encoder is 'configured to .vary a distance of
self-contained blocks' in the stream dependent on the con-
tent encoded in the stream, wherein the stream is a main
stream. Tuning, into the main stream is effected via a sec-
ondary stream' comprising at least a subset . of the data
blocks of the main stream encoded at a quality different
from a quality of the data blocks of the main stream,
wherein the self-contained blocks of the main stream .are
•inserted at positions within the main stream where differ-
ences in the quality of the data enco'ded in the main and
secondary streams are less detectable.
Yet another embodiment of the invention provides a method
for tuning into a main stream, wherein a main stream and a
secondary stream are provided. The main stream comprises a
plurality of encoded data blocks, the plurality of encoded
data blocks comprising a plurality of self-contained- blocks
including all information for decoding the block and a plu-
rality of blocks including only partial information for de-
coding. The secondary stream comprises at least a subset of
the' source data of the main stream encoded at a different,
typically -lower, quality than the data blocks of the main
stream, wherein the self-contained blocks in the main

stream are inserted at positions where differences in the
quality data encoded in the main and secondary streams are
subjectively less detectable. Upon receiving a tune-in re-
quest tuning into the. secondary stream and the main stream
occurs, and the secondary stream is decoded until a self-
contained block arrives on the main stream or a required
main.stream decoder buffer-fill level is reached. Upon ar-
rival of the self-contained block on the' main stream or
reaching the required main stream decoder buffer-fill lev-
el, decoding of the secondary stream is stopped and decod-
ing of the main stream starts.
Further embodiments of the invention provide a decoder for
receiving encoded data and for providing decoded output da-
ta. The decoder comprises an input for receiving a main
stream and a secondary stream. The main stream comprises a
plurality of .encoded data blocks, the plurality of encoded
data blocks comprising a plurality of self-contained blocks
including all information for decoding the block and a plu-
rality of blocks including only partial information for de-
coding. The secondary stream comprises at-least a subset of
the. data blocks of the main stream encoded at a lower qual-
ity than the data blocks of the main stream, wherein the
self-contained blocks in the main stream are inserted at
positions -where differences in the quality data encoded in
the main and secondary streams are less detectable. Fur-
ther, the decoder comprises a control input for receiving a
tune-in request signal and a decoding portion coupled to
the input and to the control input for producing' decoded
output data. The decoding portion is adapted to tune into
the secondary stream upon receipt of a tune-in request, to
decode the secondary stream until , a. self-contained block
arrives on the main stream or a required main stream decod-
er buffer-fill level is reached, and to stop decoding pf
the secondary stream and to start decoding of the main
stream upon arrival of the self-contained- block on the main
stream or reaching the required main stream decoder buffer-
fill level.

Embodiments of the invention concern encoders, streaming
servers,'network- components and clients or receivers for
multimedia distribution systems for using a tune-in stream
for fast channel change wherein switching from the tune-in
stream to the main stream is subjectively hidden through
content-adaptive coding.
BRIEF DESCRIPTION OF THE DRAWINGS
The embodiments of the invention will .be described in the
following with reference to the accompanying drawings,
wherein:
Fig.. 1. is a flow-diagram of a method for generating a
data stream according to an embodiment of the in-
vention; ■
Fig. 2a is a ■ block diagram of an encoder in accordance
with an embodiment of the invention;
Fig-. 2b is a block diagram of the encoder of Fig. 2a pro-
viding the main and side streams separately, i.e.
the encoder does riot comprise a multiplexer;
Fig. 3 is a flow-diagram of a method for tuning into a
main stream in accordance, with an embodiment of
the invention;
Fig.- 4a is a block .diagram of a decoder in accordance
with an embodiment of the inventions-
Fig. 4b is a block diagram of the decoder of Fig. 4a re-
ceiving the main and side streams separately,
i.e. the decoder does not comprise a demultiplex-
er; '

Fig.. 5a is a block diagram of a decoder in accordance
with another embodiment of the invention;
Fig. 5b • is a block diagram of the decoder of Fig. 5a re-
ceiving the main and side streams separately,
i.e. the decoder does not comprise a demultiplex-
er;
Fig. 6 illustrates tuning into a main stream using a
plurality of tune-in streams; and
Fig. 7 illustrates the conventional approach of tuning
into a main stream using a tune-in stream.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
In the following, embodiments of the invention will be de-
scribed. ■ One . embodiment of the--.,invention concerns an ap-
proach' for generating.a.stream of data, for example a video
stream or an audio stream wherein1 the- stream of data com-
prises a plurality of encoded data blocks which comprise a
plurality of ■ self-contained blocks including all informa-
tion for decoding the block and a plurality of blocks in-
cluding only partial information for decoding. In accor-
dance with embodiments of the invention, the stream of data
may be a video stream being encoded using intra- and inter-
coding. The encoded stream may comprise I-pictures as ran-
dom access points and P-pictures and/or B-pictures. Within
the encoded data stream, the distance ■ of self-contained
blocks, for example the I-pictures, is varied dependent on
the content of the stream.
Fig. 1 is a flow-diagram.illustrating, a method for generat-
ing a stream of data, in accordance with an embodiment of
the invention for generating a video stream. At step S100
the method starts and a video input signal is provided.
This video input signal comprises a plurality of parts, for

example a picture defining a picture within the video
stream. At step S110, a part of the video information re-
ceived, and the content of the video input signal to be en-
coded is analyzed. In the embodiment described with regard
to Fig. 1, it is determined at step S120. whether the ana-
lyzed .part of- the content is associated with a specific
scenario, like a scene boundary or ' a fast camera movement
within the scene. In case no such specific situation is
present in the analyzed part of the content, the method
proceeds to step S130, where the analyzed part is encoded
into a P-picture or a B-picture. On the other hand, in case
the above -discussed situation is recognized in the content,
the method proceeds to step S140 where the analyzed part is
encoded into an I-picture. At step S150 it is checked as to
whether additional parts to be analyzed are present and in
case this is true, the method proceeds ..to step S160 where
the next part to be analyzed in the video input signal is
selected and the, method then returns to step SllCL Other-
wise/ the - methods ends at step S170, i.e., the video input
signal is now encoded and is present in the form of a se-
quencevof I-pictures and P-pictures and/or B-pictures.
The above described approach for determining a position of
I-pictures within a video.stream results in the placement
of an I-picture within the encoded video stream dependent
on the content so that the actual distance between consecu-
tive I-pictures varies. Thus, I-pictures are inserted into
the video.stream not at a fixed distance, i.e. the static
GOP length of for example one second or. five seconds - as
used in conventional approaches - does no longer exist. Ra-
ther, the distance of consecutive I-pictures is varied. In
accordance with embodiments of the invention the distance
may be varied within a certain time window, e.g. a time
window having a maximum distance of five seconds and a min-
imum distance of one second. This Solution is advantageous
as it ' may increase the bit rate efficiency in the video
stream, because I-pictures for random ' access points are
placed at positions within the stream where the bit demand

is higher anyway, even for P-pictures, for example at scene
cuts or during high camera motion, where motion prediction
is requiring high information in the. P-pictures.
Fig.. 2 is a block diagram of an encoder in accordance with
an embodiment of the invention. Fig. 2 shows the encoder
200 which comprises a main encoder. 2.02 and a side encoder
204. The main encoder-202 comprises a content analyzer- 206.
The main encoder 202 receives a video -input signal and op-
erates in accordance with the method described above for
generating the. main stream at its output. In addition, in
fur.ther embodiments the encoder 200 may also include the
additional side encoder 204 which generates, on the basis
of the video input signal, -a so-called tune-in stream cor-
responding to the main stream, being however encoded with a
higher number of I-pictures, typically at a lower quality
to be -able to achieve a lower bitrate of the side stream
compared to the main stream. In addition, the encoder 200,
in an embodiment, may comprise a multiplexer 208 receiving
the main stream from the main encoder 202 and the tune-in
stream from the side encoder 204 and providing either a
combined' output stream 2.10 (see Fig.. 2a) or separate main
212 and tune-in 214 streams (see Fig. 2b). which is/are to
be broadcast to clients for decoding and display.
In accordance with an embodiment of the invention using the
encoder 200. as shown in Fig. 2 which comprises the two en-
coders 202 and 204, the main stream is generated for a
channel in a multi-channel transmission system, and in such
.a system tuning into the main stream is effected via the
secondary stream, the tune-in stream or side stream also
generated, by encoder 200. As discussed above, the main
stream comprises a plurality of I-pictures which are not
provided at a fixed distance, but the distance of consecu-
tive- I-pictures varies, within a certain time window. In en-
coder 202 the video content defined by the video input sig-
nal is analyzed by means-of the content analyzer 206 during
encoding and I-pictures are inserted at positions where

visual differences in resolution or visual differences- in
image quality in general are subjectively less detectable,
for example at a scene boundary or during fast camera move-
ments during a scene.
Generating the main stream in this manner is advantageous
as upon tuning into the main stream either upon turning on
a receiver or switching from another channel, a smooth
transmission from the tune-in stream to the main stream is
achieved, which may be completely undetectable by a viewer.
Further, this solution may increase the bit rate efficiency
in the main stream, because I-pictures for random access
points are placed at points in the stream where the bit.de-
mand is higher anyway, even for P-pictures. In addition,
bits for additional I-pictures at a fixed distance may be
saved without losing accessibility to the channel as acces-
sibility is ensured by providing the tune-in stream that
may have the I-pictures at a fixed distance and at a higher
rate, i.e. the number of I-pictures, as discussed above, is•
higher than the number of I-pictures in the main- stream.
This may result in the necessity to encode the tune-in
stream with a . lower quality to meet the bitrate require-
ments for transmitting the information. An additional ad-
vantage is that the above described approach is a pure
head-end solution which• preferably exists inside encoding
device 202 and requires no additional network devices or
the like. Further, it is advantageous that effectively no
additional client device complexity is added.. All the
client has to do is to monitor the main stream actively for
the first "I-picture, rather.than counting a fixed number of
pictures after tune-in in case of static transition 'pe-
riods. Alternatively, the tune-in stream may include addi-
tional . signaling about the next I-picture in the main
stream indicating when the next I-picture occurs in the
main stream. This enables the client to switch to the main
stream without monitoring same.

Placing the I-pictures within the main stream dependent on
the content is advantageous. However, in a situation' of
very long static scenes, it may be that any predefined,
maximum allowed I-picture distance (that is set to not ex-
ceed a maximum transition period length)- will be exceeded,
so that it will be necessary to place an I-picture within
the static sequence. In this situation, the switching from
the low-quality tune-in stream to the high-quality main
stream would .potentially be visible. For' such situations,
i.e., 'a situation where an I-picture must be placed irres-
pective of the content, embodiments of- the invention teach
to modify the tune-in stream by gradually enhancing the
quality.of the tune-in stream during the transition period,
for example by lowering the quantization parameters step'by
step up to the I-picture boundary of the main stream. The
quantization parameter is one of the main parameters that
controls the video quality during the encoding process. A
lower quantization parameter corresponds to a more fine
grain .quantization of the encoded data coefficients and
thus less visible encoding distortions like blocking arti-
facts.
Fig. 3 is a flow-diagram, of a method for tuning into a vid-
eo main stream in accordance with an.embodiment of the in-
vention. Fig. 3 starts from a situation where at a receiver
a video mainstream associated with a first channel is pre-
sently received, decoded and output for display, as is
shown at step .S300. At step S302 it "is monitored whether a
channel change request is obtained, for example a request
to change to a second channel. As long as no such request
is received the video main stream associated with the first
channel is continued to be decoded and output for display.
However, in case a channel change request is received at
step S302 the method proceeds to step S304. At step S304
decoding of the main stream for the ' first channel is
stopped and the receiver tunes into the tune-in stream or
side, stream an-d into the main stream for the second channel
as is indicated by step S306. After tuning into the side

stream for the second channel starting with the receipt'of
a first I-picture in the side stream decoding and output-
ting the side stream for the second channel starts at step
S308. At step S310 the main stream for the second channel
is monitored to determine in step S312. whether an I-picture
in the- main stream for the second channel was received or
not. As long as no I-picture in a main stream,was received
the method continues to decode and output information from
the .side stream and continues to monitor the main stream
for the I-picture. As soon as the first I-picture for the
main stream for the second channel is.received and is ready
for decoding the method proceeds to step S314 where receiv-
ing, decoding and outputting the side stream for the second
channel is stopped. At step S316 decoding and outputting
the main stream for the second channel is started effec-
tively completing the channel change process.
As far as Fig. 3 is,concerned, same was described with re-
.gard to a'situation where a receiver is already operating
and decoding and outputting information regarding a first
channel (see steps S300 to S304). However, in accordance
with other embodiments, this approach also works in situa-
tions where a receiver at the client's side is turned on
and starts decoding a video stream. In a similar manner as
described in steps S306 to S316 the receiver first of all
tunes into the channel selected at start-up of the:receiver
and decodes and outputs the side stream until an I-picture
in the main stream arrives.
Since -the I-picture, in the main stream is'placed within the
main stream at a" position where differences in image quali-
ty are less detectable,, the transition from the side stream
to the main stream . upon receipt of the I-picture in the
main stream is less detectable and may be completely unde-
tectable by the viewers. Thus, switching from the low-
quality side stream to the high-quality main stream is sub-
jectively hidden due to the fact that the I-pictures in the

main stream were placed in positions determined on the ba-
sis of the content of the main stream.
Fig. 4 shows a block diagram of a decoder 400 as it may be
used in accordance with an embodiment of the invention. .The
•decoder 400 comprises an input section 402. The input sec-
tion 402 receives either the main stream 408, and the side
stream 410 (see Fig. 4b) or a combined input stream 406. In
case of the combined input stream (see Fig. 4a) , the 'input
section comprises a demultiplexer 204 receiving at its in-
put, a combined input stream 406 which, by means of the de-
multiplexer 404 is split into the main stream 408 and the
side/stream -410.-
Further, the decoder 400 comprises a decoder portion 412.
The decoder portion 412 comprises a switching element 414
for selectively applying to an input of the decoder portion
either the main stream 408 or the side stream 410. Further,
the decoder portion has a control .input 416 for receiving
control signals, for example the above described channel
change request signal. Further, the decoder portion 412
monitors the main stream as is schematically illustrated by
the dashed line 418.
The decoder 400 operates in a manner as . described above
with regard to Fig. 3,' i.e. the decoder portion 412 decodes
the main stream for a specific channel as long as no con-
trol, signal' at -the input 416 indicating a channel change
request is received. Upon receipt of a channel change sig-
nal at control input 416 the decoder portion tunes into the
side stream by switching the switching element 414 to pro-
vide the side stream 410 to the decoder portion which then
decodes the' side stream once the first I—picture within the
side stream is detected and ready for decoding. The decoder
portion 412 then outputs the decoded signal 420. At the
same time, the decoder portion 412 monitors, via line 418
the main stream 408 and as soon as a first I-picture is de-
tected in the main stream and is ready for decoding it is

switched-, such that the main stream is provided to the de-
coder portion 412 and the main stream is decoded and output
at 420.
In many cases switching from the tune-in stream to the main
stream in a manner as described above with regard to Fig. -4
•is time critical to avoid any distortions. Therefore, often
two independent decoder chains for tune-in .stream and the
main stream are favored, to be able to start decoding the
main stream slightly in advance so that the switching can
be done in the uncompressed domain. However, such an ap-
proach is. disadvantageous as the use of two decoders in-
creases the client complexity. Therefore, -an additional
need,exists-to- provide a "single decoder which will save im-
plementation complexity and allows re-use of already exist-
ing decoder chips. Therefore, in accordance with an-embodi-
ment of the invention the switching point in the tune-in
stream and/or main, stream is signaled to enable the decoder
to 'switch over. Different methods for signaling can be im-
plemented,: dependent on the transport layer and ' stream
types in use.. For RTP streams, for example the RTP header
extensions can be used or for example the RTCP sender re-
ports. For MPEG-2 Transport Streams e.g. the Synchronous
Ancillary Data (SAD) mechanism or e.g. a separate stream
with another PID could be used. For AVC video stream's SEI
messages in the video stream may be used.
Using only a single decoding element and. signaling to the
s
decoding element when the switching will occur is advanta-
geous as, first, of all,, only one decoder is necessary in
the client device thereby reducing its complexity. Also,
this approach works fine -for time-shifted (delayed) tune-in
streams. In addition, there is no need for the client . to
implicitly 'calculate or estimate the correct switch-over
point. The client only needs to read the signaling messages
from the streams. The encoder sends a message that signals
the point in time, when the client is allowed to switch to
the main stream and start decoding the first main stream

data! This signal is .sent along with the stream data, e.g.
embedded in the tune-in stream data (e.g. as an RTP header
extension) or e.g. as a separate signal flow (e.g. as an
RTCP message)-
The client device is monitoring this signaling, and on ar-
rival of a message the client acts according,to this mes-
sage, i.e.- stops decoding of the tune-in stream data and
starts decoding the main stream data.
This has the advantage that the client can avoid parsing of
stream data (e.g. parsing header information of the' encoded
picture data for a picture identifier, this identifier be-
ing different for different encoding schemes like H.264 or
MPEG-2), 'and only needs to read signaling messages. Another
advantage is that the- client not only gets the information
about the arrival of the I-picture, but also when the I-
picture is ready for decoding (e.g. when the input buffer
.for the main'stream is filled to a level that avoids buf-
fer-underrun and- overflow in the subsequent stream decod-
ing) .
.Referring to Fig. 3, the client does not monitor the main
stream for an I-picture at step S310, but monitors the sig-
naling 'in step S311 to receive the information about arriv-
al (and complete reception of the picture data) and the
"ready for decoding" status.
Referring to Fig.4, the decoder portion 412 does not moni-
tor the main -stream via the dotted line 418, but the sig-
naling via the dotted line 419 to get the- above described
information.
An alternative, signaling aspect is now described. As men-
tioned above, the client needs to wait for the first I~
picture data of the main stream before being able to decode
the main stream. Any data of the main stream that is re-
ceived before the first I-picture data cannot be used and

has to be deleted.. It is advantageous to avoid the recep-
tion of such data to minimize the time when both streams
(tune-in stream and main stream) need' to be received from
the client to save bandwidth on the client connection to
the network '(e.g. a DSL line) for other ■ applications that
may- also make use of the bandwidth (that is usually con-
straint on such e.g. DSL line, compared to the core network
bandwidth).
Thus, it is advantageous to signal, when the next I-picture
(i.e. the first packets/bits of the picture) can be re-
ceived by the client. This information enables the client
to join the main stream just before the I-picture data will
arrive and avoids the .client to join the main stream ahead
of time (e.g.- in parallel to the tune-in stream, as de-
scribed in Fig. 3 step S306).
Such signaling can be done similar to the above describe
"I-picture ready for decoding" message, e.g. embedded in
the tune-in stream or as a separate data flow. Further it
may be useful to'send a number of messages, e.g. "I-picture
starts in 3 seconds", "I-picture starts in 2 seconds", "I-
picture starts in 1 second", for error robustness and to
■enable the client to cope with potential network jitter
(i.e. different network delays between- transmission of the
signaling and the stream data).
In accordance with further embodiments the decoder 4 00 may.
comprise a post-processing unit 422 which may further re-
duce the subjective difference between the tune-in stream
and the main 'stream when' switching from the tune-in stream
to the main stream. Preferably, the post-processing unit
422 manipulates the main stream decoding. To be more spe-
cific, a filtering of the decoded pictures from the main
stream may be applied, starting from the fir$t picture of
the' main stream over a specific period of time, for example
0.5 seconds. This filtering may result in a blurring of the
picture and in embodiments' the filter coefficients may. be

modified over the above mentioned period of time, thereby
reducing the blurring effect or filtering effect from pic-
ture to picture,' thereby gradually■changing from the low-
quality of the tune-in stream to the high-quality of the
main stream. Alternatively, post-processing may be achieved
by decoding the first picture after the switch only with a
subset of coefficients which is known as reduced resolution
update in the H.'263 standard. Following the first picture
for each subsequent picture an increased number of coeffi-
cients is used until the full number of coefficients is ap-
-plied and a full quality decoding of the main stream is
achieved. This approach also leads to reduced image details
in the"first pictures after the switch and thereby contri-
butes to the subjective hiding of the switch from the low-
quality stream to the high-quality stream.
The embodiments described so far operated on the basis of
receivers which obtained-both the main stream and the side
stream at the same time, however, the invention is not li-
mited to such an environment. Fig. 5 is a block diagram of
a receiver/server system which can also be used in accor-
dance with embodiments of the invention.' The . system shown
in -Fig. 5 comprises the receiver 500 which includes the de-
coder 502 and a user interface 504. In addition, a server
506 is shown. The server 506 comprises an optional demul-
tiplexer 508 (see Fig. 5a) and a selector 510. The server
506 receives at the . input of the demultiplexer 508 a com-
bined video stream 512 for one channel and by means of the
demultiplexer 508 combined input stream 512 is separated
into the main stream 514 and the tune-in stream or side-
stream 51,6. Alternatively, the main stream 514 and the
tune-in stream or side-stream 516 may be received separate-
ly so that no .demultiplexer is" needed (see Fig. 5b). These
streams are input into the selector 510 which outputs one
of the selected streams to the decoder as is shown at 518.
Further, the selector 510 is connected to the user inter-
face 504 of the receiver 500 as is shown at 520. Via line
520" the selector 510 may receive for the user interface 504

a change request signal. The functionality of the system
shown in Fig. 5 is similar to the one described above. To
be more specific, as long as no channel change request is
received at. the selector the main stream for a specific
channel is provided via' the selector .to the decoder, de-
coded at output. Upon receiving a channel change request
via line 520 .the selector tunes into the tune-in stream 516
and supplies the.tune-in stream to the decoder until at the
main stream an I-picture is obtained. Once this I-picture
is present switching from the low-quality tune-in stream to
the high-quality main stream is done as described above.
The main stream 514 may also be provided directly to the
decoder (see the dashed line in Fig. 5), i.e. not via the
server.
In accordance with another aspect of the invention simple
distortions upon switching from a tune-in stream to a main
stream may be avoided by providing one or more additional
tune-ih streams which, are encoded at'different quality le-
vels between the quality level of- the-main stream and the
quality level of the first tune-in stream. This approach
might be combined with the above-mentioned and described
approach of introducing the I-pictures at the varying dis-
tances, however, the approach of using a plurality of tune-
streams might also be used with conventional main stream
encoding approaches, i.e. with conventional main streams
having their I-pictures at fixed distances, i.e. positioned
irrespective of the content.
Fig. 6 illustrates an embodiment of the invention using one
main stream and two tune-in streams. Fig. 6(a) corresponds
to Fig. 7 and shows a situation where only a single main
stream and a•single tune-in stream, is provided. Fig. 6(b)
shows the change of quality using two tune-in streams and
Fig. 6(c) is a schematic representation of the respective
streams, wherein the vertical lines illustrate the position
of an I-picture in the respective streams. As can be seen
from Fig. 6(c), the number of I-pictures within the main

stream is the lowest, and these I-pictures are placed at a
long distance from each other. The I-pictures might either
be placed in the main stream dependent on its content or at
fixed distances. The first tune-in stream has the highest
number- of I-pictures, whereas the second tune-in stream has
a number of I-pictures, .which is between the number of I-
pictures of the main stream and the. tune-in stream.
The second tune-in stream may be used to avoid any unde-
sired jump in the presented image quality during tune-in.
The- second tune-in stream may be encoded in a way that -
during a part of a transition period - the quality is grad-
ually increased from the tune-in stream, quality Qi to the
full quality Qf of the main stream (100%) , as is shown in
Fig. 6(b). To be able to achieve Qf 'at the end of the'tran-
sition period, the second tune-in stream is using the full
resolution of the main stream. To enable a smooth transi-
tion and to avoid excessive bit rate overhead, the second
tune-in stream may use a longer GOP period than the 'tune-in
stream-.
A tune-in process'may look like it is shown in Fig. 6(b).
After initializing the tune-in stream at time t0, the
client device first of all has to wait for" the -next • I-
picture to arrive which is the. I-picture in the tune-in
stream, which arrives at tR. During the period from-tR to
tT' , the tune-in stream is decoded and presented. At time
tT' an I-picture of the second tune-in stream arrived and
is ready .for decoding. During the time period from tT' to
tTr the second tune-in stream is presented, and at time tT
the transition period ends and the. client switches to the
main stream.
In addition, embodiments allow only the second tune-in
stream' to be encoded with an adaptive GOP length and the
tune-in stream and the main streams may have .a fixed GOP
length. In other words, in such an embodiment the main
stream and the tune-in stream are conventionally encoded

and only the second tune-in stream is encoded in such a
manner that the I-pictures are placed within the second
tune-in stream dependent on the content.
It is further noted that the invention is not limited to
the use of only . two tune-in streams', rather it is possible
to have N tune-in streams instead of two, thereby enabling
a finer granularity ' and smoothness upon, increasing the
quality.
Embodiments- as described above included tune-in streams or
side streams which included both, I-pictures and P-pictures
or B-pictures. However, the invention is not limited, to
such tune-in streams. In alternative' embodiments the tune-
in streams may consist only of single tune-in pictures,
e.g. the side stream may only transmit the random access
points or I-pictures. The'intermediate pictures are decoded
using the data from the main stream, e.g. information from
the P-pictures and/or- B-pictures of the main stream. The
embodiment's described above describe two separate streams,
the main stream and the tune-in stream. However, the inven-
tion is not limited to such embodiments., rather the above
methods can also be adapted to scalable video coding (SVC)
using for example two layers, an enhancement layer having a
longer I-pictufe distance and a base layer having a shorter
I-picture'distance. In such a scenario the base layer cor-
responds to the tune-in stream and is designed like the
above-described tune-in stream. However, the above-
mentioned use of more than one tune-in stream can also be
realized by a scalable video coding approach, for example
by applying a three-layer SVC with one base layer and two
enhancement layers. Scalable video coding and using' this
approach for tuning into a main stream upon a channel
change are e.g. described in WO 2008 138546 A2 and
US 2003/0007562 Al the disclosures of which are incorpo-
rated herewith- by reference.

Subjectively hiding the switch from the low-quality stream
to the high-quality stream is achieved in the same manner
as above, i.e. during switching only the base layer is used
for decoding until an I-picture in an enhancement layer is
received. Upon receipt of this I-picture in the enhancement
layer decoding is done on the basis of the information from.
the .enhancement layer. Further, while embodiments of the
invention are described with regard to an internet protocol
TV system (IPTV system) it is noted that the invention is
not limited to such an environment. Rather, the embodiments
described above can be used and applied to any multimedia
distribution system, e.g., broadcasting systems. Since em-
bodiments ■ of the invention provide a pure head-end solu-
tion, the solution ' of tune-in streams with an adaptive
transition period length may also be applied to any multi-
media distribution system.
In accordance with a further embodiment of the invention an
additional switching picture for every I-picture may be
sent in the main stream or in the tune-in stream or inde-
pendent from the streams. The switching picture is an al-
ternative encoding of an I-picture at the same sampling in-
stance and it is only used after .switching. During normal
decoding of the main stream, in the steady state of channel
reception the normal I-picture from the main stream is
used. The switching picture is encoded in a way that also
helps to reduce the visual difference between tune-in and
main streams. It is encoded at a quality between the main
stream and the tune^in stream. Alternatively, a complete
"switching GOP" may be sent instead of a switching picture
to control the "quality ramp-up"■ from the intermediate
tune-in quality to the full main stream quality.
Further, it is noted that the embodiments were described in
combination with video data, however it is noted that the
invention is not limited to the transmission of video data,
rather- the principles described in the embodiments above
can be applied to any kind of data which is to be encoded

in a data stream. To be more specific, the above described
principles also apply to audio data or. other kind of timed
multimedia data that uses the principle of differential en-
coding, utilizing the principle of different types of
transmitted data fragments within a data stream, like full
information (that enables the client to decode the full
presentation of the encoded multimedia data) and delta (or
update) information (that contains only differential infor-
mation that the client can only use for a full presentation
of the encoded multimedia data if preceding information was
received). Examples of such multimedia data, besides video,
are graphics' data, vector graphics data, 3D graphics data
in .general,, e.g.- wireframe and texture data, or 2D or 3D
scene representation, data.
.It should' be "understood that depending, on the circums-
tances, the methods of embodiments of the invention may al-
so be implemented in software. Implementation may occur on
a digital storage medium,- in particular a disc, a DVD or a
CD with electronically readable control signals which can
interact with a programmable computer system such that the
respective method is executed. Generally, the invention
thus also consists in a computer -program product with a
program code stored on a machine-readable -carrier for per-
forming the inventive method, when the computer .program
product runs on a PC and/or a microcontroller. In other
words, the invention may thus be realized as a computer
pro-gram with a program code for performing the method when,
the computer program runs on a computer and/or a microcon-
troller.
It is noted that the above description illustrates the
principles of the invention, but: it will be appreciated
that those skilled in the art will be able to devise -vari-
ous arrangements that, although not explicitly described or
shown here, embody the principles of the invention without
departing from the spirit or scope of the .invention. It
will be appreciated by those' skilled in the art that the

block diagrams presented herein represent conceptual views
of illustrative circuitry embodying the. principles of the
invention. Similarly, it will be appreciated that any flow-
charts, flow-diagrams, transmission diagrams and the like
represent various processes which may be substantially pre-
sented in a computer readable media and so executed by- a
computer or processor' whether or not such a computer or
processor is implicitly shown. The functions 'of the various
elements shown in the figures may be provided, through the
use of dedicated hardware as well as hardware capable of
executing software in association with the appropriate
software. When provided by a processor, the functions may
be provided by a single dedicated processor,' by a single
shared processor or by a plurality of individual proces-
sors, some of which may be. shared.' Moreover, explicit use
of the term "processor" or "controller" should not be con-
strued to refer exclusively to hardware .capable of execut-
ing software, it may implicitly include, without limita-
tion, a digital signal processor hardware, . read-only memory
for storing software, random access memory and non-volatile
storage.
Embodiments of the invention were described in the context
of a- multi-channel transmission system in which the main
stream and the secondary stream are associated with a chan-
nel of a multi-channel transmission system, and a tune-in
request indicates a change from a current channel of the
multi-channel transmission system to a new channel of the
multi-channel transmission system.
However, the invention is not limited to such embodiments.
Rather, the invention, in general, is concerned with im-
proving the tune-in characteristics upon tuning into a
stream which comprises a main stream and-at least a second-
ary stream as described in detail above, wherein the stream
may be a single stream which is provided to a user, e.g.
over a network, like the Internet.

The stream containing e.g. a video contents may be provided
by a service provider such that a user may tune into the
stream at any time. In such a situation, after receiving
the tune-in request the stream including both the main and
the secondary streams is received by the user, and the sec-
ondary stream is decoded until the self-contained block ar-
rives on the main stream and the required main stream de-
coder buffer-fill level is reached.
In another embodiment of the invention, the stream is ob-
tained by a user on the user's demand, e.g. from a service
provider. The stream (e.g. video on. demand) is received by
the user and when tuning into the stream decoding of the
stream starts. Again, decoding of the stream is done on the
basis of the secondary stream until the required main
stream decoder buffer-fill level is reached. This approach
is chosen despite the fact that the stream, due to being
obtained on demand, may be provided to the user such that a
I-picture is present in the main stream upon tuning into
the stream. Nevertheless, decoding will not start imme-
diately, rather, a predefined main stream decoder buffer
fill level will be obtained to ensure continuous decoding
of the stream even in case of temporal interruptions of the
stream (e.g. due to delayed stream packets due to network
traffic) . However, the amount of data to be buffered for
the main stream is quite high. Therefore, also in such a
situation the secondary stream is used at the beginning as
the amount of information or data to be buffered in the
secondary stream decoder is lower than the amount to be
buffered in the main stream decoder. Thus, when using the
secondary stream decoding will start earlier as the re-
quired fill lever for the secondary stream buffer is
reached fast. Once the required main stream decoder buffer-
fill level is reached decoding of the main stream starts.
In the description of the embodiments of the invention, the
self-contained blocks and the non-self-contained blocks of
the streams were named as I-pictures and P- or B-pictures,

respectively. It is noted, that the term "picture", in gen-
eral, determines an encoded contents that includes data or
information that is necessary to decode the contents of the
block. In case of I-pictures all data or information is in-
cluded that is necessary to decode the complete contents of
the block, whereas in case of P- or B-pictures not all in-
formation is included that is necessary to decode a com-
plete picture, rather additional information from preceding
or following pictures is required. Alternatively, the I-,
P- and B-pictures may be named I-, P- and B-frames.

We claim:
1.A method for generating a stream of data comprising a
plurality of encoded data blocks, the plurality of en-
coded data blocks comprising a plurality of self-
contained blocks including all information for decod-
ing the block and a plurality of blocks including only
partial information for decoding, the method compris-
ing:
varying a distance of self-contained blocks in the
stream dependent on the content encoded in the stream,
wherein the stream is a main stream, and wherein tun-
ing into the main stream is effected via a secondary
stream comprising at least a subset of the data blocks
of the main stream encoded at a quality different from
a quality of the data blocks of the main stream, and
wherein the self-contained blocks are inserted at po-
sitions in the main stream where differences in the
quality of the data encoded in the main stream and in
the secondary stream are less detectable.
2. The method of claim 1, comprising
analyzing the content of the main stream during encod-
ing the data for the main stream; and
based on the analysis, determining a position of the
self-contained blocks in the main stream.
3. The method of claim 1, wherein the self-contained
blocks are inserted at a position in the main stream
where the content is more dynamic than at another po-
sition.

wherein the encoder is configured to vary a distance
of self-contained blocks in the stream dependent on
the content encoded in the stream,
wherein the stream is a main stream, and wherein tun-
ing into the main stream is effected via a secondary
stream comprising at least a subset of the data blocks
of the main stream encoded at a quality different from
a quality of the data blocks of. the main stream, and
wherein the self-contained blocks are inserted at po-
sitions in the main stream where differences in the
quality of the data encoded in the main stream and in
the secondary stream are less detectable.
10. A method for tuning into a main stream, the method
comprising:
providing a main stream comprising a plurality of en-
coded data blocks, the plurality of encoded data
blocks comprising a plurality of self-contained blocks
including all information for decoding the block and a
plurality of blocks including only partial information
for decoding;
providing a secondary stream comprising at least a
subset of the data blocks of the main stream encoded
at a lower quality than the data blocks of the main
stream, wherein the self-contained blocks are inserted
in the main stream at positions where differences in
the quality of the data encoded in the main stream and
the secondary stream are less detectable;
upon receiving a tune-in request, tuning into the sec-
ondary stream and the main stream;

4. The method of claim 3, wherein the data of the main
stream represents video content, and wherein the self-
contained blocks are inserted at scene boundaries or
during fast camera movements during a scene.
•5. The method of claim 4, wherein the main stream is en-
coded using a differential video coding scheme yield-
ing a plurality of I- or IDR-pictures as self-
contained blocks and a plurality of P-pictures or B-
pictures.
6. The method of claim 4, wherein the content is encoded
using a scalable video coding scheme yielding at least
one enhancement layer as the main stream and a base
layer as the secondary stream, the I-picture distance
in the base layer being shorter than the I-picture
distance in the enhancement layer.
7. The method of claim 1, wherein the distance of the
self-contained blocks in the stream is varied within a
predefined time window.
8. The method of claim 1, further comprising:
generating for each self-contained block in the main
stream a switching block or for each group of blocks
in the main stream a group of switching blocks, a
switching block being encoded with a quality between
the quality of the data blocks of the main stream and
the secondary stream.
9. An encoder for generating a stream of data comprising
a plurality of encoded data blocks, the plurality of
encoded data blocks comprising a plurality of self-
contained blocks including all information for decod-
ing the block and a plurality of blocks including only
partial information for decoding,

decoding the secondary stream until a self-contained
block arrives on the main stream or until a required
main stream decoder buffer-fill level is reached; and
upon arrival of the self-contained block on the main
stream or reaching the required main stream decoder
buffer-fill level, stopping decoding of the secondary
stream and starting decoding of the main stream.
11. The method of claim 10, wherein the main stream and
the secondary stream are provided to a receiver,
wherein the secondary stream comprises a plurality of
self-contained blocks, wherein a distance of the self-
contained blocks in the secondary stream is shorter
than the distance of the self-contained blocks in the
main stream.
12. The method of claim 11, wherein the secondary stream
further comprises a plurality of blocks including only
partial information for decoding.
13. The method of claim 10, wherein the main stream and
the secondary stream are provided via a server to a
receiver, wherein in response to the tune-in request
the secondary stream is provided from the server to
the receiver for decoding, until the self-contained
block arrives on the main stream or until the required
main stream decoder buffer-fill level is reached.
14. The method of claim 10, further comprising post-
processing the decoded main stream to reduce an ab-
ruptness of the transition from the low-quality sec-
ondary stream to the high-quality main stream.
15. The method of claim 14, wherein post-processing com-
prises filtering the decoded main stream, wherein a
filter coefficient is adapted over a predefined number
of blocks to gradually increase the quality, or where-

in post-processing comprises starting the decoding of
the main stream with a reduced set of decoding coeffi-
cients and adjusting the set of decoding coefficients
over a predefined number of blocks to gradually in-
crease the quality of the main stream.
16. The method of claim 10, further comprising:
providing an additional secondary st ream comprising at
least a subset of the data blocks of the main stream
encoded at a quality between the quality of encoding
the data blocks of the main stream and the secondary
stream,
wherein the secondary stream is decoded until a self-
contained block arrives on the additional secondary
stream, and
wherein the additional secondary stream is decoded un-
til arrival of the self-contained block in the main
stream.
17. The method of claim 16, wherein a plurality of addi-
tional secondary streams is provided, each additional
secondary stream comprising at least a subset of the
data blocks of the main stream encoded with a differ-
ent quality between the quality of encoding the data
blocks of the main stream and the secondary stream.
18. The method of claim 10, further comprising:
providing for each self-contained block in the main
stream or in the secondary stream a switching block or
for each group of blocks in the main stream or the
secondary stream a group of switching blocks, a
switching block being encoded with a quality between
the quality of data blocks of the main stream and the
secondary stream.

19. The method of claim 18, wherein the switching blocks
or the group of switching blocks are provided together
with the main stream or the secondary stream or are
provided independent from the main stream and the sec-
ondary stream.
20. The method of claim 10, further comprising:
signaling a switching point in the secondary stream or
the main stream to a decoder used for decoding the
secondary stream and the main stream; and
switching the decoder from a mode for decoding the
secondary stream to a mode for decoding the main
stream before starting decoding of the main stream.
21. The method of claim 10, wherein the main stream and
the secondary stream are associated with a channel of
a multi-channel transmission system, wherein the tune-
in request indicates a change from a current channel
of the multi-channel transmission system to a hew
channel of the multi-channel transmission system, and
wherein the secondary stream is decoded until the
self-contained block arrives on the main stream and
the required main stream decoder buffer-fill level is
reached.
22. The method of claim 10, wherein the main stream and
the secondary stream are associated with a stream,
wherein the tune-in request initiates an initial tun-
ing into the stream, and wherein the secondary stream
is decoded until the self-contained block arrives on
the main stream and the required main stream decoder
buffer-fill level is reached
23. The method of claim 10, wherein the main stream and
the secondary stream are associated with a stream

which is obtained on demand of a user, wherein the
tune-in request initiates an initial tuning into the
stream, and wherein the secondary stream is decoded
until the required main stream decoder buffer-fill
evel is reached.
24. A decoder for receiving encoded data and providing de-
coded output data, the decoder comprising:
an input for receiving a main stream and a secondary
stream, the main stream comprising a plurality of en-
coded data blocks; the plurality of encoded data
blocks comprises a plurality of self-contained blocks
including all information for decoding the block and a
plurality of blocks including only partial information
for decoding, wherein the secondary stream comprises
at least a subset of the data blocks of the main
stream encoded at a lower quality than the data blocks
of the main stream, and wherein the self-contained
blocks are inserted at positions in the main stream
where differences in the quality of the data encoded
in the main stream and the secondary stream are less
detectable;
a control input for receiving a tune-in request sig-
nal; and
a decoding portion coupled to the input and to the
control input for producing decoded output data, the
decoding portion being adapted to tune into the sec-
ondary stream upon receiving a tune-in request at the
control input, to decode the secondary stream until a
self-contained block arrives on the main stream or un-
til a required main stream decoder buffer-fill level
is reached, and to stop decoding of the secondary
stream and to start decoding of the main stream upon
arrival of the self-contained block on the main stream

or reaching the required main stream decoder buffer-
fill level.
25. A computer readable medium for storing instruction
which, when being executed by a computer, carry out a
method for generating a stream of data comprising a
plurality of encoded data blocks, the plurality of en-
coded data blocks comprising a plurality of self-
contained blocks including all information for decod-
ing the block and a plurality of blocks including only
partial information for decoding, wherein a distance
of self-contained blocks in the stream is varied, de-
pendent on the content encoded in the stream,
wherein the stream is a main stream, and wherein tun-
ing into the main stream is effected via a secondary
stream comprising at least a subset of the data blocks
of the main stream encoded at a quality different from
a quality of the data blocks of the main stream, and
wherein the self-contained blocks are inserted at po-
sitions in the main stream where differences in the
quality of the data encoded in the main stream and in
the secondary stream are less detectable.

A method generates a stream of data, wherein the stream
comprises a plurality of encoded data blocks. The encoded
data blocks comprise a plurality of self-contained blocks
including all information for decoding the block and a plurality
of blocks including only partial information for decoding.
The distance of the self-contained blocks in the
stream is varied dependent on the content encoded in the
stream. The stream is a main stream, and wherein tuning
into the main stream is effected via a secondary stream
comprising at least a subset of the data blocks of the main
stream encoded at a quality different from a quality of the
data blocks of the main stream. The self-contained blocks
are inserted at positions in the main stream where differences
in the quality of the data encoded in the main stream
and in the secondary stream are less detectable.