Abstract:
When processing a Japanese BTSC transmission, a main channel and a sub channel are processed separately. Because more components and steps are used to process the sub channel, processing the sub channel takes longer than the main channel. Therefore, a delay is inserted into the main channel. This delay is equal to the sum of the delays resulting from sub channel processing, less the delay pre-inserted into the main channel by a broadcaster. In an embodiment, the delay inserted is 42 samples. The processed main channel and sub channel are used together so as to produce left and right audio signals. All filters are designed to be very flat in the passband with steep rejection in the stop band; filters with the best phase linearity are chosen to allow good phase compensation via simple sample-delay insertion. This results in optimal stereo separation at the L and R decoded outputs.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to signal processing of a Japanese audio broadcast signal.  
         [0003]     2. Related Art  
         [0004]     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 mono 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, provides the mono signal of the original audio program content. This summed signal may be received by mono television sets.  
         [0005]     To create the stereo signal, the JBTSC system also uses an L−R signal, which is the difference between left and right channels. This signal is referred to as the sub channel, which is FM modulated, or FM carrier channel. While the sub channel alone cannot be used by the television set, it is essential to reconstructing the stereo signal.  
         [0006]     A third signal, called the control channel, is inserted into the transmission. The control channel carries information indicating the mode of transmission. Therefore, what is needed is a system for processing the three channels of the JBTSC audio transmission.  
       SUMMARY OF THE INVENTION  
       [0007]     The main channel and the sub channel in a JBTSC transmission are processed separately by a processing system and method. The sub channel is processed by a sub path. In an embodiment, the sub path includes a bandpass filter centered at approximately 2 f H , so that only the sub channel passes through. The sub channel is then modulated into an in-phase (“I”) signal and a quadrature-phase (“Q”) signal by a set of multipliers. Each of these signals is filtered with a low-pass filter to remove double frequency terms produced by the multipliers. The I and Q signals are combined and demodulated by an FM demodulator and then low-pass filtered. The signal is also processed by a deemphasis circuit, which negates the effect of a preemphasis imposed by a broadcaster.  
         [0008]     The main channel is processed by a main path. In an embodiment, the main path includes a low-pass filter identical to the low-pass filter in the sub path. The low-pass filter in the main path rejects all but the main channel. The main channel is also processed by a deemphasis circuit.  
         [0009]     In an embodiment, all filters are designed to be very flat in the passband with steep rejection in the stop band; additionally, filters with the best phase linearity are chosen to allow good phase compensation via simple sample-delay insertion. This results in optimal stereo separation at the L and R decoded outputs.  
         [0010]     Because more steps and components are involved when processing the sub channel than when processing the main channel, the sub channel takes longer to be processed. Since the main channel and the sub channel are combined to produce the output signals, the phases of each channel must match. Otherwise, the decoder outputs L and R will have poor stereo separation. Therefore, a delay is inserted into the main path of the receiver to compensate for delays resulting from processing in the sub path. In the Japanese BTSC standard, a delay of 20 μs (equivalent to 5 samples at 250 kHz sampling rate) is automatically inserted into the main channel prior to transmission. For this reason, the delay in the main path of the receiver is determined by adding the delays produced by each component in the sub path, and subtracting the 5-sample delay inherent in the main channel. In an embodiment, the delay inserted into the main path of the receiver is equal to 42 samples.  
         [0011]     After the delay is inserted, the sum of the results of the sub path and the main path is divided by 2 to produce the left stereo channel of the audio transmission. Similarly, the result of the sub path is subtracted from the result of the main path, the difference being divided by 2, to produce the right stereo channel of the audio transmission.  
         [0012]     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  
       [0013]     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.  
         [0014]      FIG. 1  is an illustration of the relationship between three channels in the JBTSC standard&#39;s spectrum.  
         [0015]      FIG. 2  is a block diagram of an embodiment of the present invention.  
         [0016]      FIG. 3  is a flowchart of a method according to an embodiment of the present invention.  
         [0017]      FIG. 4  is flowchart of a sub path method according to an embodiment of the present invention.  
         [0018]      FIG. 5  is flowchart of a main path method according to an embodiment of the present invention.  
     
    
       [0019]     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  
       [0020]     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.  
         [0021]     As shown in  FIG. 1 a  JBTSC audio transmission includes a main channel  102 , a sub channel  104 , and a control signal  106 . Main channel  102  is also referred to as the sum, since it carries the L+R audio signal. Sub channel  104  is FM modulated at 2 f H , f H  being the horizontal scanning frequency. This modulating signal is either L−R (if stereo mode) or the second audio program (if dual mono mode). Sub channel  104  is typically centered at 2 f H , f H  being the horizontal scanning frequency. If the transmission is in stereo or dual mono mode, control signal  106  includes an AM carrier at 3.5 f H , whose AM sidebands&#39; frequencies indicate whether the transmission is in stereo or dual mono.  
         [0022]      FIG. 2  is a block diagram of a processing system  200  according to an embodiment of the present invention. Processing system  200  includes a sub path  202 , a main path  204 , and a separator  206 . Sub path  202  includes a bandpass filter  208 , a first filter path  210 , a second filter path  212 , an FM demodulator  214 , a lowpass filter  216 , and a deemphasis circuit  218 .  
         [0023]     An audio transmission  220 , input to processing system  200 , is split between sub path  202  and main path  204 . In the sub path, bandpass filter  208  filters audio transmission  220 . In an embodiment, bandpass filter  208  is a 65 th -order FIR filter centered at approximately 2 f H  so that only subchannel  104  passes through. Bandpass filter  208  is designed to be flat in the passband with steep rejection in the stop band to reject signals from main channel  102  and control channel  106 . In an embodiment, to assist in demodulation, both the in-phase and quadrature-phase (I and Q) version of the signal are applied to FM demodulator  214 . In this embodiment, sub channel  104  is split between first filter path  210  and second filter path  212 . First filter path  210  produces I signal  226 . First filter path  210  includes an in-phase multiplier  222  and an in-phase low-pass filter  224 . In an embodiment, in-phase multiplier  222  multiplies channel  104  by cos(4πf H t). In-phase low-pass filter  224  is then used to reject the double frequency term in the signal produced by in-phase multiplier  222 . In-phase low-pass filter outputs I signal  226 . In an embodiment, filter  224 , a 32nd-order FIR filter, is substantially flat in the passband, so as to preserve sidebands in the sub channel. This filter is constrained to have maximum rejection around the 2×image. Above that frequency, constraints can be relaxed due to the fact that there is no input energy there.  
         [0024]     Second filter path  212  produces Q signal  228 . Second filter path  212  includes a quadrature-phase multiplier  230  and a quadrature-phase low-pass filter  232 . In an embodiment, quadrature-phase multiplier  230  multiplies sub channel  104  by sin(4πf H t). Quadrature-phase low-pass filter  230  is then used to reject the double frequency term in the signal produced by quadrature-phase multiplier  230 . Quadrature-phase low-pass filter outputs Q signal  228 .  
         [0025]     I signal  226  and Q signal  228  are both input to FM demodulator  214 . FM demodulator  214  applies a difference equation to demodulate the FM signal. In an embodiment, the difference equation is the first-order difference equation: 
 
 FMDemod=[Q ( n )* I ′( n )− I ( n )* Q ′( n )]/[ Q ( n )* Q ( n )+ I ( n )* I ( n )], 
 
 where I′(n)=I(n)−I(n−1). One of skill in the art will recognize that a higher-order difference equation may also be used. FM demodulator  214  outputs demodulated FM signal  234 . 
 
         [0027]     Low-pass filter  216  receives demodulated FM signal  234 . In an embodiment, low-pass filter  216  filters out everything above, for example, 13 kHz. In an embodiment, low-pass filter  216  is a 10 th -order elliptical filter. One of skill in the art will recognize that different filters may be substituted as needed. Low-pass filter  216  outputs signal  236 .  
         [0028]     Signal  236  is next input to deemphasis circuit  218 . In an FM system, the higher frequencies contribute more to the noise than the lower frequencies. Because of this, all FM systems adopt a system of preemphasis where the higher frequencies are increased in amplitude before the transmission is modulated. Thus, when the transmission is received, the higher frequencies must be deemphasized in order to recover the original baseband signal. In an embodiment, deemphasis circuit  218  is set at approximately 75 μs. Deemphasis circuit  218  outputs signal  238 . Signal  238  is equal to the difference between the left and right stereo signals, or L−R, and is also referred to as the sub path signal S.  
         [0029]     When audio transmission  220  is input to processing system  200 , audio transmission  220  is split between sub path  202  and main path  204 . Main path  204  includes a low-pass filter  240 , a deemphasis circuit  242 , and a delay block  244 .  
         [0030]     Low-pass filter  240  is identical to low-pass filter  216  from sub path  202 , and filters out all but main channel  102 . The output of low-pass filter  238  is sum signal  246 . Deemphasis circuit  242  is identical to deemphasis circuit  218  from sub path  202 , and performs the same function.  
         [0031]     Delay block  244  inserts a timing delay into sum signal  246 . This timing delay is inserted to account for the time required to process and output difference signal  238  in sub path  202 . The timing delay is needed because, if the sum and difference signals are out of phase, stereo separation between L and R outputs will be poor.  
         [0032]     In the JBTSC standard, a 20 μs delay is automatically inserted into the main channel of the audio transmission by a broadcaster. This is done because a bandpass filter is typically needed to separate the sub channel from the main channel, and the typical delay resulting from such a bandpass filter is approximately 20 μs.  
         [0033]     The total delay that needs to be corrected for by delay block  244  is the sum of the delays resulting from components of sub path  202 , less the delay pre-inserted into the main channel by the broadcaster. In an embodiment, the components of sub path  202  that add to the total delay are bandpass filter  208  and low-pass filters  224  and  232 . Low-pass filter  216  in sub path  202  is identical to low-pass filter  238  in main path  204 . Therefore, low-pass filter  216  does not contribute any additional delay. In an example embodiment, bandpass filter  208  is a Remez filter of the 63 rd  order, resulting in a delay of 32 samples. In the same embodiment, for example, low-pass filters  224  and  232  are Remez filters of the 32 nd  order, resulting in a delay of 15 samples. In this embodiment, the delay resulting from components of sub path  202  is approximately 47 samples.  
         [0034]     In the JBTSC standard, the incoming sample rate is 250 kHz, resulting in each sample equating to approximately 4 μs. Since each sample is approximately equal to 4 μs, the 20 μs delay inserted by the broadcaster equates to approximately 5 samples. Thus, for this embodiment, the total delay inserted into sum signal  246  by delay block  244  is (47−5) samples, or 42 samples. Due to mismatches or imperfections in the initial encoding process, the final delay added may vary slightly from the calculated amount. For example, in the embodiment above, the total delay inserted into main channel  102  may be adjusted to 43 samples. One of skill in the art will recognize that different values for the total delay may be substituted to correspond to the delays produced by different filters used. Delays produced by the filters will depend on the type and order of filters used.  
         [0035]     After the delay is inserted, sum signal  245  is output by delay block  244 . Sum signal  245  is equal to the sum of left and right stereo signals, or L+R, and may also be referred to as main path signal M.  
         [0036]     Sum signal  245  and difference signal  238  are both received by separator  206 . Since sum signal  245  is equal to L+R, and difference signal  238  is equal to L−R, the left channel L of the stereo signal may be obtained by adding together sum signal  245  and difference signal  238 , and dividing the result in half. Using the notation given above, L=(M+S)/2. Similarly, the right channel R may be obtained by subtracting difference signal  238  from sum signal  245 , and dividing the result in half. Using the notation given above, R=(M−S)/2. The left channel L is output through left output  248 , and the right channel R is output through right output  250 .  
         [0037]      FIG. 3  is a flowchart of a method  300  according to an embodiment of the present invention. In step  302 , the sub channel  104  of a JBTSC signal is processed.  FIG. 4  is a flowchart that further details step  302 . In step  402 , transmission  220  is filtered by bandpass filter  208  to separate, for example, sub channel  104 . In step  404 , an I signal is produced from sub channel  104 . Similarly, in step  406 , a Q signal is produced from sub channel  104 . In step  408 , the I and Q signals are demodulated by, for example FM demodulator  214 . This produces a demodulated signal, such as, for example, demodulated FM signal  234 . In step  410 , the demodulated signal is filtered by a low-pass filter. Finally, in step  412 , the signal is deemphasized to regain the original baseband signal.  
         [0038]     In step  304 , the main channel of the JBTSC transmission is processed. In an embodiment, this step is performed concurrently with step  302 .  FIG. 5  is a flowchart further detailing step  304 . In step  502 , transmission  220  is filtered to produce the main channel, such as main channel  102 . In step  504 , the main channel is deemphasized to regain the original baseband signal.  
         [0039]     In step  306 , a delay is inserted into the main channel. This delay is equal to the delay resulting from step  302  less a delay inherent in the main channel of the transmission. Step  306  may occur separately from step  304 . In an alternative embodiment, step  306  occurs at the same time as step  304 .  
         [0040]     In step  308 , left and right stereo components of the transmission are produced from the results of step  302  and step  306 .  
         [0041]     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.