NICAM audio signal resampler
View Patent ↗A NICAM audio signal re-sampler may include a non-linear interpolator configured to interpolate in a non-linear manner between sequential digital samples that are based on a stream of demodulated NICAM audio samples. A phase differential calculator may be included that compares phase information at different resolutions.
1. A digital sample rate converter comprising:
a digital up sampler configured to receive a first stream of digital samples of an analog signal at a first frequency and to generate a second stream of digital samples at a second frequency that is substantially higher than the first frequency and that substantially tracks the first stream of digital samples; and
a non-linear interpolator configured to interpolate between two sequential digital samples in the second stream of digital samples in a non-linear manner, wherein the non-linear interpolator is configured to interpolate at points in time that are based on a phase difference between the first stream of digital samples and a local sample clock.
2. The digital sample rate converter of claim 1 wherein the non-linear interpolator is configured to interpolate between the two sequential digital samples by determining a non-linear function that substantially fits three sequential digital samples in the second stream that include the two sequential digital samples.
3. The digital sample rate converter of claim 2 wherein the non-linear function is a parabolic function.
4. The digital sample rate converter of claim 2 further including a digital delay line configured to generate at least two versions of the second stream of digital samples, each delayed by a different amount of time with respect to the second stream of digital samples.
5. The digital sample rate converter of claim 4 further comprising a phase differential calculator configured to calculate the phase differential, said phase difference calculator including a phased-lock loop.
6. The digital sample rate converter of claim 4 further comprising a synchronizer configured to synchronize interpolations made by the non-linear interpolator with the local sample clock.
7. The digital sample rate converter of claim 5 wherein the synchronizer includes a 2-word FIFO.
8. The digital sample rate converter of claim 4 wherein the first stream of digital samples are demodulated NICAM audio samples at a frequency of approximately 32 kHz.
9. The digital sample rate converter of claim 7 wherein the local sample clock has a frequency of approximately 31.25 kHz.
10. The digital sample rate converter of claim 7 wherein the local sample clock has a frequency of approximately 32 kHz.
11. The digital sample rate converter of claim 7 wherein the local sample clock has a frequency of approximately 44.1 kHz.
12. The digital sample rate converter of claim 7 wherein the local sample clock has a frequency of approximately 46.875 kHz.
13. The digital sample rate converter of claim 7 wherein the local sample clock has a frequency of approximately 48 kHz.
14. The digital sample rate converter of claim 7 wherein the second frequency is between 128 Khz to 1.024 Mhz.
15. The digital sample rate converter of claim 13 wherein the second frequency is approximately 384 kHz.
16. The digital sample rate converter of claim 1 wherein the digital up sampler includes a sample interpolator configured to divide each digital sample in the first stream into an integer multiple of samples.
17. The digital sample rate converter of claim 15 wherein the sample interpolator is configured to divide each digital sample in the first stream into between 4 and 32 samples.
18. The digital sample rate converter of claim 16 wherein the sample interpolator is configured to divide each digital sample in the first stream into 12 samples.
19. The digital sample rate converter of claim 15 wherein the sample interpolator is configured such that one of the integer multiple of samples is based on the digital sample in the first stream and the other integer multiple samples are substantially zero.
20. The digital sample rate converter of claim 18 wherein the digital up sampler includes a digital low pass filter and wherein the digital low pass filter is configured to filter the integer multiple of samples.