Digital audio processing system and method
A method includes receiving first data corresponding to a first signal sampled at a first sample rate, decimating the first data to provide a second signal sampled at a second sample rate, and recovering a pilot signal from the second signal. The method also includes evaluating the pilot signal to determine an error value, where the error value is based on a comparison of a sample of the pilot signal to zero. The method also includes adjusting the second sample rate based on the error value.
1. A digital audio processing method comprising:
receiving first data corresponding to a first signal sampled at a first sample rate;
decimating the first data to provide a second signal sampled at a second sample rate;
recovering a pilot signal from the second signal;
evaluating the pilot signal to determine an error value, wherein the error value is based on a comparison of a sample of the pilot signal to zero; and
adjusting the second sample rate based on the error value.
2. The method of claim 1 , wherein the error value is based on a value of the sample of the pilot signal and a difference between the value and a prior sample value.
3. The method of claim 1 , wherein the comparison of the sample to zero is performed on every Nth sample of the pilot signal, wherein N is four or thirty-two.
4. The method of claim 1 , further comprising:
adjusting a phase value associated with the second signal by an offset value, the offset value based on a detected error of a prior phase value associated with the second signal;
mapping the adjusted phase value to a nearest predetermined phase value selected from a plurality of predetermined phase values; and
outputting an indication of a symbol, wherein the symbol indicates a difference between the nearest predetermined phase value and a prior nearest predetermined phase value.
5. The method of claim 4 , wherein the symbol is a Near Instantaneous Companded Audio Multiplex (NICAM) symbol.
6. The method of claim 4 , wherein the plurality of predetermined phase values includes zero degrees, ninety degrees, one hundred eighty degrees, and two hundred seventy degrees.
7. The method of claim 6 , further comprising:
determining a phase difference between the adjusted phase value and the nearest predetermined phase value; and
modifying the offset value based on the phase difference.
8. A method, comprising:
decimating an input signal at a sample rate to produce first output;
filtering the first output to recover a pilot signal;
evaluating the pilot signal to determine an error value, wherein the error value is based on a comparison of a sample of the pilot signal to zero; and
adjusting the sample rate based on the error value.
9. The method of claim 8 , wherein the error value is based on a value of the sample of the pilot signal and a difference between the value and a prior sample value.
10. The method of claim 8 , wherein the comparison of the sample of the pilot signal to zero is performed on every Nth sample of the pilot signal, wherein N is four or thirty-two.
11. The method of claim 8 , further comprising:
adjusting a phase value associated with the first output by an offset value, the offset value based on a detected error of a prior phase value associated with the first output;
mapping the adjusted phase value to a nearest predetermined phase value selected from a plurality of predetermined phase values; and
outputting an indication of a symbol, wherein the symbol indicates a difference between the nearest predetermined phase value and a prior nearest predetermined phase value.
12. The method of claim 11 , further comprising:
determining a phase difference between the adjusted phase value and the nearest predetermined phase value; and
modifying the offset value based on the phase difference.
13. The method of claim 11 , wherein the symbol is a Near Instantaneous Companded Audio Multiplex (NICAM) symbol.
14. A method, comprising:
decimating a first inphase signal and a first quadrature signal at a sample rate with a decimator to output a second inphase signal and a second quadrature signal;
recovering a pilot signal from the second inphase signal and the second quadrature signal with a pilot filter;
evaluating the pilot signal to determine an error value with a phase detector, wherein the error value is based on a comparison of a sample of the pilot signal to zero; and
adjusting the sample rate at the decimator based on the error value.
15. The method of claim 14 , wherein the first inphase signal and the first quadrature signal are output of a coordinate rotation digital computer mixer.
16. The method of claim 15 , wherein input to the coordinate rotation digital computer mixer comprises an input signal and an input frequency.
17. The method of claim 16 , wherein the input signal is a modulated digital signal.
18. The method of claim 14 , wherein the second inphase signal and the second quadrature signal pass through a channel filter to the pilot filter.
19. The method of claim 14 , wherein the second inphase signal and the second quadrature signal pass through a channel filter to a demodulator.
20. The method of claim 14 , wherein the pilot filter comprises an absolute value circuit and a summer.