Reciprocal index lookup for BTSC compatible coefficients
View Patent ↗An algorithm calculates spectral compression/expansion filter coefficients using a value proportional to a reciprocal of a feedback/feedforward signal and stores the coefficients to a lookup table. The lookup table is indexed by a pre selected set of coefficient bits to generate a filter coefficient function. A first portion of the lookup table stores a plurality of discrete values at index points of a line segment corresponding to a filter coefficient function approximation, so as to generate an initial discrete value corresponding to the filter coefficient function at a value of the high order bits and a second portion stores a plurality of slope values, which indicates a slope of the filter coefficient function. A linear circuit interpolates/decimates an approximation of the compression/expansion filter coefficient function based on the slope value, the initial discrete value and a preselected set of low order bits of the reciprocal value.
1. A method for decoding an encoded BTSC composite audio signal comprising:
digitally separating a L−R signal and a L+R signal, both being encoded in the composite signal;
digitally applying deemphasis to the L+R signal;
digitally processing the L−R signal, wherein the step of digitally processing includes computing spectral expansion coefficients and using said coefficients to spectrally expand the L−R signal into a spectrally expanded output signal, wherein the coefficients for spectrally expanding the L−R signal are determined based on a band-limited feed-forward compressed L−R signal, and wherein a reciprocal of the RMS value of the feed-forward signal is used to determine the spectral expansion coefficients; and
combining the de-emphasized L+R signal with the digitally processed L−R signal to produce separate I, and R audio signals.
2. The method of claim 1 wherein the step of digitally processing includes decimation of the digitally processed L−R signal and the de-emphasized L+R signal.
3. The method of claim 1 wherein the coefficients are computed by interpolating between the reciprocal of the RMS value of the feedforward signal and values retrieved from a lookup table, the lookup table indexed by high order bits of the reciprocal of the RMS feedforward signal.
4. A method for decoding an encoded BTSC composite audio signal comprising:
digitally separating a L−R signal, a L+R signal and a SAP signal all being encoded in the composite signal;
digitally applying deemphasis to the L+R signal;
digitally processing the L−R signal;
digitally processing the SAP signal, wherein the step of digitally processing the SAP signal includes computing spectral expansion coefficients and using said coefficients to spectrally expand the SAP signal into a spectrally expanded output signal, wherein the coefficients for spectrally expanding 20 the SAP signal are determined based on a feedforward signal, which is based on a band-limited compressed L−R signal, and wherein a reciprocal of the RMS value of the feed-forward signal is used to determine the spectral expansion coefficients; and
combining the de-emphasized L+R signal with the digitally processed L−R signal to produce separate L and R audio signals.
5. The method of claim 4 wherein the step of digitally processing includes decimation of the digitally processed L−R signal, the de-emphasized L+R signal and the SAP signal.
6. The method of claim 4 wherein the coefficients are computed by interpolating between the reciprocal of the RMS value of the feedforward signal and values retrieved from a lookup table, the lookup table indexed by high order bits of the reciprocal of the RMS feedforward signal.
7. A method for decoding a digital BTSC-compatible stereo television audio signal to generate a left and right audio output signal, the method comprising:
digitally processing a feedforward portion of the digital BTSC signal at a predetermined sample rate using digitally modeled transfer functions to generate spectral expansion coefficients, the digitally modeled transfer functions being compliant with BTSC standard continuous time transfer functions, wherein the step of digitally processing is accomplished using bilinear transformation of analog transfer functions;
using the spectral expansion coefficients to expand the digital BTSC signal; and
using digital modeling techniques to model amplitude and phase characteristics of ideal analog decoder transfer functions.
8. The method of claim 7 further comprising:
Adjusting the gain of the expanded digital BTSC signal with the reciprocal of the feedforward portion of the digital signal to form a gain adjusted signal; and
decimating the gain adjusted signal at the predetermined sample rate.
9. The method of claim 7 wherein the digitally modeled amplitude and phase characteristics do not provide phase compensation for non-idealities.
10. The method of claim 7 wherein the step of digital processing includes digitally modeling a BTSC standard variable spectral expansion continuous time update transfer function.
11. The method of claim 7 wherein the step of digitally processing includes digitally modeling a BTSC standard gain control continuous time update algorithm.
12. The method of claim 7 wherein the step of digitally processing includes using an impulse invariance transformation of analog transfer functions.
13. The method of claim 7 wherein the step of digitally processing includes using a digital variable expansion filter controlled by a variable expansion feedforward loop modeled using bilinear transformations of analog transfer functions.
14. The method of claim 7 wherein the step of digitally processing includes using a digital variable expansion filter controlled by a variable expansion feedforward loop modeled using impulse invariance transformation of analog transfer functions.
15. The method of claim 7 wherein the step of digitally processing includes using a digital wideband feedforward gain control bandpass filter modeled using bilinear transformation of analog transfer functions.
16. The method of claim 7 wherein the step of digitally processing includes using a digital wideband feedforward gain control bandpass filter modeled using impulse invariance transformation of analog transfer functions.
17. The method of claim 7 wherein the step of digitally processing includes using a digital wideband feedforward RMS detector.
18. The method of claim 7 wherein the step of digitally processing includes using a digital wideband feedback 1/x function.
19. The method of claim 7 wherein the step of digitally processing includes 20 processing the signal using a digital variable expansion feedforward bandpass filter modeled using the bilinear transformation of analog transfer functions.
20. The method of claim 7 wherein the step of digitally processing includes processing the signal using a digital variable expansion feedforward bandpass filter modeled using the impulse invariance transformation of analog transfer functions.
21. The method of claim 7 wherein the step of digitally processing includes processing the signal using a digital variable expansion feedforward rms detector.
22. The method of claim 7 wherein the method is performed using a digital signal processor.
23. The method of claim 7 wherein the method is performed using an application specific integrated circuit.
24. The method of claim 7 wherein the method is performed using a field programmable gate array.