Patent Publication Number: US-9426574-B2

Title: Automatic audio source switching

Description:
BACKGROUND 
     This disclosure relates to an audio/video system and, more specifically to a video display with a supplemental audio system. 
     Many modern televisions (“TVs”) have significant audio/video (“A/V”) source switching capabilities. As a result, customers often choose to connect their A/V sources (e.g. cable box, DVD player) directly to the TV using, for example, HDMI cables. This eliminates the need for an external home theater A/V receiver to switch audio/video signals. The HDMI inputs and audio outputs of TVs are technically capable of supplying a multichannel audio output (e.g. Dolby Digital 5.1) from the TV to a supplemental audio system being used with the TV. However, the audio signal available at a TV&#39;s audio output is often not multichannel. Instead, the audio output from the TV is typically a down-mixed 2 channel signal (e.g. PCM 2.0). This down-mixing (e.g. via summation) results in lost audio information (e.g. bass and dynamic range) that can limit the sound quality of a supplemental sound system that uses a TV&#39;s audio output as an input signal. 
     SUMMARY 
     In one aspect, a method for automatically switching an audio source includes the steps of receiving an audio signal from a video display device, and receiving a digital audio signal from one of a plurality of audio/video source devices which each can supply audio and video information to the video display device. The digital audio signal is compared with the audio signal from the video display device. The digital audio signal is output to a supplemental audio system of the video display device when the comparing step indicates that the digital audio signal and the audio signal from the video display device contain substantially similar audio programs. 
     Embodiments may include one or more of the following features. The audio signal received from the video display device is an analog signal. The audio signal received from the video display device is a digital signal. The video display device is a television. The audio signal received from the video display device is supplied to the video device from one of the audio/video source devices. The digital audio signal received from the source device has 6 or more channels. The digital audio signal received from the source device has more channels than the audio signal received from the video display device. The receiving step includes receiving a digital audio signal from each of the plurality of audio/video source devices. The comparing step utilizes information beyond what is included in the audio signal and digital audio signal. The information that is beyond what is included in the audio signal and digital audio signal includes the presence or absence of a wireless signal from a remote control. 
     In another aspect, an apparatus for automatically switching an audio source includes an audio receiver that can receive (a) an audio signal from a video display device, and (b) a digital audio signal from one of a plurality of audio/video source devices which each can supply audio and video information to the video display device. The audio receiver compares the digital audio signal with the audio signal from the video display device and outputs the digital audio signal to a supplemental audio system of the video display device when the digital audio signal and the audio signal from the video display device contain substantially similar audio programs. 
     Embodiments may include one or more of the following features. The audio signal received from the video display device is an analog signal. The audio signal received from the video display device is a digital signal. The video display device is a television. The audio signal received from the video display device is supplied to the video device from one of the audio/video source devices. The digital audio signal received from the source device has 5 or more channels. The digital audio signal received from the source device has more channels than the audio signal received from the video display device. 
     In yet another aspect, a method for automatically switching an audio source includes the steps of receiving a digital audio signal from one of a plurality of audio/video source devices which each can supply video information to a video display device, and receiving an indication from the video display device that the digital audio signal should be transmitted to a supplemental audio system. The digital audio signal is output to the supplemental audio system. 
     An embodiment may include the following feature. The indication is an audio signal that substantially matches the digital audio signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic arrangement of an audio/video system; 
         FIG. 2  is a flow diagram; 
         FIG. 3  is a signal cross-correlation circuit; 
         FIG. 4  provides more detail for the peak detector block of  FIG. 3 ; 
         FIG. 5  is a graph demonstrating the detected peaks in a signal; and 
         FIG. 6  is another example of an audio/video system. 
     
    
    
     DETAILED DESCRIPTION 
     Many consumers choose to add a supplemental audio system for use with their video display (e.g. a TV) to provide enhanced audio reproduction while watching a video program on their video display. The disclosure below discusses how to automatically switch the highest quality audio signal to the supplemental speaker system when A/V sources (e.g. DVD player, cable box) are directly connected to the TV. 
       FIG. 1  discloses an audio/video system  60  which includes a video display device  62  such as a liquid crystal or plasma display. The display  62  includes integrated speakers (acoustic drivers)  63  and an infra-red (IR) receiver  67 . A pair of audio/video source devices, such as a cable box  68  and a digital video disc (DVD) player  70 , are attached to the display  62  by, for example, respective HDMI cables  72  and  74 . Additional A/V source devices can be connected to the display  62 . Any of the A/V source devices can be connected to the display  62  by an analog connection (e.g. component video with a separate audio connection) instead of a digital connection (e.g. HDMI). The A/V source devices can each supply audio (e.g. Dolby Digital 5.1 with six channels) and video information to the display  62 . Another example of an A/V device that can be connected to the display  62  is a computer (not shown). The display  62  can also be connected directly to the internet which can supply A/V information to the display for presentation. Use is made of a source switching capability included in the display  62  to select AV information from A/V device  68  or  70  to present to the user. The user operates a wireless remote control  88  (e.g. IR or RF) to transmit a signal to the IR receiver  67  to select the desired A/V information from device  68  or  70  for presentation by the display  62 . Alternatively, the user can operate buttons on the display  62  to select the desired A/V device  68  or  70 . 
     The display outputs an audio signal over a cable  66  to an audio receiver  64 . The information in the output audio signal was supplied to the display  62  by the selected A/V device  68  or  70 . If cable  66  is a digital audio cable (such as optical or coaxial digital audio), then the output audio signal is in digital form (likely a PCM 2.0 signal). If cable  66  is a traditional stereo audio cable pair, then the audio signal output by display  62  is in analog form. The output audio signal is transmitted over cable  66  and received by a controller  61  in an audio receiver  64 . The controller  61  is also connected to and can receive respective digital audio signals from the cable box  68  and DVD player  70  over respective digital audio cables  80  and  82 . The audio signal output to the controller  61  by each of the cable box  68  and the DVD player  70  would typically be a high quality, multichannel digital audio signal (e.g. a Dolby Digital 5.1 audio signal). These high quality digital audio signals can provide a higher quality acoustic performance than the audio signal output by the display  62  over cable  66  because the latter is usually down-mixed. The reason the display  62  down-mixes the received audio signals is to optimize the audio output from the integrated speakers  63 . Speakers  63  typically have significantly reduced performance as compared to an external audio system, and therefore the downmixed signals are sometimes greatly reduced in dynamic range and spectral extension. This downmixed signal cannot provide the dynamic range and bass performance that can be provided by the high quality digital audio signals provided directly from the AV sources  68  and  70 . 
     A supplemental speaker system  76  is connected to the controller  61  of the audio receiver  64  by a cable  78 . The speaker system  76  may have only a single acoustic driver (mono audio), or it may have multiple acoustic drivers arranged in, for example, a stereo, 5.1 or 7.2 audio system. A 5.1 system has the following speakers: left front, center, right front, left surround, right surround and bass (low frequency effects). The speaker system could also be a 1.0 or 1.1 type of system which is sometimes referred to as a “soundbar” (with or without a separate bass speaker). The speaker system preferably includes audio processing (such as a digital signal processor) and one or more power amplifiers As such, in an alternative example (not shown) the audio receiver  64  and the supplemental speaker system  76  can be combined into a single unit in which the functions of the controller  61  are performed by the audio processor of the speaker system  76 . This type of arrangement can result in reduced costs for the entire system. 
     Referring now to  FIGS. 1 and 2 , the logic used by the controller  61  to provide the best available audio signal to the supplemental speaker system  76  will be described. Once the audio receiver  64  is powered up, the subroutine in  FIG. 2  starts at a step  90 . At a step  92  the controller  61  checks to see if it is receiving an audio signal from the display  62  over the cable  66 . When a signal is being received, the controller  61  decodes and processes this signal as required and passes it to the speaker system  76  over cable  78  at a step  94 . This step provides audio output while the controller  61  is checking to see if there is a higher quality (multi-channel) audio signal available directly from the A/V sources  68  and  70 . At a step  96  the controller  61  checks to see if the received audio signal on cable  66  contains greater than two channels. When the signal on cable  66  is greater than two channels, this signal is continued to be processed and sent to the speaker system  76  as it is a multi-channel signal. If the received audio signal from cable  66  is a mono or two channel signal, then controller  61  compares this signal with the respective digital audio signals, if any, being received from cables  80  and  82  (and decoded) to look for a match between the signal on cable  66  and one of the signals on cables  80  and  82  (step  98 ). In other words, the controller  61  determines if the audio program on cable  66  is substantially similar to any of the audio programs on cables  80  and  82 . When the signal on cable  66  does not match any of the signals on cables  80  or  82 , the signal on cable  66  is continued to be processed and sent to the speaker system  76 . 
     The matching of signals described at the end of the previous paragraph can be done as follows. The controller  61  looks for matching characteristics between the signal on cable  66  and each of the signals on cables  80  and  82 . Matching characteristics can be refined and specifically made insensitive to the expected differences between the mono or two channel signal on cable  66  and the multi-channel signals which may be present on each of cables  80  and  82 . These differences include that the signal on cable  66 , if digital, will likely (a) not include a low frequency effects (LFE) channel, (b) have experienced dynamic range compression, and (c) have been down-mixed from a multi-channel signal to a two channel or mono signal. The effects of down-mixing can be compensated for by calculating the acoustic energy sum over all channels for each signal being compared (down-mixing is approximately energy conserving). The short term spectrum of the energy sum for each signal is compared. The likely absence of a LFE channel in the signal on cable  66  can be compensated for by ignoring frequency content below 120 Hz in the signals on cables  80  and  82 . Likely dynamic range compression of the signal on cable  66  can be compensated for by evaluating the positions in time of energy changes and not the energy changes themselves. 
     When the audio program on cable  66  substantially matches the audio program on one of cables  80  and  82 , the controller  61  transitions (e.g. by cross-fading) the audio signal output to the supplemental audio system  76  over cable  78  from the signal on cable  66  to the matched signal on cable  80  or  82  (step  100 ). This transition from a “search phase” to a “matched phase” may take a relatively long amount of time as the audio signal on cable  66  should be properly paired with the video being presented on the display  62 . In other words, the controller  61  does not have to be very fast at detecting a match between the signal on cable  66  and one of the signals on cables  80  and  82  because the system can play the signal on cable  66  in the meantime with confidence that the video and audio being played will match. Care should be taken during this transition to minimize or eliminate any switching audio artifacts (e.g. popping). The energy sum over all channels for each signal (mentioned in the previous paragraph) can be used to match the loudness between the audio signal (stream) on cable  66  and the audio signal (stream) on cable  80  or  82  at the transition (switching) time. Cross-correlation in time between the audio signal on cable  66  and the audio signal on cable  80  or  82  can determine a time lag (if any) between the two audio signals. An appropriate delay can then be applied by the controller  61  to time align the two audio signals before making the transition. 
     At a step  102  the controller  61  checks to see if the audio signal being received on cable  66  still matches the same signal on cable  80  or  82 . This matching check is done as was explained two paragraphs above. When the match continues, the logic loops back to step  100  and checks for this match again. When this match no longer exists, the logic loops back to the start of the subroutine to the step  92 . This subroutine continues to run until the audio receiver is powered down. This transition from the matched phase to the search phase (moving from step  102  to step  94 ) should occur relatively quickly. Typically, this transition is caused by an A/V source switch by a user of the A/V system  60  (e.g. switching from the cable box  68  to the DVD player  70 ). In order to keep the video and audio matched, the transition back to using the audio signal on cable  66  (back to step  94 ) for rendering audio should occur as quickly as possible to minimize the amount of time that the video and audio do not match. 
     The two matching audio streams on cable  66  and one of cables  80  and  82  will typically not be fully identical, so the system makes judgments about the degree of similarity. This can be done, as discussed above, by calculating a quantitative measure of similarity and comparing this to a threshold. This method can be improved by taking non-audio signal information into account. One such piece of information is the presence or absence of wireless remote control activity which can be detected through, for example, an IR sensor  65 . When IR activity is observed by the sensor  65 , the system assumes an A/V source switch may be taking place and increases the threshold for audio signal matching. The system lowers this threshold when no IR remote signal is present. 
     Another way to describe the example shown in  FIGS. 1 and 2  is as follows. The receiver  64  receives a digital audio signal over one of cables  80  and  82  from one of a plurality of audio/video source devices  68  and  70  which each can supply video information to a video display device  62  over respective cables  72  and  74 . The receiver  64  receives an indication from the video display device  62  that the digital audio signal should be transmitted to a supplemental audio system  76 . This indication is the audio signal that the receiver  64  receives over cable  66  from the display  62 . When the audio signal on cable  66  and the digital audio signal substantially match, the digital audio signal is output to the supplemental audio system over a cable  78 . 
     Referring now to  FIGS. 3-5 , an example of how to compare a signal on cable  66  with a signal on cable  80  or  82  will be discussed. The following hardware and logic is preferably housed in receiver  64 . In  FIG. 3 , a 5.1 channel signal  103  received over cable  80  includes the following components: L, L s , C, R s , R and LFE. The LFE (low frequency effects) signal  104  is discarded. The Ls, C and Rs are each passed through respective amplifiers  106  which each apply a 0.7 gain to their input signal. The L, L s  and C signals are then passed to a summer  110  while the C, R s  and R signals are passed to a summer  112 . The signals out of summers  110  and  112  are respectively passed to bandpass filters  114  and  116  which each pass signals in about the 400 Hz to 10 kHz range. The signals out of filters  114  and  116  are each respectively squared in blocks  118  and  120 . The signals out of blocks  118  and  120  are combined in a summer  122 . The signal out of summer  122  has a square root function applied to it in block  124 , and then a peak detector  126  (described in further detail below) operates on the signal. Finally, the signal out of the peak detector  126  is passed to a sparse cross-correlation block  128  (described in further detail below). It should be noted that a signal on cable  82  ( FIG. 1 ) would receive similar signal processing as that described in this paragraph in a separate signal path (not shown) before being fed into block  128 . 
     A two channel stereo signal  130  received over cable  66  includes the following components: L t  and R t . The Lt and Rt signals are respectively passed to bandpass filters  132  and  134  which each pass signals in about the 400 Hz to 10 kHz range. The signals out of filters  132  and  134  are each respectively squared in blocks  136  and  138 . The signals out of blocks  136  and  138  are combined in a summer  140 . The signal out of summer  140  has a square root function applied to it in block  142 , and then a peak detector  144  (described in further detail below) operates on the signal. Finally, the signal out of the peak detector  144  is passed to the sparse cross-correlation block  128  (described in further detail below). 
     With reference to  FIGS. 3 and 4 , each of the peak detectors  126  and  144  described above operate as follows. In  FIG. 4  a peak detector  126  is shown. A signal X(t) that is fed into the peak detector  126  is first low pass filtered at a block  146 . This low pass filter is a 2 nd  order Butterworth filter with a cutoff of about 1 kHz. The signal out of the filter  146  is given as X LPF (t). Then X LPF (t) is differentiated at a block  148  to obtain X LPF (t). Anywhere that X LPF (t) crosses zero by going from a positive number to a negative number corresponds to a peak in X LPF (t) (by definition of a derivative). As such, these zero crossings are used to locate the peaks of X LPF (t). The zero crossings or peaks as a function of time are determined in a block  150 . The output of block  150  is used to control the transmission of X LPF (t). When a peak is detected, X LPF (t) is transmitted. When a peak is not detected, a stream of zeros is transmitted. This transmission of zeros and X LPF (t) results in X PEAK (t). 
     Turning to  FIG. 5 , in order to reduce the amount of data that needs to be processed, insignificant peaks are discarded. A graph  152  includes a “t” axis (time)  154  and an X PEAK (t) axis  156 . The signal at this point is equal to the maximum points  158  of X LPF (t) at certain locations and zero everywhere else. Any small peaks that are preceded by large peaks are discarded. This is accomplished by using an exponentially decaying threshold  160 . Whenever a peak is detected, the threshold is reset to that peak value. The threshold is then allowed to decay until another peak is found that exceeds the threshold. In this example the first three peaks from left to right are utilized because they are not less than the threshold  160 . The last two peaks from left to right are discarded because they are less than the threshold  160 . 
     Referring to  FIGS. 3 and 5 , the sparse cross-correlation block  128  utilizes an algorithm that receives from the peak detector  126  or  144  a sequence of numbers that correspond to the non-discarded peak values and locations (in time). The algorithm in block  128  uses this information to estimate the correlation between the stereo audio signal  130  on cable  66  and any multi-channel audio signal which may be present on cables  80  and  82  ( FIG. 1 ). 
     The next step in the block  128  algorithm is normalization. Each frame of data received is made to have unit energy (i.e. X norm =X peaks ÷∥X peaks ∥), where ∥X peaks ∥ is the energy of X peaks  i.e., it is the square root of the sum of the squares of X peaks  found in a 100 milliseconds (ms) sample of the signal (called a frame of data). At this point in the block  128  algorithm, a multichannel audio signal on cable  80  or  82  is considered to not be correlated with the a stereo audio signal on cable  66  if the multichannel signal has (a) less than half the energy of the stereo signal, or (b) more than ten times the energy of the stereo signal. If a multi-channel audio signal is still a possible match with the stereo audio signal after normalization, the cross-correlation of the signals is approximated. Let Y[n] and X[n] be sampled data signals, where N is sample number. A cross-correlation is typically computed as: 
     
       
         
           
             
               
                 
                   
                     
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     At this point, however, there are no time sequences: only the peak values and peak locations are known, so we cannot perform the cross-correlation as it is typically done and must approximate it using only the peak values and peak locations. The benefit is that this greatly reduces the amount of computation that is needed. The cross-correlation of the multichannel signal and stereo signal is approximated as follows. 
     Let: 
     
         
         
           
             X 51 =peak values of a 5.1 (multichannel source) Mpeaks 
             X LR =peak values of a stereo audio source from a display N peaks 
             τ 51 =time locations of peaks of a 5.1 source 
             τ LR =time locations of peaks of a stereo audio source
 
The convolution equation given above can be approximated as follows. First, compute all the cross-product terms by computing the outer product of the two vectors of peak values
 
           
         
       
    
               [             X   51     ⁡     [   1   ]                   X   51     ⁡     [   2   ]               ⋮               X   51     ⁡     [   M   ]             ]     [               X   LR     ⁡     [   1   ]               X   LR     ⁡     [   2   ]           …           X   LR     [   N   〛           =           [               X   51     ⁡     [   1   ]       ⁢       X   LR     ⁡     [   1   ]                   X   51     ⁡     [   1   ]       ⁢       X   LR     ⁡     [   2   ]             …             X   51     ⁡     [   1   ]       ⁢       X   LR     ⁡     [   N   ]                       X   51     [   2   ]     ⁢       X   LR     ⁡     [   1   ]                   X   51     [   2   ]     ⁢       X   LR     ⁡     [   2   ]             …             X   51     [   2   ]     ⁢       X   LR     ⁡     [   N   ]                 ⋮       ⋮                   ⋮                 X   51     [   M   ]     ⁢       X   LR     ⁡     [   1   ]                   X   51     [   M   ]     ⁢       X   LR     ⁡     [   2   ]             …             X   51     [   M   ]     ⁢       X   LR     ⁡     [   N   ]               ]               
Next, compute the time alignment value of each of the entries in the cross-product matrix above,
 
     
       
         
           
             
               
                 
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             1. Next, the entries of the above time alignment matrix are rounded so that the product terms that are within about 1 ms of each other are considered to be collocated. 
             2. Product terms that have the same time alignment are added together to create an approximation to the cross-correlation. If the peak of this cross-correlation is greater than 0.75 then the multichannel signal and stereo signal are considered to be matched for that frame. 
           
         
       
    
     If the controller  61  is currently sending the signal from cable  66  to the speaker system  76 , the controller  61  will switch to sending the multichannel signal from cable  80  or  82  to the speaker system  76  if at least nine out of ten frames are considered to be matched (cross-correlated). If the controller  61  is currently sending a multichannel signal from cable  80  or  82  to the speaker system  76 , the controller  61  will switch to sending the signal from cable  66  to the speaker system  76  if less than two out of ten frames are considered to be matched (cross-correlated). 
     In order for the signal matching described above with reference to  FIGS. 3-5  to work well, the peaks extracted from the multichannel signal on e.g. cable  80  and the stereo signal on cable  66  should be time-aligned within about 100 ms. Typically the time delays are larger than this and it is necessary to estimate the time delays in parallel to the matching algorithm and adjust the time delays accordingly. This can be don by running a separate cross-correlation algorithm similar to the one discussed above is on a much larger frame size (e.g. about 1 second) to estimate the time alignment of the multichannel and stereo signals. The results of this time alignment determination set the delay values of variable time delay for the leading signal prior to being send to signal comparison block. 
     Turning to  FIG. 4 , another example will be described. Components in  FIG. 4  that are similar to like components in  FIG. 1  are given the same reference numerals as in  FIG. 1 .  FIG. 4  discloses an audio/video system  60  which includes a video display device  62  that includes integrated speakers (acoustic drivers)  63  and an infra-red (IR) receiver  67 . A pair of audio/video source devices, such as a cable box  68  and a digital video disc (DVD) player  70 , are attached to an audio receiver  64  by, for example, respective HDMI cables  81  and  83 . The A/V source devices can each supply a signal to the receiver  64  which includes video and audio information. A controller  61  in the receiver  64  receives the signals from cables  81  and  83  and passes the respective video information in the signals to respective cables  72  and  74  which are each connected to a video input of the display  62 . In other words, the video information on cable  81  is passed over cable  72 , and the video information on cable  83  is passed over cable  74 . Use is made of a source switching capability included in the display  62  to select video information from A/V device  68  or  70  to present to the user. The user operates a wireless remote control  88  (e.g. IR or RF) to transmit a signal to the IR receiver  67  to select the desired video information from device  68  or  70  for presentation by the display  62 . Alternatively, the user can operate buttons on the display  62  to select the desired A/V device  68  or  70 . 
     Assume the user makes a selection of video information on cable  72  to present on the display  62  (a video sink). The display  62  and receiver  64  (a video repeater) will go through an HDCP “handshake” over HDMI cable  72 . The controller  61  checks to see if it is receiving an audio signal from the display  62  over the cable  66 . The subroutine shown in  FIG. 2  and described above is then followed. Accordingly, the controller  61  takes audio information that it has received over cable  81  and passes the audio information to the supplemental audio system  76  over cable  78 . In this way the audio is automatically switched to supplemental audio system  76  so that it matches the video that is being presented on the display  62 . 
     Another way to describe the example shown in  FIG. 4  is as follows. The receiver  64  receives a digital audio signal over one of cables  81  and  83  from one of a plurality of audio/video source devices  68  and  70  which each can supply video information to a video display device  62  over respective cables  72  and  74 . The receiver  64  receives an indication from the video display device  62  over cable  66  that the digital audio signal should be transmitted to a supplemental audio system  76 . The digital audio signal is output to the supplemental audio system over a cable  78 . 
     A number of implementations have been described. Nevertheless, it will be understood that additional modifications may be made without departing from the spirit and scope of the inventive concepts described herein, and, accordingly, other embodiments are within the scope of the following claims.