Abstract:
A Japanese BTSC audio broadcast signal has three modes of transmission: mono, stereo, or dual mono. The control channel centering at 3.5 fH is one of the three channels in the J-BTSC signal, and contains information to indicate to the decoder which one of the three modes the audio transmission is in. The present invention uses a bandpass filter directly in the AM band, followed by envelope filtering and a decision circuit. Therefore, the need for AM demodulation and AM carrier detection is eliminated.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to signal processing in a Japanese audio broadcast signal. 
   2. Related Art 
   The Japanese Broadcast Television Systems Committee (“JBTSC”) standard audio broadcast signal has three modes of transmission. These modes are mono, stereo, and dual mono. To serve both stereo and non-stereo television sets, the JBTSC standard requires the left (“L”) and right (“R”) channels of a stereo signal to be summed and transmitted as one signal in the space normally occupied by the mono audio signal. The summed L+R output, called the main channel signal, provides the mono signal of the original audio program content. This summed signal may be received by non-stereo television sets. 
   In stereo mode, the JBTSC system sends an L−R signal (herein referred to as “sub”), which is the difference between left and right channels. While this signal alone cannot be used by the television set, it is essential to reconstructing the stereo signals (L and R). In dual mono mode, the second mono audio program is transmitted in the sub channel. 
   A third channel, called the control channel, is inserted into the transmission to indicate whether the broadcast is in mono, stereo, or dual mono mode. This channel is AM modulated and requires a detector to figure out the actual broadcast mode. 
   SUMMARY OF THE INVENTION 
   A Japanese BTSC audio broadcast signal has three modes of transmission: mono, stereo, or dual mono. In order to distinguish between the three modes, the present invention bandpass filters directly in the AM band. This eliminates the need for AM demodulation and AM carrier detection. 
   In an embodiment, a control signal is input to two different bandpass filters. In an embodiment, the first bandpass filter is centered at 982.5 Hz from the AM carrier, which indicates stereo transmission. In an embodiment, the second bandpass filter is centered at 922.5 Hz from the AM carrier, which indicates dual mono transmission. Each of these bandpass filters is followed by its own envelope tracker. A decision circuit receives the outputs of both of the envelope trackers. 
   In order to determine the mode of the transmission, the decision circuit first compares the amplitudes of the outputs of the two envelope trackers. Because of the frequencies of the bandpass filters, the outputs are referred to as a stereo-filtered signal and a dual-mono filtered signal, respectively. If the amplitude of the stereo-filtered output is greater than an upper threshold, for example, three times the amplitude of the dual mono-filtered output, the transmission is determined to be in stereo mode. If the amplitude of the dual mono-filtered output is greater than, for example, three times the amplitude of the stereo-filtered output, the transmission is determined to be in dual mono mode. 
   If neither signal is at least, for example, three times larger than the other, the decision circuit again compares the amplitudes using a lower programmable threshold. In this comparison, if either signal amplitude is greater than the lower threshold, for example, 1.5 times the amplitude of the other signal, the transmission is determined to be in a state of transition. In this case, the new mode is determined to be the same as a previously determined mode. The programmable upper and lower thresholds form a hysterisys that prevents premature mode switching during mode transitions. 
   If neither of the amplitudes are greater than, for example, 1.5 times the amplitude of the other, the transmissions are determined to be in mono mode. 
   Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
     The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
       FIG. 1  is an illustration of the relationship between three channels (main, sub, and control) used in the JBTSC standard&#39;s composite spectrum. 
       FIG. 2  is a block diagram of a conventional JBTSC processing system. 
       FIG. 3  is a block diagram of a JBTSC processing system according to the present invention. 
       FIG. 4  is a block diagram of a decision circuit according to an embodiment of the present invention. 
       FIG. 5  is a flowchart of a method according to an embodiment of the present invention. 
       FIG. 6  is a timing diagram of signals produced by an embodiment of the present invention. 
   

   The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Overview 
   While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the present invention. It will be apparent to a person skilled in the pertinent art that this invention can also be employed in a variety of other applications. 
   Signal Mode Determination 
   As shown in  FIG. 1 , a JBTSC audio transmission includes a main channel  102 , a sub channel  104 , and a control channel  106 . Main channel  102  is also referred to as the sum channel, since it carries the L+R audio signal. Sub channel  104  is FM modulated and can carry L−R (stereo mode) or the second mono program (dual mono). If control channel  106  contains an AM carrier, then the broadcast is in stereo or dual mono mode. Otherwise, the transmission is in mono mode. 
   Control channel  106  is typically centered at 3.5 f H , f H  being the horizontal scanning frequency. If there is a 60% AM carrier in control channel  106 , then the transmission is in either stereo or dual mono mode. Otherwise, without this carrier, the transmission is in mono mode. When control channel  106  includes an AM carrier, sidebands  108  can be at either 982.5 Hz, which indicates stereo mode, or 922.5 Hz, which indicates dual mono mode. 
   In an embodiment, the mode of JBTSC transmissions can be detected with an AM demodulator.  FIG. 2  is a block diagram of a system  200  according to this embodiment. Carrier detector  202  detects the presence of an AM carrier in control channel  106 . If this carrier exists, the AM demodulator  204  demodulates control channel  106  to baseband with a signal at either 982.5 Hz (stereo) or 922.5 Hz (dual mono). This baseband signal  206  is input to two bandpass filters. First bandpass filter  208  is centered at a first frequency. In an embodiment, the first frequency is 982.5 Hz. First envelope tracker  212  receives the output of first bandpass filter  208 , and encases the signal in a smooth signal envelope. Decision circuit  214  receives the output of first envelope tracker  212 . 
   Second bandpass filter  210  is centered at a second frequency. In an embodiment, the second frequency is 922.5 Hz. Second envelope tracker  216  receives the output of the second bandpass filter  210 , and encases the signal in a smooth signal envelope. Decision circuit  214  receives the output of second envelope tracker  216 . Decision circuit  214  then determines the mode of the JBTSC transmission. 
     FIG. 3  is a block diagram of a system  300  according to an embodiment of the present invention wherein bandpass filtering is performed directly in the AM band. In this embodiment, the need for AM demodulation and AM carrier detection is eliminated. System  300  includes a first signal path  302 , a second signal path  304 , and a decision circuit  306 . First signal path  302  includes a first bandpass filter  308  and a first envelope tracker  310 . In an embodiment, first bandpass filter  308  is centered at (3 f H -982.5 Hz). First bandpass filter  308  allows an AM sideband at this frequency to pass through, but prevents an AM sideband at (3 f H -922.5 Hz) from passing through. 
   Second signal path  304  includes a second bandpass filter  312  and a second envelope tracker  314 . In an embodiment, second bandpass filter  312  is centered at (3 f H -922.5 Hz). Second bandpass filter  312  allows an AM sideband at this frequency to pass through, but prevents an AM sideband at (3 f H -982.5 Hz) from passing through. 
   An input signal  316 , such as control signal  106 , goes to both first path  302  and second path  304 . First bandpass filter  308  filters input signal  316  to create filtered signal  318 . First envelope tracker  310  encases filtered signal  318  in a smooth signal envelope to create signal  320 . Second bandpass filter  312  filters input signal  316  to create filtered signal  322 . Second envelope tracker  314  encases filtered signal  322  in a smooth signal envelope to create signal  324 . Signal  320  and signal  324  are each input into decision circuit  306 . 
   With bandpass frequencies as listed above, if the transmission is in stereo mode, the amplitude of stereo-filtered signal  320  will be larger than the amplitude of dual mono-filtered signal  324 . Similarly, if the transmission is in dual mono mode, the amplitude of dual mono-filtered signal  324  will be larger than the amplitude of stereo-filtered signal  320 . If the transmission is in mono mode, the amplitudes of signals  320  and  324  will both be relatively small and comparable to each other. 
   The relative sizes of the output amplitudes are used in decision circuit  306  to distinguish between the three signal modes.  FIG. 4  is a block diagram detailing decision circuit  306 . In this example, the amplitude of stereo-filtered signal  320  is referenced as w 1 (n). Likewise, the amplitude of dual mono-filtered signal  324  is referenced as w 2 (n). A mode determination signal  402 , output by decision circuit  306 , is referenced as decision(n). In this example, the three signal modes are referenced as stereo, dualmono, and mono. 
     FIG. 5  is a flowchart of a method  500  according to an embodiment of the present invention. Method  500  may be used by decision circuit  306 . 
   In step  504 , an upper threshold U for one of the signals is set equal to, for example, 3 times the value of the amplitude of the other signal. 
   In step  506 , a lower threshold L for the first signal is set equal to, for example, 1.5 times the value of the amplitude of the other signal. 
   In step  508 , a comparison is made between the amplitude of stereo-filtered signal  320  and three times the amplitude of dual mono-filtered signal  324 . If the amplitude of stereo-filtered signal  320  is more than three times larger than the amplitude of dual mono-filtered signal  324 , then decision circuit  306  will determine that the JBTSC transmission is in stereo mode. For example, using the notation above, if [w 1 (n)&gt;U*w 2 (n)], then decision (n)=stereo. 
   If a stereo signal is not present, that is, if [w 1 (n)&lt;U*w 2 (n)], method  500  proceeds to step  510 . In step  510 , a second comparison is made. If the amplitude of dual mono-filtered signal  324  is more than three times larger than the amplitude of stereo-filtered signal  320 , then decision circuit  306  will determine that the JBTSC transmission is in dual mono mode. For example, if [w 2 (n)&gt;U*w 1 (n)], then decision(n)=dualmono. 
   If neither of the above comparisons produce a definitive result, method  500  continues to step  512 . In step  512 , a comparison involving the lower threshold L is made. If the amplitude of stereo-filtered signal  320  is greater than 1.5 times the amplitude of dual mono-filtered signal  324 , decision circuit  306  determines that the JBTSC transmission is in a state of transition. In this instance, instead of switching back and forth between transmission modes, decision circuit  306  will determine that the signal mode at issue is the same as the most recent signal mode used. For example, if [w 1 (n)&gt;L*w 2 (n)], decision(n)=decision(n- 1 ). 
   Likewise, in step  514 , decision circuit  306  determines that the JBTSC transmission is in a state of transition, if the amplitude of dual mono-filtered signal  324  is greater than 1.5 times the amplitude of stereo-filtered signal  320 . For example, if [w 2 (n)&gt;L*w 1 (n)], decision(n)=decision(n- 1 ). With this method, the transmission mode used will only change when the new transmission mode is stable. 
   Step  514  may be performed separately from step  512 . Alternatively, step  514  may be performed concurrently with step  512 , wherein the two are joined with an “or” statement. For example, if [w 1 (n)&gt;L*w 2 (n)] or if [w 2 (n)&gt;L*w 1 (n)], decision(n)=decision(n- 1 ). 
   If none of the above conditions are met, that is, the comparisons do not produce a definitive result, decesion circuit  306  determines that the JBTSC transmission is in mono mode (decision(n)=mono). That is, the audio transmission is in mono mode when the amplitude of stereo-filtered signal  320  is lower than a lower threshold times the amplitude of dual mono-filtered signal  324  and the amplitude of dual mono-filtered signal  324  is lower than the lower threshold times the amplitude of stereo-filtered signal  320 . 
     FIG. 6  is a timing diagram of mode determination signal  402 , stereo-filtered signal  320 , and dual mono-filtered signal  324 . The values shown are in accordance with the embodiment described above. In  FIG. 6 , a higher value for mode determination in signal  402  corresponds to stereo mode. Similarly, a lower value for mode determination in signal  402  corresponds to dual mono mode. 
   As shown in  FIG. 6 , when stereo-filtered signal  320  has an amplitude that is at least three times larger than the amplitude of dual mono-filtered signal  324 , mode determination signal  402  corresponds to stereo mode. As the amplitude of stereo-filtered signal  320  begins to decrease, and the amplitude of dual mono-filtered signal  324  begins to increase, mode determination signal  402  does not immediately change. During this transition period, the comparison made in step  512  of method  500  applies. Since the amplitude of stereo-filtered signal  320  is still greater than 1.5 times the amplitude of dual mono-filtered signal  324 , decision circuit  306  outputs the same mode determination signal as previously output. Here, that mode determination is stereo mode. 
   Once the amplitude of dual-mono filtered signal  324  increases to at least three times the amplitude of stereo-filtered signal  320 , decision circuit  306  recognizes that the JBTSC transmission is in a stable dual mono mode. At this point, the amplitude of mode determination signal  402  changes to reflect completion of the transition to dual mono mode. 
   Conclusion 
   While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.