BTSC ENCODER
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 . An adaptive digital signal weighting system for use with a digitally represented difference signal representing the difference between two stereophonic audio signals, the system including a signal path for transmitting an electrical information signal containing information relating to the difference signal of a predetermined bandwidth through said system at a predetermined sample rate so as to preserve the signal content of said information signal, said system further comprising:
a digital filter arrangement disposed in said signal path and constructed and arranged so as to vary the gain impressed on the portion of said information signal within a first select spectral region within said predetermined bandwidth by a first variable gain factor, said first variable gain factor varying in response to and as a function of a first control signal;
a first control signal generator constructed and arranged so as to digitally generate said first control signal only in response to and in accordance with the signal energy of said digitally represented difference signal within a second select spectral region including at least a part of said first select spectral region;
a digital gain controller, disposed in said signal path and coupled to said digital filter arrangement, and constructed and arranged so as to vary the signal gain impressed on said information signal substantially throughout said predetermined bandwidth by a second variable gain factor, said second variable gain factor varying in response to and as a function of a second control signal; and
a second control signal generator constructed and arranged so as to digitally generate said second control signal in response to and as a function of the signal energy of said digitally represented difference signal substantially within a third select spectral region within said predetermined bandwidth;
wherein the digital filter arrangement, first control signal generator, digital gain controller, and the second control signal generator each operate at a predetermined sample rate so as to preserve the signal content of the information signal, and the sampling rate is chosen so as to be equal to an integer multiple of the frequency of a pilot tone that can be added to the difference signal to identify the encoded signal to a receiver.
2 . A digital system for encoding an electrical information signal of a predetermined bandwidth at a predetermined sample rate so as to preserve the signal content of said information and so that said information signal can be recorded on or transmitted through a channel with a frequency dependent dynamic range as an encoded difference signal representing the difference between two stereophonic audio signals, the channel having a narrower dynamically-limited portion in a first spectral region than in at least one other spectral region of said predetermined bandwidth, said system comprising:
an input constructed and arranged so as to receive said information signal;
a signal transmission path, coupled to said input and constructed and arranged so as to transmit said information signal received at said input;
an output coupled to said input through said signal transmission path for providing said information signal as encoded by said system;
a digital gain controller coupled to said signal path for varying the signal gain impressed on said information signal substantially throughout said predetermined bandwidth, said signal gain varying in response to and as a function of a first control signal;
a digital filter arrangement, coupled to said signal path and said digital gain controller, and constructed and arranged so as to impress a second variable gain on the portion of said information signal substantially within said first spectral region so as to preemphasize said portion with respect to the remaining portions of said information signal, said second variable gain varying in response to and as a function of a second control signal;
a first control signal generator constructed and arranged so as to digitally generate said first control signal in response to and as a function of the signal energy of said information signal substantially within a second spectral region of said predetermined bandwidth; and
a second control signal generator constructed and arranged so as to digitally generate said second control signal only in response to and in accordance with the signal energy of said information signal within a third spectral region of said predetermined bandwidth including at least a part of said first spectral region;
wherein the digital gain controller, said digital filter arrangement, said first control signal generator and said second control signal generator each operate at a predetermined sample rate so as to preserve the signal content of said information signal, and the sampling rate is chosen so as to be equal to an integer multiple of the frequency of a pilot tone that can be added to the encoded signal to identify the encoded signal to a receiver.
3 . An adaptive digital signal weighting system for use with a digitally represented difference signal representing the difference between two stereophonic audio signals, the system comprising:
a signal path for transmitting an electrical information signal of a predetermined bandwidth through said system;
a variable coefficient digital filter arrangement (a) constructed and arranged so as to filter said information signal, the filter arrangement being characterized by a variable coefficient transfer function, and (b) constructed and arranged so as to vary the gain impressed on the portion of said information signal within a first select spectral region within said predetermined bandwidth by a first variable gain factor, the variable coefficients of said variable coefficient transfer function and said first variable gain factor varying in response to and as a function of a first control signal;
a first control signal generator constructed and arranged so as to digitally generate said first control signal only in response to and in accordance with the signal energy of said information signal within a second select spectral region including at least a part of said first select spectral region;
a digital gain controller disposed in said signal path and coupled to said variable coefficient digital filter arrangement, and constructed and arranged so as to vary the signal gain impressed on said information signal substantially throughout said predetermined bandwidth by a second variable gain factor, said second variable gain factor varying in response to and as a function of a second control signal; and
a second control signal generator constructed and arranged so as to digitally generate said second control signal in response to and as a function of the signal energy of said information signal substantially within a third select spectral region within said predetermined bandwidth;
wherein the digital filter arrangement, first control signal generator, digital gain controller and second control signal generator each operate at a predetermined sample rate so as to preserve the signal content of said information signal, and the sampling rate is chosen so as to be equal to an integer multiple of the frequency of a pilot tone that can be added to the difference signal to identify the difference signal to a receiver.
4 . The system according to claim 1 , wherein the pilot frequency is substantially 15,734 Hz.
5 . The system according to claim 1 , wherein DC signal energy is absent from the signal content of the information signal so as to prevent ticking.
6 . The system according to claim 2 , wherein the pilot frequency is substantially 15,734 Hz.
7 . The system according to claim 2 , wherein DC signal energy is absent from the signal content of the information signal so as to prevent ticking.
8 . The system according to claim 3 , wherein the pilot frequency is substantially 15,734 Hz.
9 . The system according to claim 3 , wherein DC signal energy is absent from the signal content of the information signal so as to prevent ticking.
10 . A digital implementation of an analog adaptive signal weighting system for use with a digitally represented difference signal representing the difference between two stereophonic audio signals, the system including a signal path for transmitting an electrical information signal containing information relating to the difference signal of a predetermined bandwidth through said system, said analog adaptive weighting system being of the type including first analog control signal generator including a first analog filter component and a second analog control signal generator including a second analog filter component, said digital implementation further comprising:
a digital filter arrangement disposed in said signal path and constructed and arranged so as to vary the gain impressed on the portion of said information signal within a first select spectral region within said predetermined bandwidth by a first variable gain factor, said first variable gain factor varying in response to and as a function of a first control signal;
a first digital control signal generator constructed and arranged so as to digitally generate said first control signal only in response to and in accordance with the signal energy of said digitally represented difference signal within a second select spectral region including at least a part of said first select spectral region, said first digital control signal generator including a first digital filter component having an amplitude response such that the difference between the amplitude response of the first digital filter component and the amplitude response of the first analog filter component is minimized irrespective of their respective phase responses;
a digital gain controller, disposed in said signal path and coupled to said digital filter arrangement, and constructed and arranged so as to vary the signal gain impressed on said information signal substantially throughout said predetermined bandwidth by a second variable gain factor, said second variable gain factor varying in response to and as a function of a second control signal;
a second digital control signal generator constructed and arranged so as to digitally generate said second control signal in response to and as a function of the signal energy of said digitally represented difference signal substantially within a third select spectral region within said predetermined bandwidth, said second digital control signal generator including a second digital filter component having an amplitude response such that the difference between the amplitude response of the second digital filter component and the amplitude response of the second analog filter component is minimized irrespective of their respective phase responses; and
wherein the digital filter arrangement, first control signal generator, digital gain controller, and the second control signal generator each operate at a predetermined sample rate so as to preserve the signal content of the information signal.
11 . The digital implementation according to claim 10 , wherein said first digital filter component comprises a spectral bandpass filter.
12 . The digital implementation according to claim 10 , wherein the first digital filter component comprises an IIR filter.
13 . The digital implementation according to claim 10 , wherein the first digital filter component comprises a cascade of IIR filters.
14 . The digital implementation according to claim 10 , wherein said second digital filter component comprises an IIR filter.