The present invention is related to audio encoding/decoding and in particular to an approach of encoding and decoding data, which permits initializing a decoder such as it may be required when switching between different codec configurations.

Embodiments of the invention may be applied to scenarios, in which properties of transmission channels may vary widely depending on access technology, such as DSL, WiFi, 3G. LTE and the like. Mobile phone reception may fade indoors or in rural areas. The quality of wireless internet connections strongly depends on the distance to the base station and access technology, leading to fluctuations of the bitrate. The available bitrate per user may also change with the number of clients connected to one base station.

It is the object of the invention to provide for a concept which permits delivery of audio content in a flexible manner.

According to the invention, this object is achieved by an audio decoder according to claim 1, an apparatus for generating encoded audio output data according to claim 9, , a method for decoding audio input data according to claim 18, a method for generating encoded audio data according to claim 22, and a computer program according to claim 25.

Embodiments of the invention provide an audio decoder for decoding a bit stream of encoded audio data, wherein the bit stream of encoded audio data represents a sequence of audio sample values and comprises a plurality of frames, wherein each frame includes associated encoded audio sample values, the audio decoder comprising:

a determiner configured to determine whether a frame of the encoded audio data is a special frame comprising encoded audio sample values associated with the special frame and additional information, wherein the additional information comprise encoded audio sample values of a number of frames preceding the special frame, wherein the encoded audio sample values of the preceding frames are encoded using the same codec configuration as the special frame, wherein the number of preceding frames is sufficient, to initialize the decoder to be in a position to decode the audio sample values associated with the special frame if the special frame is the first frame upon start-up of the decoder; and

an initializer configured to initialize the decoder if the determiner determines that the frame is a special frame, wherein initializing the decoder comprises decoding the encoded audio sample values included in the additional information before decoding the encoded audio sample values associated with the special frame.

Embodiments of the invention provide an apparatus for generating a bit stream of encoded audio data representing a sequence of audio sample values of an audio signal, wherein the bit stream of encoded audio data comprise a plurality of frames, wherein each frame includes associated encoded audio sample values, wherein the apparatus comprises:

a special frame provider configured to provide at least one of the frames as a special frame, the special frame comprising encoded audio sample values associated with the special frame and additional information, wherein the additional information comprise encoded audio sample values of a number of frames preceding the special frame, wherein the encoded audio sample values of the preceding frames are encoded using the same codec configuration as the special frame, and wherein the number of preceding frames is sufficient to initialize a decoder to be in a position to decode the audio sample values associated with the special frame if the special frame is the first frame upon start-up of the decoder; and

an output configured to output the bit stream of encoded audio data.

Embodiments of the invention provide a method for decoding a bit stream of encoded audio data, wherein the bit stream of encoded audio data represents a sequence of audio sample values and comprises a plurality of frames, wherein each frame includes associated encoded audio sample values, comprising:

determining whether a frame of the encoded audio data is a special frame comprising encoded audio sample values associated with the special frame and additional information, wherein the additional information compnse encoded audio sample values of a number of frames preceding the special frame, wherein the encoded audio sampie

values of the preceding frames are encoded using the same codec configuration as the special frame, wherein the number of preceding frames is sufficient to initialize a decoder to be in a position to decode the audio sample values associated with the special frame if the special frame is the first frame upon start-up of the decoder; and

initializing the decoder if it is determined that the frame is a special frame, wherein the initializing comprises decoding the encoded audio sample values included in the additional information before decoding the encoded audio sample values associated with the special frame.

Embodiments of the invention provide a method for generating a bit stream of encoded audio data representing a sequence of audio sample values of an audio signal, wherein the bit stream of encoded audio data comprise a plurality of frames, wherein each frame includes associated encoded audio sample values, comprising:

providing at least one of the frames as a special frame, the special frame comprising encoded audio sample values associated with the special frame and additional information, wherein the additional information comprise encoded audio sample values of a number of frames preceding the special frame, wherein the encoded audio sample values of the preceding frames are encoded using the same codec configuration as the special frame, and wherein the number of preceding frames is sufficient to initialize a decoder to be in a position to decode the audio sample values associated with the special frame if the special frame is the first frame upon start-up of the decoder; and

generating the bit stream by concatenating the special frame and the other frames of the plurality of frames.

Embodiments of the invention are based on the finding that immediate replay of a bit stream of encoded audio data representing a sequence of audio sample vaiues of an audio signal and comprising a plurality of frames can be achieved if one of the frames is provided as a special frame including encoded audio sample values associated with preceding frames, which are necessary to initiate a decoder to be in a position to decode the encoded audio sample vaiues associated with the special frame. The number of frames necessary to initiate the decoder accordingly depends on the codec configuration used and is known for the codec configurations. Embodiments of the invention are based on the finding that switching between different codec configurations can be achieved in a beneficial manner if such a special frame is arranged at a position where switching between the coding configurations shall take place. The special frame may not only include encoded audio sampie values associated with the special frame, but further information that allows switching between codec configurations and immediate replay upon switching. In embodiments of the invention, the apparatus and method for generating encoded audio output data and the audio encoder are configured to prepare encoded audio data in such a manner that immediate reply upon switching between codec configurations can take place at the decoder side. In embodiments of the invention, such audio data generated and output at the encoder side are received as audio input data at the decoder side and permit immediate replay at the decoder side. In embodiments of the invention, immediate replay is permitted at decoder side upon switching between different codec configurations at the decoder side.

In embodiments of the invention, the initializer is configured to switch the audio decoder from a current codec configuration to a different codec configuration if the determiner determines that the frame is a special frame and if the audio sample values of the special frame have been encoded using the different codec configuration.

In embodiments of the invention, the decoder is configured to decode the special frame using the current codec configuration and to discard the additional information if the determiner determines that the frame is a special frame and if the audio sample values of the special frame have been encoded using the current coded configuration.

In embodiments of the invention, the additional information comprise information on the codec configuration used for encoding the audio sample values associated with the special frame, wherein the determiner is configured to determine whether the codec configuration of the additional information is different from the current codec configuration.

In embodiments of the Invention, the audio decoder comprises a crossfader configured to perform crossfading between a plurality of output sample values obtained using the current codec configuration and a plurality of output sample values obtained by decoding the encoded audio sample values associated with the special frame. In embodiments of the invention, the crossfader is configured to perform crossfading of output sample values obtained by flushing the decoder in the current codec configuration and output sample values obtained by decoding the encoded audio sample values associated with the special frame.

in embodiments of the invention, an earliest frame of the number of frames comprised in the additional information is not time-differentially encoded or entropy encoded relative to any frame previous to the earliest frame and wherein the special frame is not time-differentially encoded or entropy encoded relative to any frame previous to the earliest frame of the number of frames preceding the special frame or relative to any frame previous to the special frame.

In embodiments of the invention, the special frame comprises the additional information as an extension payload and wherein the determiner is configured to evaluate the extension payload of the special frame. In embodiments of the invention, the additional information comprise information on the codec configuration used for encoding the audio sample values associated with the special frame.

In embodiments of the invention, the encoded audio data comprise a plurality of segments, wherein each segment is associated with one of a plurality of portions of the sequence of audio sample values and comprises a plurality of frames, wherein the special frame adder is configured to add a special frame at the beginning of each segment.

In embodiment of the invention, the encoded audio data comprise a plurality of segments, wherein each segment is associated with one of a plurality of portions of the sequence of audio sample values and comprises a plurality of the frames, wherein the apparatus for generating a bit stream of encoded audio data comprises a segment provider configured to provide segments associated with different portions of the sequence of audio sample values and encoded by different codec configurations, wherein the special frame provider is configured to provide a first frame of at least one of the segments as the special frame; and a generator configured to generate the audio output data by arranging the at least one of the segments following another one of the segments. In embodiments of the Invention, the segment provider is configured select a codec configuration for each segment based on a control signal. In embodiments of the invention, the segment provider is configured to provide m encoded versions of the sequence of audio sample values, with m ≥ 2, wherein the m encoded versions are encoded using different codec configurations, wherein each encoded version comprises a plurality of segments representing the plurality of portions of the sequence of audio sample values, wherein the special frame provider is configured to provide a special frame at the beginning of each of the segments.

in embodiments of the invention, the segment provider comprises a plurality of encoders, each configured to encode at least in part the audio signal according to one of the plurality of different codec configurations. In embodiments of the invention, the segment provider comprises a memory storing the m encoded versions of the sequence of audio sample values.

In embodiments of the invention, the additional information are in the form of an extension payload of the special frame.

In embodiments of the invention, the method of decoding comprises switching the audio decoder from a current codec configuration to a different codec configuration if it is determined that the frame is a special frame and if the audio sample values of the special frame have been encoded using the different codec configuration.

In embodiments of the invention, the bit stream of encoded audio data comprises a first number of frames encoded using a first codec configuration and a second number of frames following the first number of frames and encoded using a second codec configuration, wherein the first frame of the second number of frames is the special frame.

In embodiments of the invention, the additional information comprise information on the codec configuration used for encoding the audio sample values associated with the special frame, and the method comprises determining whether the codec configuration of the additional information is different from the current codec configuration using which encoded audio sample values of frames in the bit stream, which precede the special frame, are encoded.

In embodiments of the invention, the method of generating a bit stream of encoded audio data comprises providing segments associated with different portions of the sequence of audio sample vaiues and encoded by different codec configurations, wherein a first frame of at least one of the segments is provided as the special frame.

Thus, in embodiments of the invention, crossfading is performed in order to permit seamless switching between different codec configurations. In embodiments of the invention, the additional information of the special frame comprise the pre-roll frames necessary in order to initialize a decoder to be in a position to decode the special frame. In other words, in embodiments of the invention, the additional information comprise a

copy of that frames of encoded audio sample values preceding the special frame and encoded using the same codec configuration as the encoded audio sample values represented by the special frame necessary to initialize the decoder to be in position to decode the audio sample values associated with the special frame.

In embodiments of the invention, special frames are introduced into encoded audio data at regular temporal intervals, i.e. in a periodic manner. In embodiments of the invention, a first frame of each segment of encoded audio data is a special frame. In embodiments, the audio decoder is configured to decode the special frames and following frames using the codec configuration indicated in the special frame until a further special frame indicating a different codec configuration is encountered.

In embodiments of the invention, the decoder and the method for decoding are configured to perform a crossfade when switching from one codec configuration to another codec configuration, in order to permit seamless switching between multiple compressed audio representations.

In embodiments of the invention, the different codec configurations are different codec configurations according to the AAC (Advanced Audio Coding) standard, i.e. different codec configurations of the AAC family codecs. Embodiments of the invention may be directed to switching between codec configurations of the AAC family codecs and codec configurations of the AMR (Adaptive Multiple Rate) family codecs.

Thus, embodiments of the invention permit for immediate replay at decoder side and switching between different codec configurations so that the manner in which audio content is delivered may be adapted to the environmental conditions, such as a transmission channel with variable bitrate. Thus, embodiments of the invention permit for providing the consumer with the best possible audio quality for a given network condition.

Embodiments of the invention are subsequently discussed referring to the accompanying drawings, in which:

Fig. 1 shows a schematic view of an embodiment of an apparatus for generating encoded audio output data;

Fig. 2 shows a schematic view for explaining an embodiment of a special frame;

Fig. 3 shows a schematic view of different representations of an audio signal;

Fig. 4a and Fig. 4b show schematic views of apparatuses for generating encoded audio output data;

Fig. 5 shows a schematic view of an audio decoder;

Fig. 6 shows a schematic block diagram for explaining an embodiment of an audio decoder and a method for decoding;

Fig. 7 shows a schematic block diagram for explaining switching of an audio decoder between different codec configurations;

Fig. 8 shows a schematic diagram for explaining AAC (Advanced Audio Coding) decoder behavior;

Fig. 9 shows switching from a first stream 1 to a second stream 2; and

Fig. 10 shows an exemplary syntax element providing additional information.

Generally, embodiments of the invention aim at the delivery of audio content, possibly combined with video delivery, over a transmission-channel with variable bitrate. The goal may be to provide a consumer with the best possible audio quality for a given network condition. Embodiments of the invention focus on the implementation of AAC family codecs into an adaptive streaming environment.

In embodiments of the invention, as used herein, audio sample values which are not encoded represent time domain audio sample values such as PCM (pulse code modulated) samples. In embodiments of the invention, the term encoded audio sample value refers to frequency domain sample values obtained after encoding the time domain audio sample values, in embodiments of the invention, the encoded audio sample values or samples are those obtained by converting of the time domain samples into a spectral representation, such as by means of a MDCT (modified discrete cosine transformation), and encoding the result, such as by quantizing and Huffman coding. Accordingly, in embodiment of the invention, encoding means obtaining the frequency domain samples

from the time domain samples and decoding means obtaining the time domain samples from the frequency domain samples. Sample values (samples) obtained by decoding encoded audio data are sometimes referred to herein as output sample values (samples).

Fig. 1 shows an embodiment of an apparatus for generating encoded audio output data. Fig. 1 shows a typical scenario of adaptive audio streaming, which embodiments of the invention may be applied to. An audio input signal 10 is encoded by various audio encoders 12, 14, 16 and 18, i.e. encoders 1 to m. The encoders 1 to m may be configured to encode the audio input signal 10 simultaneously. Typically, encoders 1 to m may be configured such that a wide bit rate range can be achieved. The encoders generate different representations, i.e. coded versions, 22, 24, 26 and 28 of the audio input signal 10, i.e. representations 1 to m. Each representation includes a plurality of segments 1 to k, wherein the second segment of the first representation has been given reference number 30 for exemplary purposes only. Each segment comprises a plurality of frames (access units) designated by the letters AU and a respective index 1 to n indicating the position of the frame in the respective representation. The eighth frame of the first representation is given reference number 40 for exemplary purposes only.

The encoders 12, 14, 16 and 18 are configured to insert stream access points (SAPs) 42 at regular temporal intervals, which define the sizes of the segments. Thus, a segment, such as segment 30, consists of multiple frames, such as AU5, AU6, AU7 and AU8, wherein the first frame, AU5, represents a SAP 42. In Fig. 1 , the SAPs are indicated by hatching. Each representation 1 to m represents a compressed audio representation (CAR) for the audio input signal 10 and consists of k such segments. Switching between different CARs may take place at segment borders.

On decoder side, a client may request one of the representations which fits best for a given situation, e.g. for given network conditions. If for some reason the conditions change, the client should be able to request a different CAR, the apparatus for generating the encoded output data should be able to switch between different CARs at every segment border, and the decoder should be abie to switch to decode the different CAR at every segment border. Hence, the client would be in a position to adapt the media bit rate to the available channel bit rate in order to maximize quality while minimizing buffer under runs ("re-buffering"). If HTTP (Hyper Text Transfer Protocol) is used to download the segments, such a streaming architecture may be referred to as HTTP adaptive streaming.

Current implementations include Apple HTTP Live Streaming (HLS), Microsoft Smooth Streaming, and Adobe Dynamic Streaming, which all follow the basic principle. Recently. MPEG released an open standard: Dynamic Adaptive Streaming over HTTP (MPEG DASH), see "Guidelines for Implementation: DASH-AVC/264 Interoperability Points". http://dashif.Org/w/2013/08/DASH-AVC-264-v2.00-hd-mca.pdf. HTTP typically uses TCP/IP (Transmission Control Protocol/Internet Protocol) as the underlying network protocol. Embodiments of the invention can be applied to all of those current developments.

A switch between representations (encoded versions) shall be as seamless as possible. In other words, there shall not be any audible glitch or click during the switch. Without further measures provided for by embodiments of the invention, this requirement can only be achieved under certain constraints and if special care is taken during the encoding process.

In Fig. 1, the respective encoder which a segment originates from is indicated by a respective mark put within a circle. Fig. 1 further shows a decision engine 50, which decides which representation to download for each segment. A generator 52 generates encoded audio output data 54 from the selected segments which are given reference numbers 44, 46 and 48 in Fig. 1 by concatenating the selected segments. The encoded audio output data 54 may be delivered to a decoder 60 configured to decode the encoded audio output data into an audio output signal 62 comprising audio output samples.

I/We Claim:
1. Audio decoder (60) for decoding a bit stream of encoded audio data,
wherein the bit stream of encoded audio data represents a sequence of
audio sample values and comprises a plurality of frames (40), wherein each
frame (40) includes associated encoded audio sample values, the audio
decoder (60) comprising:
a determiner (130) configured to determine whether a frame of the encoded
audio data is a special frame (42, 80) comprising encoded audio sample
values associated with the special frame (42, 80) and additional information
(82), wherein the additional information (82) comprise encoded audio
sample values of a number of frames (86) preceding the special frame,
wherein the encoded audio sample values of the preceding frames are
encoded using the same codec configuration as the special frame, wherein
the number of preceding frames, corresponding to pre-roll frames,
corresponds to the number of frames needed by the decoder (60) to build
up the full signal during start-up of the decoder so as to be in a position to
decode the audio sample values associated with the special frame (42, 80)
if the special frame is the first frame upon start-up of the decoder; and
an initializer configured to initialize the decoder (60) if the determiner
determines that the frame is a special frame, wherein initializing the
decoder comprises decoding the encoded audio sample values included in
the additional information before decoding the encoded audio sample
values associated with the special frame (42, 80),.
wherein the initializer is configured to switch the audio decoder (60) from a
current codec configuration to a different codec configuration (84) if the
determiner (130) determines that the frame is a special frame (42, 80) and
if the audio sample values of the special frame have been encoded using
the different codec configuration, and
wherein the decoder is configured to decode the special frame (42, 80)
using the current codec configuration and to discard the additional
information if the determiner determines (130) that the frame is a special
frame (42, 80) and if the audio sample values of the special frame have
5 been encoded using the current codec configuration,
wherein, prior to being initialized, the decoder is flushed upon detection of
a codec configuration change and the resulting output samples are stored
in a flush buffer.

2. Audio decoder of claim 1, wherein the additional information comprise
information on the codec configuration (84) used for encoding the audio
sample values associated with the special frame (42, 80), wherein the
determiner is configured to determine whether the codec configuration of
the additional information is different from the current codec configuration.
3. Audio decoder (60) of one of claims 1 and 2, comprising a crossfader (318)
configured to perform crossfading between a plurality of output sample
values obtained using the current codec configuration and a plurality of
output sample values obtained by decoding the encoded audio sample
values associated with the special frame (42, 80).
4. Audio decoder of claim 3, wherein the crossfader (318) is configured to
perform crossfading of output sample values obtained by flushing the
decoder (60) in the current codec configuration and output sample values
obtained by decoding the encoded audio sample values associated with
the special frame (42, 80).
5. Audio decoder of one of claims 1 to 4, wherein an earliest frame of the
number of frames (86) comprised in the additional information (82) is not
time-differentially encoded or entropy encoded relative to any frame
previous to the earliest frame and wherein the special frame (42, 80) is not
time-differentially encoded or entropy encoded relative to any frame
previous to the earliest frame of the number of frames preceding the special

frame (42, 80) or relative to any frame previous to the special frame (42, 80). 6. Audio decoder of one of claims 1 to 5, wherein the special frame (42, 80) 5 comprises the additional information as an extension payload and wherein the determiner is configured to evaluate the extension payload of the special frame (42, 80). 7. Method for decoding a bit stream of encoded audio data, wherein the bit 10 stream of encoded audio data represents a sequence of audio sample values and comprises a plurality of frames (40), wherein each frame (40) includes associated encoded audio sample values, comprising: determining whether a frame of the encoded audio data is a special frame 15 (42, 80) comprising encoded audio sample values associated with the special frame (42, 80) and additional information (82), wherein the additional information (82) comprise encoded audio sample values of a number of frames (86) preceding the special frame, wherein the encoded audio sample values of the preceding frames are encoded using the same 20 codec configuration as the special frame, wherein the number of preceding frames, corresponding to pre-roll frames, corresponds to the number of frames needed by a decoder (60) to build up the full signal during start-up of the decoder so as to be in a position to decode the audio sample values associated with the special frame (42, 80) if the special frame is the first 25 frame upon start-up of the decoder; initializing the decoder (60) if it is determined that the frame is a special frame, wherein the initializing comprises decoding the encoded audio sample values included in the additional information before decoding the 30 encoded audio sample values associated with the special frame (42, 80); switching the audio decoder (60) from a current codec configuration to a different codec configuration (84) if it is determined that the frame is a
special frame (42, 80) and if the audio sample values of the special frame have been encoded using the different codec configuration; and decoding the special frame (42, 80) using the current codec configuration 5 and discarding the additional information if it is determined that the frame is a special frame (42, 80) and if the audio sample values of the special frame have been encoded using the current codec configuration, wherein, prior to being initialized, the decoder is flushed upon detection of 10 a codec configuration change and the resulting output samples are stored in a flush buffer. 8. Method of claim 7, wherein the bit stream of audio data comprises a first number of frames encoded using a first codec configuration and a second 15 number of frames following the first number of frames and encoded using a second codec configuration, wherein the first frame of the second number of frames is the special frame. 9. Method of one of claims 7 or 8, wherein the additional information comprise 20 information on the codec configuration (84) used for encoding the audio sample values associated with the special frame (42, 80), the method comprising determining whether the codec configuration of the additional information is different from the current codec configuration using which encoded audio sample values of frames in the bit stream, which precede 25 the special frame, are encoded. 10. Computer program for performing, when running on a computer or a processor, the method of one of claims 7 to 9.