Asynchronous sample rate converter
An asynchronous sample rate converter and method for converting an input signal to a resampled output signal is disclosed. An efficient and cost-effective sample rate converter for converting an input signal of arbitrary sample rate to a resampled output signal of a second sample rate is disclosed. A hardware-efficient sample-rate converter for resampling an audio input signal with an arbitrary sample rate to an output audio signal with a known sample rate for use in an audio processor is disclosed.
1. A sample rate converter for converting an input signal sampled at a first sample rate to an output signal sampled at an output sample rate different from the first sample rate, the sample rate converter comprising:
a frequency synthesizer configured to produce a clock signal;
a cross enable unit configured to accept the clock signal and the input signal and to generate a count disparity signal and a de-jittered signal sampled at substantially a mean of the first sample rate from the input signal and the clock signal; and
a linear interpolation unit configured to generate the output signal at a sample rate of the clock signal from the de-jittered signal, the clock signal, and the count disparity signal,
wherein the linear interpolation unit includes a first sigma delta loop,
wherein the linear interpolation unit calculates a difference between successive samples of the de-jittered signal on each clock cycle of the de-jittered signal and inputs the difference to the first sigma delta loop,
wherein the first sigma delta loop divides the difference by a predetermined value to obtain an integral part and a remainder part, and
wherein the predetermined value is based on the output sample rate.
2. The sample rate converter in accordance with claim 1 , wherein the first sigma delta loop adds the integer part to a previous resampled output signal on each cycle of the clock signal to generate a current sample of a resampled output signal.
3. The sample rate converter in accordance with claim 1 , wherein the first sigma delta loop recycles the remainder part into a correction signal.
4. The sample rate converter in accordance with claim 1 , further comprising a FIR filter module configured to generate a filtered intermediate signal from the de-jittered signal, the clock signal, and one or more resampled clock signals,
wherein the FIR filter module includes a low pass filter, and
wherein the linear interpolation unit is further configured to accept the filtered intermediate signal.
5. The sample rate converter in accordance with claim 4 , wherein the low pass filter is a low pass polyphase FIR filter.
6. The sample rate converter in accordance with claim 4 , wherein the FIR filter module further includes a limiter configured to prevent overflow of the filtered intermediate signal.
7. The sample rate converter in accordance with claim 1 , wherein the cross enable unit includes:
a second sigma delta loop configured to calculate a temporal correction value from the count disparity signal; and
a resampled clock generator configured to generate one or more resampled clock signals from the temporal correction value.
8. The sample rate converter in accordance with claim 7 , wherein the cross enable unit further includes a FIFO buffer configured to store samples of the count disparity signal and input signal and to release samples to the second sigma delta loop.
9. The sample rate converter in accordance with claim 1 , wherein the cross enable unit includes:
an averaging block configured to generate an averaged count disparity signal from the count disparity signal;
a second sigma delta unit configured to calculate a temporal correction value from the averaged count disparity signal; and
a resampled clock generator configured to generate one or more resampled clock signals from the temporal correction value.
10. The sample rate converter in accordance with claim 9 , wherein the cross enable unit further includes a FIFO buffer, configured to store samples of the count disparity signal and the input signal and to release the samples to the averaging block.
11. The sample rate converter in accordance with claim 10 , wherein the averaging block includes a moving average FIR filter configured to at least partially calculate the averaged count disparity signal.
12. A method for converting an input signal sampled at a first sample rate to an output signal sampled at an output sample rate different from the first sample rate, the method comprising:
producing a clock signal;
accepting the clock signal and the input signal;
generating a count disparity signal and a de-jittered signal sampled at substantially a mean of the first sample rate from the input signal and the clock signal;
generating the output signal at a sample rate of the clock signal from the de-jittered signal, the clock signal, and the count disparity signal,
wherein generating the output signal includes:
calculating a difference between successive samples of the de-jittered signal on each clock cycle of the de-jittered signal; and
dividing the difference by a predetermined value to obtain an integral part and a remainder part,
wherein the predetermined value is based on the output sample rate.
13. The method in accordance to claim 12 , wherein the predetermined value is the count disparity signal.
14. The method in accordance to claim 12 , further comprising adding the integer part to a previous resampled output signal on each cycle of the clock signal to generate a current sample of a resampled output signal.
15. The method in accordance to claim 12 , further comprising recycling the remainder part into a correction signal.