The disclosed BTSC encoder includes a left high pass filter means for receiving a digital left channel audio signal and for digitally high pass filtering the digital left channel audio signal and thereby generating a digital left filtered signal; a right high pass filter means for receiving a digital right channel audio signal and for digitally high pass filtering the digital right channel audio signal and thereby generating a digital right filtered signal; a matrix means for receiving the digital left and digital right filtered signals, and including means for summing the digital left and digital right filtered signals and thereby generating a digital sum signal, and including means for subtracting one of the digital left and digital right filtered signals from the other of the digital left and digital right filtered signals and thereby generating a digital difference signal; a difference channel processing means for digitally processing the digital difference signal; and a sum channel processing means for digitally processing the digital sum signal.
1. A system for producing a digital composite modulated BTSC signal compliant with the BTSC standard, the system comprising:
(A) a digital BTSC encoder operative to generate digital sum and difference signal components that are BTSC-compliant, wherein the digital BTSC encoder includes,
(i) a sum channel processing section operative to process a digital sum signal and produce a digital conditioned sum signal according to the BTSC standard; and
(ii) a difference channel processing section including a compression unit with a gain control feedback line, the difference channel processing section operative to encode a digital difference signal and produce a digital encoded difference signal according to the BTSC standard, wherein the digital difference signal represents the difference between left (L) and right (R) audio signals in left and right channels, respectively, wherein the difference channel processing section comprises a clipper;
(iii) a lowpass filter operative to receive the digital difference signal and to produce an attenuation of signal energy above 15 kHz in the digital difference signal, and wherein the attenuation reduces instability in the gain control feedback line and the digital encoded difference signal, for avoiding audible artifacts and/or improving left (L) and right (R) channel separation upon decoding of the digital composite modulated BTSC encoded signal, and wherein the lowpass filter comprises first and second sections disposed in the difference channel processing section, and wherein the first section of the lowpass filter is disposed in the difference channel processing section before the compression unit and the clipper, and wherein the second section of the lowpass filter is disposed in the difference channel processing section after the compression unit and the clipper and before the feedback line of the compression unit;
wherein the digital BTSC encoder has a digital domain magnitude response and digital domain inter-channel phase difference that are substantially equal to the analog domain magnitude response specified by the BTSC standard and analog domain inter-channel phase difference specified by the BTSC standard, respectively; and
(B) a digital composite modulator operative to receive the digital encoded difference signal and the digital conditioned sum signal and to produce the digital composite modulated BTSC signal.
2. The system of claim 1 , wherein the left channel comprises a left digital audio channel, wherein the right channel comprises a right digital audio channel, and wherein the first section of the low pass filter includes a first portion disposed in the left digital audio channel and a second portion disposed in the right digital audio channel.
3. The system of claim 1 , wherein the errors between the digital domain magnitude response of the digital BTSC encoder and the analog domain magnitude response specified by the BTSC standard, and between the digital domain inter-channel phase difference of the digital BTSC encoder and the analog domain inter-channel phase difference specified by the BTSC standard, are small enough to allow stereo separation upon decoding, from 100 Hz to 8 kHz, no worse than 10 dB less than that required by the BTSC broadcast standard.
4. A system for producing a digital composite modulated BTSC signal compliant with the BTSC standard, the system comprising:
(A) a digital BTSC encoder operative to generate digital sum and difference signal components that are BTSC-compliant, wherein the digital BTSC encoder includes,
(i) a sum channel processing section operative to process a digital sum signal and produce a digital conditioned sum signal according to the BTSC standard; and
(ii) a difference channel processing section including a compression unit with a gain control feedback line, the difference channel processing section operative to encode a digital difference signal and produce a digital encoded difference signal according to the BTSC standard, wherein the digital difference signal represents the difference between left (L) and right (R) audio signals in left and right channels, respectively, wherein the difference channel processing section comprises a clipper;
(iii) lowpass filter means operative to receive the digital difference signal and to produce an attenuation of signal energy above 15 kHz in the digital difference signal, and wherein the attenuation reduces instability in the gain control feedback line and the digital encoded difference signal, for avoiding audible artifacts and/or improving left (L) and right (R) channel separation upon decoding of the digital composite modulated BTSC encoded signal, and wherein the lowpass filter means comprises first and second sections disposed in the difference channel processing section, and wherein the first section of the lowpass filter means is disposed in the difference channel processing section before the compression unit and the clipper, and wherein the second section of the lowpass filter means is disposed in the difference channel processing section after the compression unit and the clipper and before the feedback line of the compression unit;
wherein the digital BTSC encoder has a digital domain magnitude response and digital domain inter-channel phase difference that are substantially equal to the analog domain magnitude response specified by the BTSC standard and analog domain inter-channel phase difference specified by the BTSC standard, respectively; and
(B) a digital composite modulator operative to receive the digital encoded difference signal and the digital conditioned sum signal and to produce the digital composite modulated BTSC signal.
5. The system of claim 4 , wherein the left channel comprises a left digital audio channel, wherein the right channel comprises a right digital audio channel, and wherein the first section of the low pass filter means includes a first portion disposed in the left digital audio channel and a second portion disposed in the right digital audio channel.
6. The system of claim 4 , wherein the errors between the digital domain magnitude response of the digital BTSC encoder and the analog domain magnitude response specified by the BTSC standard, and between the digital domain inter-channel phase difference of the digital BTSC encoder and the analog domain inter-channel phase difference specified by the BTSC standard, are small enough to allow stereo separation upon decoding, from 100 Hz to 8 kHz, no worse than 10 dB less than that required by the BTSC broadcast standard.
7. A computer-executable program product for producing a digital composite modulated BTSC signal compliant with the BTSC standard, the computer-executable program product comprising a non-transitory computer-readable medium with resident computer-readable instructions, the computer-readable instructions comprising instructions for:
(i) processing a digital sum signal and producing a digital conditioned sum signal according to the BTSC standard;
(ii) encoding a digital difference signal and producing a digital encoded difference signal according to the BTSC standard, wherein the digital difference signal represents the difference between left (L) and right (R) audio signals in left and right channels, respectively, wherein encoding includes compressing and providing gain control by a feedback line in accordance with the BTSC standard;
(iii) receiving the digital difference signal, and with a low pass filter producing an attenuation of signal energy above 15 kHz in the digital difference signal, and wherein the attenuation reduces instability in the gain control feedback line and the digital encoded difference signal, for avoiding audible artifacts and/or improving left (L) and right (R) channel separation upon decoding of the digital composite modulated BTSC encoded signal, and wherein the instructions for producing the attenuation of signal energy above 15 kHz cause the attenuation to occur with the low pass filter disposed in a difference channel processing section of a digital BTSC encoder, wherein the lowpass filter comprises first and second sections disposed in the difference channel processing section, wherein the difference channel processing section comprises a compression unit and a clipper, and wherein the instructions for producing the attenuation of signal energy above 15 kHz cause the attenuation to occur with the first section of the lowpass filter disposed in the difference channel processing section before the compression unit and the clipper, and also with the second section of the lowpass filter disposed in the difference channel processing section after the compression unit and the clipper and before the feedback line of the compression unit;
wherein the digital domain inter-channel magnitude difference between the digital encoded difference signal and the digital conditioned sum signal is substantially equal to the analog domain inter-channel magnitude difference specified by the BTSC standard, and
wherein the digital domain inter-channel phase difference between the digital encoded difference signal and the digital conditioned sum signal is substantially equal to the analog domain inter-channel phase difference specified by the BTSC standard.
8. The computer-executable program product of claim 7 , wherein the instructions further comprise instructions for receiving the digital encoded difference signal and the digital conditioned sum signal and modulating them together to produce the digital composite modulated BTSC signal.
9. The computer-executable program product of claim 7 , wherein the left channel comprises a left digital audio channel, wherein the right channel comprises a right digital audio channel, and wherein the instructions for producing the attenuation of signal energy above 15 kHz cause the attenuation to occur in a first portion of the first section of the low pass filter disposed in the left digital audio channel and in a second portion of the first section of the low pass filter disposed in the right digital audio channel.
10. The computer-executable program product of claim 7 , wherein the wherein the instructions provide that the errors between the digital domain magnitude response of a digital BTSC encoder and the analog domain magnitude response specified by the BTSC standard, and between the digital domain inter-channel phase difference of the digital BTSC encoder and the analog domain inter-channel phase difference specified by the BTSC standard, are small enough to allow stereo separation upon decoding, from 100 Hz to 8 kHz, no worse than 10 dB less than that required by the BTSC broadcast standard.