Description
CIRCUIT ARRANGEMENT PROVIDING A HIGH-DEVIATION MODE AND METHOD OF OPERATING A BROADCAST SIGNAL RECEIVER APPARATUS
Embodiments of the Invention refer to a circuit arrangement for releasing a high-deviation mode of an audio processing unit of a broadcast signal receiver apparatus.- Other embodiments refer to a method of operating a broadcast signal receiver apparatus having a high-deviation mode.
A broadcast television signal contains a composite video signal describing a picture content and one or more sound signals describing the audio content of a television channel. A plurality of video standards is defined for the video signal and a plurality of audio standards is defined for the sound signal.
Some audio standards, for example the A2 audio standard, use two separate FM (frequency modulated) souhd carriers. In case of stereo transmission, a main sound carrier carries a main sound signal being a composite of the left and right audio channels and a second sound carrier may carry a further sound signal representing one of the audio channels or a difference signal of the left and right audio channels. In the case of monaural transmission, the main sound carrier carries the monaural channel whereas the second sound carrier may be absent. The NICAM (near instantaneous compounded audio multiplex) standard provides a main sound carrier for providing the monaural signal and a sub carrier carrying a digital signal representing one digital stereo sound channel, two independent digital mono sound channels, a pure data channel or a combined monaural sound and data channel. Other audio standards for television broadcast and audio broadcast provide non-monaural audio information modulated on a single sound carrier." For example, a composite of the left and right audio channels may be transmitted as baseband signal and a difference signal may be modulated on a
r
sub-carrier occupying a frequency range outside that of the baseband signal.
In some regions modulation of the sound signal can change with time within the same broadcast channel. For example, monaural transmission and stereophonic transmission may succeed each other in the same broadcast 'channel. For example, in emerging countries the main sound signal may often over-modulate the main sound carrier during a phase of monaural transmission, for example for attracting attention during the transmission of commercials. Over-modulation of up to 1000 percent of the standardized nominal FM deviation can be observed.

Gonventionally, TV sets provide a high-deviation mode, where the TV set accepts ultrahigh over-modulated sound carriers at the expense of stereo playback capabilities, since over-modulation and stereophonic transmission exclude each other In many standards, e.g. referring to the B/G, D/K letter codes. The viewer may release tjie high-deviation mode by manual setting.
. The object of the present invention is providing a more user-friendly broadcast receiving apparatus. This object is achieved with the subject-matter of the independent claims. Further embodiments are specified in the dependent claims, respectively.
Details and advantages of the Invention will become more apparent from the following description of embodiments In connection with the accompanying drawings. Features of the^ various embodiments may be combined unless they exclude each other.
BRIEF DESCRIPTION OF THE SERVERAL VIEWS OF THE DRAWINGS
Figure 1 is a schematic block diagram of a broadcast signal receiver
apparatus operable in a high-deviation mode And including a circuit arrangement for releasing the high-deviation mode in accordance with an embodiment of the invention.
Figure 2A is a schematic block diagram of a stereo detection unit in accordance with an embodiment using a filter for determining the presence of non-monaural audio information.
Figure 2B is a schematic.block diagram of a stereo detection unit in accordance with an-embodiment using a signal detector for determining the presence of non-monaural audio information.
Figure 2C is a schematic block diagram of a stereo detection unit In accordance with an embodiment using the demodulator FM threshold effect for determining the presence of non-monaural audio Information.
Figure 2D is a schematic diagram of a detector unit of the stereo detection unit of Figure 2C in accordance with another embodiment.

Figure 3A Is a schematic frequency diagram illustrating the effect of a low-pass filter of a carrier detection unit on a digital input signal branched off from a signal containing a composite video signal and a sound carrier.
Figure 3B is a schematic frequency diagram for illustrating the effect of a band-pass filter of a carrier detection unit in the case the digital input signal of the carrier detection unit contains a sound carrier at an analyzing frequency.'
Figure 3C illustrates the effect of the'band-pass filter of a carrier detection unit in the case the input signal' of the carrier detection unit does not contain a sound carrier at the analyzing frequency.
Figure 4 is a schematic block diagram of an audio processing unit according
to an embodiment related to a separated carrier detection circuit.
Figure 5A is a schematic frequency diagram of a signal containing a composite video signal, a main sound carrier and a sub sound carrier for illustrating effects of the Invention.
Figure 5B is a schematic frequency diagram of a signal containing a composite video signal and an over-modulated main sound carrier for illustrating effects of the Invention.
Figure 6 is a schematic state diagram of an audio processing unit for
illustrating effects of embodiments of the invention.
Figure 7 is a simplified flow-chart of a method of operating a broadcast signal
receiver apparatus in accordance with a further embodiment.
DETAILED DESCRIPTION
In the following, embodiments of the invention are described. It is important to note, that all described embodiments in the following may be combined in any way, i.e. there is no limitation that certain described embodiments^ may not be combined with others. Further, it should be noted that same reference signs throughout the figures denote same or similar elements. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the Invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the

present invention is defined by the appended claims. It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.
Figure 1 refers to a broadcast signal receiver apparatus 900 that receives an
analogue broadcast signal for example via an RF (radio frequency) antenna 901, a
cable connector 902 or a satellite reception equipment 903. The broadcast signal
may transm.lt a plurality of broadcast channels using a plurality of carrier
frequencies, wherein a broadcast chaririel may be a radio channel or a television
channel. Each broadcast 'channel may include one or more sound carriers
containing the audio contents. A television broadcast channel also contains a
composite' video signal containing. information describing brightness and
chrominance of pictures in ,a video stream transmitted in the respective, television
broadcast channel. ';
The broadcast signal receive'r apparatus 900.may be a stand-alone radio receiver apparatus or television tuner, or may be integrated in a TV set, a video recording system, a handheld communications unit equipped with TV functionality or a computer with TV functionality, for example a multi-media computer. The broadcast signal may be a radio signal or a television signal in conformity with a television standard as defined by the CCIR (Comite Consultatif International des Radiocommunications) letter code and using the NTSC (National Television System Committee) system, the PAL (phase alternating line) system, or the SECAM (Sequentiel couleur a memoire) colour system, for example PAL-N, PAL-I, PAL-B, PAL-D, PAL-K, NTSC-M, SECAM-L, SECAM-B or SECAM-G.
A receiver unit 905 tunes to a carrier frequency of a broadcast channel selected by the user or during an; automatic channel scan and samples the received analogue broadcast signal. A digital signal processing unit 100 receives the sampled broadcast signal. A pre-processing unit 110 may be provided for signal conditioning. In the case of a television signal, a video processing unit 115 provides further signal processing of the composite video signal and outputs video data VIp to an image display unit. From the Information modulated on one or more sound carriers an audio processing unit 120 may recover sound signals L, R for the left and the right audio channels. The audio processing unit 120 may further recover digital data transmitted via one of the sound carriers.
In a first transmission mode (normal mode, multi-audio mode) the broadcast station transmits non-monaural audio information, for example stereo or dual

audio Information. The non-monaural audio information may or may not contain additional digital data not referring to audio information. The audio information may be modulated on one sound carrier or on two sound carriers. For example, both a main sound signal and a sub-sound signal may be modulated on a first sound carrier, wherein the main sound signal may be transmitted in as baseband signal on the first sound carrier and the sub-sound signal may be transmitted using a sub-sound carrier modulated on the first sound carrier outside the baseband signal with the main sound signal. According to another embodiment, the main sound signal is modulated ori a first sound carrier and the sub-sound signal is modulated on a second sound carrier, wherein both the first and the second sound carrier are modulated on-the same broadcast carrier.
In a second transmission mode, the broadcast station transmits a main sound signal on the first sound carrier in su:ch a way that the main sound signal over-modulates the first sound carrier, wherein the level of the main sound signal exceeds the value necessacy to produce 100 percent modulation of the main sound carrier.
In a first receiver mode (normal mode, multi-audio mode) the audio processing unit 120 outputs non-monaural audio information modulated, for example, on the first sound carrier and the sub-carrier or on the first and second sound carriers of the broadcast channel. In a second receiver mode (high-deviation mode), the audio processing unit 120 only outputs the respective main sound signal and discards signals derived from a signal path demodulating the sub-carrier or the second sound carrier. The main sound signal is conditioned, e.g. attenuated or attenuated and equalized, or, before demodulation, filtered with a broader filter than in the first receiver mode to compensate for effects in the demodulated main sound signal resulting from.'over-modulation.
A stereo detection unit 150 receives a digital input signal containing a sub sound signal modulated on the first or second sound carrier when the broadcast station transmits in the first transmission mode. The input signal in substance correspDnds to a base band version of the broadcast signal modulated on the carrier frequency of the broadcast channel. In case of transmission of non-monaural audio information, the input signal contains at least the sub sound signal modulated on the first or second sound carrier or a digital signal modulated on a carrier. The stereo detection unit 150 outputs a stereo detection signal indicating presence of the sub sound signal in the input signal. A control unit 190 receives the stereo detection signal and, assumed that the audio

processing unit 120 currently operates In the second receiver mode (high-deviation mode), switches the audio processing unit 120 into the first receiver mode thereby releasing the high-deviation mode.
According to an embodiment illustrated in Figure 2A, the stereo detection unit 150 comprises a sub-sound signal filter unit 151a configured to let pass the sub-sound signal contained in the digital signal. A sub-sound signal detection unit 151b outputs the stereo detection signal when an output signal of the sub-sound signal filter unit 151a indicates presence of the sub-sound signal. The sub-sound signal may be modulated on the first, sound carrier. For example the sub-sound signal detection unit 151b allows for indicating a sub-sound signal coded according to the BTSC, or EIAJ standard or allows for indicating presence of the L-R sub-signal in an FM radio signal. According to another embodiment, the sub-sound signal may be modulated on a second sound carrier.
Figure 2B refers to an embodiment, where the stereo detection unit 150 comprises a digital signal detection unit 151c that indicates presence of a digitally modulated sub-signal modulated on a carrier contained in the digital signal. For example, the digital signal detection unit 151c indicates presence of a signal coded according to the NICAM standard.
Figure 2C refers to the detection of non-monaural audio information modulated on the first and a second sound carrier contained in the digital signal. To this purpose, the stereo detection unit 150 may comprise a carrier detection unit 15Id configured to output, as the stereo detection signal, a carrier detection signal indicating presence of the second sound carrier or the sub-sound carrier in the digital signal. Providing stereo detection by detection of the presence of a narrow carrier instead of a broad' sound signal may be more reliable, for example in situations where the spectrum of the over-modulated main sound carrier overlaps with a bandpass filter range provided for detection of the second sound signal. According to an embodiment, this approach may be applied to two sound carrier standards according to the B/G and D/K letter codes.
For determining the presence of the second sound carrier, the carrier detection unit 15Id may use the demodulator FM threshold effect. All components of the carrier detection unit 15Id may be provided separated from the audio processing unit 120. Other embodiments may provide sharing of resources of the carrier detection unit 15Id and a sub sound demodulation path of the audio processing unit 120.
^1

The carrier detection unit 15Id receives a digital input signal which is a modulated information signal in substance corresponding to a broadcast signal frequency-shifted by the tuning frequency. The modulated information signal contains a digital complex base-band version of the sound signal contained in the broadcast channel which a receiver unit is tuned to and contains two orthogonal components. A low-pass pre-filter unit 152 may be provided for suppressing out-of-band portions that may be contained in the modulated information signal, for example the frequency-shifted video , signal component. The upper cut-off frequency of the pre-filter unit 152 Is sufficiently high for allowing a significant portion of the side band energy of a base-band representation of a sound signal to pass. For example, the significant portion may be about 98 percent and the upper cut-off frequency of the pre-filter unit 152 may correspond to the Carson bandwidth of the modulated Information signal. On the other hand, the upper cut¬off frequency of the pre-filter unit 152"is sufficiently low to suppress out-of-band portions and to separate a Jjase-band representation of the sound signal in the modulated Information signal from video signal components. According to an embodiment, the upper cut-off frequency of the pre-filter unit 152 is In the range of 50 kHz to 180 kHz, for example about 150 kHz. According to an embodiment, the pre-filter unit 152 outputs a low-pass filtered digital signal that may contain a stereophonic signal, two different monaural signals or a monaural signal and a digital signal modulated onto a main and a sub sound carrier, respectively, or a monaural signal which may over-modulate the main sound carrier. "
An oscillator unit 155 may supply an analyzing frequency that corresponds to the frequency offset of the sub sound carrier of the standard employed for the currently received broadcast-signal. The oscillator unit 155 may be controlled to output different analyzing frequencies. According to an embodiment the analyzing frequency may be supplied to the pre-filter unit 152 to make the circuit arrangement effective as a sort of shiftable band pass with regard to the input signal.
A demodulator unit 154 demodulates the filtered input signal at the analyzing frequency to generate a test signal. The demodulator unit 154 includes an FM (frequency modulation) demodulator and may also include an AM (amplitude modulation) demodulator or may be a configurable unit whose mode of operation can be switched between AM and FM demodulation. If the pre-flltered digital signal contains the sub sound carrier, the test signal contains a base-band version of the signal modulated onto the sub sound carrier. Otherwise, the test

signal In substance contains broadband noise having a bandwidth substantially greater than a sound signal.
The analyzing frequency corresponds to the sub sound carrier, when a deviation between the .analyzing frequency and the sub sound carrier frequency is not greater than half the minimum frequency distance between two sound carriers defined in the various standards.
The broadband noise results from the demodulator FM threshold effect, according to which above a certain input signal level, the SNR (signal-to-noise ratio) in a demodulator output signal moderately increases with the input signal level, whereas below that certain input signal level, the SNR in the demodulator output signal falls steeply with decreasing input signal levels. Where the digital signal does not contain the sub sound carrier or contains only a weak sound signal, the demodulator primarily outputs broadband noise. By contrast, where the digital signal does contain the sulj sound carrier, the demodulator unit 154 outputs a band-limited signal. Small deviations between the analyzing frequency and the sub sound carrier frequency in substance only contribute to a DC (direct current) portion in the demodulator output signal, whereas at a significant deviation between the analyzing frequency and a signal carrier or in absence of any signal carrier, the demodulation generates a strong broadband noise in the test signal.
The carrier detection unit 15Id further comprises a band-pass filter unit 156 which filters the test signal and which has a pass band outside a maximum bandwidth of an information signal which is modulated on the sub sound carrier and which may be an audio signal or a data signal. The filtered test signal is output to a detection unit 158. When the test signal contains the sound or Information signal, the filter unit 156 attenuates it and not or only low signal power can be detected in the filtered test signal at the output of the filter unit 156. Otherwise, when the test signal is dominated by broadband noise due to the demodulator FM threshold effect, a portion of the broadband noise will fall within the pass band of the filter unit 156 and a noise signal with more signal power can be detected in the filtered test signal.
The detector unit 158 detects the noise contained in the filtered test signal and outputs a carrier detection signal when the noise falls below a predefined threshold. The filter unit 156 may be a high-pass filter or a band-pass filter and has a lower cut-off frequency that is higher than the upper limiting frequency of a typical sound signal modulated on the sub sound carrier. According to an
^1

embodiment, the lower cut-off frequency of the filter unit 156 is derived from the upper cut-off frequency of the pre-filter unit 152. For example, the lower cut-off frequency of the filter unit 156 is not lower or in the range of the upper cut-off frequency of the pre-filter unit 152.
According to an embodiment, the sound signal modulated on the second sound carrier contains sound information from 0 Hz to 15 kHz. In this case, the lower cut-off frequency of the filter unit 156 may be 15 kHz or higher, for example at least 10 percent above the upper limiting frequency of the sound signal. The filter unit 156 may be a band-pass filter w.hose upper cut-off frequency is determined such that expectable spurious disturbances or interference frequencies appear outside the pass band. For example, interference frequencies typically appear at harmonics of the line frequency or other repetitive signals in video signals. Though attenuated in the sound signal branch, the signals may still occur In the demodulated signal. According to another embodiment, the filter unit 156 may be a band-pass filter whose upper cut-off frequency depends on or results from the sample rate according to the Nyquist theorem.
Figure 2D illustrates an embodiment of a detector unit 158. The detector unit 158 may comprise a peak detection unit 158a and a threshold decision unit 158b. The peak detection unit 158a may detect a peak amplitude in the filtered signal. The threshold decision unit 158b may switch a signal level of the carrier detection signal in response to the peak amplitude of the filtered signal falling below or exceeding a predefined threshold value. In accordance with another embodiment, the detector unit 158 may comprise an integrator unit integrating the filtered signal during a predetermined time period, wherein the threshold decision unit 158b compares the integrated filtered signal with a predefined threshold value.
The diagrams of Figures 3A to 3C illustrate the effect of the functional sub-units of the carrier detection unit 15Id of Figure 2C in more detail. Figure 3A schematically shows the spectral range 380 of the modulated inforrhation signal, which is the digital input signal of the carrier detection unit 151d. The modulated information signal has an upper limiting frequency fa. The cut-off frequency fb of the pass band 390 of the pre-filter unit 152 of Figure 3A is selected to be greater than the limiting frequency fa and lower than a lower limiting frequency of out-of-band portions which may be contained in the digital complex base-band signal and which may result from the demodulation of the sampled broadcast signal in the previous stage.

i'lgure isti snows tne spectral range 320 of the output signal of the demodulator unit 154 of Figure 2C in case a sub sound carrier is present in the modulated information signal at the analyzing frequency. The test signal corresponds to a base-band sound signal with a limiting frequency fO, which is typically about 15 kHz, wherein fO is lower than the first limiting frequency fa of the modulated Plnformation signal. The pass band 310 of the filter unit 156 of Figure 2C is selected such that it does not overlap the spectral range of a base-band sound signal. For example, the lower cut-off frequency fl may be defined by the cut-off frequency fb of the pre-filter unit 152 bf Figure 2C. In addition, the pass band 310 may be selected such thatt predictable interference signals resulting- from the processing in the previous stages and appearing at -frequencies fS may be suppressed. For example, the pass band 310 may have an upper cut-off frequency f2 between 200 and 400 kHz, for example about 300 kHz.
Figure 3B refers to the functionality oT the filter unit 156 of Figure 2C In the case that the sub sound carrier Is present in the signal supplied to the demodulator unit 154. The signal power is concentrated in the spectral range 320 and no or only a weak signal is present within the pass band 310. A detector unit evaluating the filtered test signal detects no signal or only a low signal amplitude or signal power.
Figure 3C refers to a situation where no sound carrier is present at the analyzing frequency. Due to the FM threshold effect, the demodulator unit 154 of Figure 2C generates broadband noise. The noise spectrum 330 overlaps with the pass band 310 such that noise is present within the filter pass band 310. The detector unit 158 detects hig^h noise signal power or high noise signal amplitudes in the filtered signal. Providing the filter unit as a band-pass filter with an upper cut-off frequency below predictable spurious frequencies such as harmonics of line frequencies increases the reliability of noise detection.
Figure 4 refers to an embodiment of an audio processing unit 120 comprising a first demodulator unit 124a in a main sound demodulator path and a second demodulator unit 124b in a sub sound demodulator path. The first demodulator unit r24a generates a base-band version of a main sound signal containing, for example, a sum signal of the right and left audio channels and the second demodulator unit 124b generates a base-band version of a sub sound signal containing, for example, a differential signal of the right and left audio channels, or one of both. In the sub sound demodulator path a stereo detection unit 150 indicates whether or not a non-monaural sound signal is present. The stereo

detection unit 150 may share resources with the second demodulator unit 124b. For example, the stereo detection unit 150 may use side information generated in the second demodulator unit 124b. An audio signal recovering unit 128 may be provided that recovers the left and right audio channel signals from the main and Sub sound signals and that may output further digital data encoded, for example, in the sub sound signal.
According -to an embodiment, in the main sound demodulator path the audio processing unit 120 comprises an over-modulation detection unit 160 that detects an over-modulation condition on the main sound carrier. According to an embodiment, the over-modulation detection unit 160 evaluates audio signal levels from the 'demodulated signal or from the audio channels, for example by evaluating the envelope or peak values of the demodulated signal. The over-modulation detector unit 1,60 may ou-tput an over-modulation detectiori signal to the control unit 190 and the control "unit 190 may switch the audio processing unit 120 into the second rec^eiver mode (high-deviation mode) upon detection of an over-modulation condition. According to another embodiment, high-deviation mode is only employed when both over-modulation Is detected and, simultaneously, no second sound carrier is detected.
The audio processing unit 120 may further comprise signal condition means that, in the high-deviation mode, compensates for effects resulting from over-modulation, for example by attenuating and/or equalizing. For example, the audio signal recovering unit 128 may be configurable such that it automatically, without user Interaction, attenuates the audio levels in the left and right audio channels by a predetermined amount, for example, 3dB br 6dB in the high-deviation mode. According to another embodiment, a compensation unit 162 may be provided in the main sound demodulator path. The compensation unit 162 automatically reduces- the sound level of the main sound signal or both audio channels and may compensate for distortions resulting from over-modulation. In addition or alternatively, the width of a filter provided in the main sound demociulator path, for example the width of a first configurable filter 122a, may be increased to avoid distortions.
The compensation unit 162 may be effective only in the high-deviation mode and is by-passed or transparent in the stereophonic mode or a standard monaural mode. In the high-deviation mode the compensation unit 162 may attenuate and/or equalize the main sound signal. During regular monaural transmission, the signals for the left and right channel may be provided in a conventional way

uy superposing zac mam sound signal and tiie sub sound signal in the"usual way. In accordancfe with an embodiment the control unit 190 may cause to discard information from the sub sound demodulator path. In the alternative or in addition to these compensation approaches, in the high-deviation mode the first demodulator unit 124a may be controlled to use other parameters for demodulatiori than in the normal mode to compensate for effects resulting from over-modulation.
According to a further embodiment, a noise detection unit 170 may be provided in the main sound demodulator path. For example, the noise detection unit 170 may use a side information generated in the first demodulator unit 124a. For example, the noise detection unit 170 may indicate whether or not noise dominates in a frequency range where a main sound signed can be expected, for example by evaluating frequency and amount of -amplitude changes, which are both high in the presence of noise. According to another embodiment, the noise detection unit 170 may be configured to detect noise in a frequency range that is occupied by an over-modulated main sound signal but that is not occupied by a not over-modulated main sound signal. The control unit 190 may then release the second receiver inode and the audio processing unit 120 may output monaural audio information without compensating for effects resulting from over-modulation. Detecting the presence of noise in the main sound demodulator path allows releasing the high-deviation mode even where non-over-modulated, standard monaural transmission with no second sound carrier or sub-sound carrier follows an over-modulation condition.
For example, where no antenna signal or no sound carrier signal is available, e.g. during a channel scan, the. first demodulator unit 124a may produce high-level noise that may cause the over-modulation detection unit 160 to Indicate an over-modulation condition and the audio processing unit 120 may be switched into the high-deviation mode. The high-deviation mode would not be automatically released if, in the following period, the broadcast station transmits a monaural signed where ho second sound carrier can be detected. The noise detection unit 170 allows for releasing the high-deviation mode even In this scenario.
According to a further embodiment, information indicating that the receiver unit has switched to another broadcast channel may be used to decide whether or not to release the high-deviation mode. For example, the control unit 190 may receive a channel change signal, an over-modulation detection signal, a carrier detection signal and a noise detection signal output by the noise detection unit 170 and

may control switching between a high-deviation and a normal receiving mode by-controlling sub-units of the audio processing unit 120. For example, switching back to the normal mode by releasing the high-deviation mode may be triggered by a channel change, or upon detection of the second sound carrier or upon noise detection in the main sound demodulator path. Transitions are possible and desirable where a TV program transmits parts of its program, e.g. commercials, by using over-modulation and other parts, e.g. the regular,program, in compliance with the TV standards.
In accordance with an embodiment, the audio processing unit 120 comprises a first XJonfigurable filter 122a as main sound filter that lets pass the first sound carrier coiitaining the main sound signal and that blocks the second sound carrier and the composite video signal. The control unit 190 may control the first configurable filter 122a to'apply a wider pas? band in the second receiver mode (high-deviation mode) than in the first receiver mode. According to a further embodiment, the audio processing unit 120 contains a second configurable filter 122b as sub sound filter that lets pass the second sound carrier containing the sub sound signal and that blocks the first sound carrier and the composite video signal. The control unit 190 may control the second configurable filter 122b to apply a narrower pass band in the second receiver mode (high-deviation mode) than in the first receiver mode. The configurable filters allow for good stereo separation in the case of stereophonic transmission and high reliable sub carrier detection in high-deviation mode.
Figure 4 refers to a parallel arrangement of configurable filters and demodulators for two sound carrier systems. Equivalent considerations apply to sound carrier systems where the sub-sound signal is m.odulated on the main sound carrier and the sub sound demodulator path may branch off from the main sound demodulator path after the first demodulator unit 124a.
Compared to conventional approaches that evaluate the audio level of the main sound carrier in order to decide whether the main sound carrier is not longer over-modulated, the circuit arrangement with the audio processing unit 120 is more reliable, since temporarily small audio levels down to silence are often content-related and do not necessarily indicate a change of the transmission modulation index. Instead, the circuit arrangement comprising the audio processing unit 120. the stereo detection unit 150 and the control unit 190 reliably detects the conditions for the release of high-deviation mode without further user interaction and user settings.

Each of the signal processing unit 100, the audio processing unit 120, the stereo detection unit 150, and the control unit 190 and the sub-units thereof may be realized in hardware, in software or as a combination thereof. Some or all of the units and sub-units thereof may be integrated in a common package, for example an IC (integrated circuit), an ASIC (application specific integrated circuit) or a DSP (digital signal processor).
Figure 5A illustrates the spectrum of,an analogue TV broadcast signal with a composite video signal 510 and a regular sound signal encoded in accordance with the A2 letter code as used in the B/G and D/K standards. A main sound signal 520 containing the composite left channel and right channel signal is modulated on a main sound carrier fl and a sub sound signal 530 containing the right audio channel signal Is modula'ted on the sub sound carrier f2. The main and sub sound carriers fl, f2 have a frequency offset with reference to the picture signal 510. The main sound carrier has a frequency offset of 5.5 or 6.5 MHz and the sub sound carrier may have a frequency offset of 6.2578125 MHz, 6.7421875 MHz or 5.7421875 MHz. byway of example. The pass bands 512, 522, 532 for the picture filter, the main sound filter and the sub sound filter are typically defined in the respective standard.
Figure 5B shows the spectrum in case of an over-modulated main sound signal 522. Typically over-modulgition of the main sound signal implies monaural transmission and no sub sound carrier appears in the spectrum of Figure 5B. Since the spectrum of the over-modulated main sound signal 522 may overlap with the pass band 532 of a standard sub sound filter, a detection of the presence of the second sound carrier, which Is only based on signal energy, may become unreliable. While for stereophonic transmission the pass bands 522, 532 of the main and sub sound signals as shown In Figure 5A show best results, in high-deviation mode a main sound filter with a broader pass band 522a allows better restoring of a sound signal over-modulating the main sound carrier and a sub sound filter with a narrower pass-band allows for a more reliable carrier detection.
Figure 6 illustrates the state changes of the audio processing unit 120. In the first receiver mode 601 the audio processing unit outputs both the main sound signal modulated on the first sound carrier and audio information or digital information modulated on the first or a second sound carrier. In the second receiver mode 602 the audio processing unit discards further audio Information modulated on the

first or second sound carrier, for example derives the signal supplied to audio outputs from the main sound signal only or indicates to a further processing unit that it has no further audio or digital information available. The first state change 612 from the normal mode to the high-deviation mode is triggered by over-modulation detection alone or by both over-modulation and simultaneous absence of a second sound signal. The second state change 621 from the high-deviation mode to the standard mode may be triggered by a channel change performed by the user, by detection of the presence of non-monaural audio information, or by noise detection, in the main sound demodulator path, for example in a frequency range where the main sound signal.can be expected or in a frequency range occupied by over-modulated but not occupied by a non-over-modulated main sound sigrial.
Figure 7 refers to a method of operating a broadcast signal receiver apparatus. An audio processing unit is controlled to demodulate a received broadcast signal to output a main sound sign,al over-modulating a first sound carrier (702). The received signal is tested for presence of non-monaural audio information (704). Upon detection of non-monaural audio information, the audio processing unit is controlled to output a main sound signal modulated on the first sound carrier and further audio information modulated on the first or a second sound carrier (706).
According to an embodiment, presence of non-monaural audio information may be detected by evaluating presence of a second sound carrier. Testing for the presence of the second sound signal carrier may comprise demodulating the received broadcast signal at a predefined analyzing frequency corresponding to the sub sound carrier frequency, wherein a test signal is obtained that contains high-level broadband noise when the digital signal does not contain the sub sound signal carrier at the analyzing frequency and no or low-level broadband noise otherwise. The test signal is filtered using a filter unit which pass band is outside a maximum bandwidth of a sound signal modulated on the second sound signal carrier, wherein a filtered test signal is generated from the test signal and wherein a signal level in the filtered test signal below a predetermined threshold indicates presence of the second sound carrier.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the described embodiments. This application is intended to cover any adaptations or

variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.

Claims
1. A circuit arrangement comprising
an audio processing unit configured to demodulate a digital signal and to
output, in a first receiver mode, non-mopaural audio information contained in the
digital signal and modulated on at least a first sound carrier, the audio processing
unit being further configured to condition, in a second receiver mode, a main
sound signal over-modulating the first sound carrier to compensate for effects
resulting from over-modulation; i
a stereo detection linit configured to output a stereo detection signal indicating presence of non-monaural audio information in the digital signal; and
a co-ntrol unit configured to switch the audio processing unit into the first receiver mode in response to the stereo detection signal.
2. The circuit arrangement of claim 1, wherein the stereo detection unit comprises
a sub-sound signal filter unit configured to let pass a second sound signal contained in the digital signal, and
a sub-sound.signal detection unit configured to output the stereo detection signal when an output signal of the sub-sound signal filter unit Indicates presence of the second sound signal.
3. The circuit arrangement of any of claims 1 or 2, wherein the stereo detection
unit comprises
a digital signal detection unit configured to indicate presence of a digitally modulated sub-signal modulated on a carrier contained in the digital signal.
4. The circuit arrangement .'of any of claims 1 to 3, wherein in the first receiver
mode the audio processing unit outputs non-monaural audio Information
modulated on the first and a second sound carrier and
the stereo detection unit comprises a carrier detection unit configured to output, as the stereo detection signal, a carrier detection signal Indicating presence of the second sound carrier in the digital signal.
5. The circuit arrangement of claim 4, wherein
the carrier detection unit Is adapted to use a demodulator FM threshold effect for determining the presence of the second sound carrier.
6. The circuit arrangement of any of claims 4 or 5, wherein the carrier detection
unit comprises

a aemoauiaior unit conligured to demodulate tne digital signal a^^^^ analyzing frequency to generate a test signal, the test signal containing high-level broadband noise when the digital signal does not contain a carrier signal at the analyzing frequency and no or low-level broadband noise otherwise;
a filter unit configured to filter the test signal and having a pass band outside a majiimum bandwidth of an Information signal modulated on the second sound carrier to generate a filtered test signal from the test signal; and
a detector unit configured to generate the carrier detection signal when a signal level of the filtered test signal falls below a predetermined threshold.
7. The circuit arrangement of claim or 6; wherein
the 'test signal contains the information signal when the analyzing frequency corresponds to a frequency of the second sound carrier.
8. The circuit arrangement of any of claims 1 to 7, further comprising
an over-modulation detection unit configured to detect over-modulation of the first sound carrier, wherein
the control unit is further configured to switch the audio processing unit into the second receiver mode in response to detection of an over-modulation of the first sound carrier.
9. The circuit arrangement of any of claims 1 to 8, further comprising
a noise detection unit configured to detect whether or not noise predominates in a frequency range assigned to the first sound carrier, wherein
the control unit is further configured to control the audio processing unit to release the second receiver mode and to output monaural audio Information without compensating for effects resulting from over-modulation, when the noise detection unit indicates that noise predominates in the frequency range assigned to the first sound carrier.
10. The circuit arrangement of any of claims 1 to 9, further comprising
a first configurable filter applied to the digital signal before demodulation to obtafn the main sound signal, wherein the first configurable filter Is controlled to appfy a wider pass band In the second receiver mode than In the first receiver mode.
11. The circuit arrangement of claim 10, further comprising
a second configurable filter applied to the digital signal before demodulation to obtain the non-monaural audio information, wherein the second
^ I)

configurable filter is controlled to apply a narrower pass band in the second receiver mode'than in the first receiver mode.
12. An integrated circuit comprising the circuit arrangement according to any of
claims 1 to 11.
f ■ . 1 -.
13. A television apparatus comprising the circuit arrangement according to any of claims 1 to 11, wherein the broadcast channel is a television broadcast channel comprising a television signal carrier and at least the first sound carrier.
14. A method of operating a broadcast signal receiver apparatus, the method comprising
controlling an audio processing unit to operate in a second receiver mode to recover a main sound signal over-mod.ulating a first sound carrier contained in a digital signal and to compensate for effects resulting from over-modulation;
evaluating, in a stere© detection unit, the digital signal for presence of non-monaural audio Information,
switching, upon detection of the non-monaural audio information, the audio processing unit into a first receiver mode to output non-monaural audio information contained in the digital signal.
15. The method of claim 14, wherein
evaluating the digital signal for presence of non-monaural audio information comprises bandpass-filtering the digital signal to let pass a sub-sound signal contained in the digital signal,
evaluating whether or not a sub-sound signal is present in. the bandpass-filtered digital signal.
16. The-method of any of claims 14 or 15, wherein
evaluating the digital signal for presence of non-monaural audio Information comprises evaluating whether or not the digital signal contains a digitally modulated sub-signal modulated on a carrier.
17. The method of any of claims 14 to 16, wherein
evaluating the digital signal for presence of non-monaural audio information comprises evaluating the digital signal for presence of a second sound carrier, and
switching Into the first receiver m.ode comprises outputting non-monaural audio Information modulated on the first and the second sound carriers.

18. The method of claim 17, wherein evaluating the digital signal for presence of
the second sound carrier comprises
demodulating the received digital signal at a predefined analyzing ^equency, wherein a test signal is obtained that contains high-level broadband noise when the digital signal does not contain a carrier signal at the analyzing frequency and no or low-level broadband noise otherwise;
filtering the test signal using a filter unit having a pass band outside a maximum bandwidth of an information signal modulated on the second sound carrier to generate a filtered test signal from the test signal, wherein a signal level in the filtered signal below a predetermined threshold indicates presence of the second soifnd signal carrier.
19. The method of any of clkims 14 or-18, further comprising
testing, when the audio processing unit is in the first receiver mode, whether or not the received,'digital signal contains ah over-modulated first sound carrier, and if so,
switching the audio processing unit Into the second receiver mode.
20. The method of any of claims 14 to 19, further comprising '
testing, when the audio processing unit is in the first receiver mode, whether or not the received digital signal contains an over-modulated first sound carrier and, simultaneously,, only monaural audio information is present, and if so,
switching the audio processing unit into the second receiver mode.
21. The method of any of claims 14 to 20, further comprising
filtering the digital signal before demodulation to obtain the main sound signal, wherein in the second receiver mode a wider pass band is applied than in the first receiver mode to compensate for effects resulting from over-modulation.
22. The method of claim 21, further comprising
filtering the digital signal before demodulation to obtain non-monaural audio information, wherein a narrower pass band is applied In the second receiver mode than in the first receiver mode.
23. A television apparatus comprising
an audio processing unit configured to demodulate a digital signal, wherein . in a first receiver mode the audio processing unit outputs non-monaural audio

information contained in tlie digital signal and modulated en at lejst a first sound carrier and, In a second receiver mode, conditions a main sound signal over-modulating the first sound carrier to compensate for effects resulting from over-modulation;
a stereo detection unit configured to output a stereo detection signal indicating presence of non-monaural audio information in the digital signal; and
a control unit configured to switch the audio processing unit into the first receiver mode in response to the stereo detection signal.
24. The television apparatus of claim 23, wherein
in the first receiver mode the audio processing unit outputs non-monaural audio information modulated on the first and a second sound carrier and
the stereo detection unit comprises a carrier detection unit configured to output a carrier detection 'signal Indi-cating presence of the second sound carrier in the digital signal as the stereo detection signal.
K - ■
25. A television apparatus comprising
audio processing means for demodulating a digital signal, wherein in a first receiver mode the audio ■ processing means outputs non-monaural audio information contained in the digital signal and modulated onat least a first sound carrier and, in a second receiver mode, conditions a main sound signal over-modulating the first sound carrier to compensate for effects resulting from over-modulation;
stereo detection means for outputting a stereo detection signal indicating presence of non-monaural audio information in the digital signal, the stereo detection means connected to the audio processing means; and
control means for switching the audio processing means into the first receiver mode in response to the stereo detection signal, the control means connected to the audio processing means and the stereo detection means.