Sample rate converter

A method for digitally converting a digital input signal sampled at a first sample rate .function..sub.INPUT into a digital output signal having a second sample rate frequency .function..sub.OUTPUT, the method comprising the following steps:

• selecting a predefined single-stage low-pass filter from a plurality of single-stage low-pass filters, wherein the selected filter is designed for use in connection with the conversion of signals having sample rate frequencies falling within a predetermined continuous sample rate frequency range, and .function..sub.INPUT and .function..sub.OUTPUT fall within said sample rate frequency range and wherein a normalized cutoff frequency for each of said predefined filters is optimized for a plurality of critical discrete input and output sample rate frequency conversion pairs identified for each filter, and wherein the normalized cutoff frequency for each of the predefined low pass filters provides a zero pitch error for any conversion of an input signal having an .function..sub.INPUT sample rate to an output signal having an .function..sub.OUTPUT sample rate, the .function..sub.INPUT and the .function..sub.OUTPUT sample rates being selected from one of the plurality of critical discrete input and output sample rate frequency conversion pairs identified for the filter;
• calculating an interpolation (L) and a decimation (M) factor for the selected filter based on the .function..sub.INPUT and the .function..sub.OUTPUT sample rate frequencies;
• generating an intermediate signal having an intermediate sample rate frequency .function..sub.INTERMEDIATE by inserting L-1 zero-valued samples between each existing data sample in the input digital signal;
• filtering the intermediate signal with the selected filter, wherein the filter operates on every Lth data point and skips the intervening L-1 zero data points;
• and
• generating the output signal having a sample rate frequency of .function..sub.OUTPUT by decreasing the sample rate frequency of the filtered intermediate signal by removing M-1 out of every M data samples in the filtered intermediate signal.

A sample rate converter for digitally converting a digital input signal sampled at a first sample rate frequency .function..sub.INPUT into a digital output signal having a second sample rate frequency .function..sub.OUTPUT, wherein .function..sub.INPUT and the .function..sub.OUTPUT fall anywhere within a predetermined continuous sample rate frequency range, the sample rate converter comprising:

• means for establishing an upper and a lower acceptable sample rate frequency for the predetermined sample frequency range based upon a minimum predetermined quality characteristic required for the output signal where the predetermined quality characteristic is a pitch error associated with the output signal;
• interpolation means for increasing the sample rate frequency .function..sub.INPUT of the input signal by a factor of L, resulting in an intermediate signal having an intermediate sample rate frequency;
• filter means for low-pass filtering the intermediate signal so as to remove portions of the signal falling above a normalized cutoff frequency, the filter means being comprised of a single-stage low pass filter and the normalized cutoff frequency being optimized for a plurality of predefined critical discrete input and output sample rate frequency conversion pairs;
• decimation means for decreasing the sample rate frequency of the filtered intermediate signal by a factor of M, resulting in the output signal having a sample rate frequency of .function..sub.OUTPUT ;
• and
• wherein L and M are calculated based on .function..sub.INPUT, .function..sub.OUTPUT and the normalized cutoff frequency for the single-stage low pass filter.

A method for digitally converting a digital input signal sampled at a first sample rate frequency .function..sub.INPUT into a digital output signal having an output sample rate frequency .function..sub.OUTPUT, where .function..sub.INPUT and the .function..sub.OUTPUT fall anywhere within a predetermined continuous sample rate frequency range, the method comprising the following steps:

• (a) identifying a plurality of critical discrete input and output sample rate frequency conversion pairs;
• (b) based upon said plurality of critical conversion pairs, calculating a normalized cutoff frequency .function..sub.C-NORM that is optimized for said plurality of critical discrete input and output sample rate frequency conversion pairs, and wherein the normalized cutoff frequency provides a zero pitch error for any conversion of an input signal having an .function..sub.INPUT sample rate to an output signal having an .function..sub.OUTPUT sample rate, the .function..sub.INPUT and the .function..sub.OUTPUT sample rates being selected from one of the plurality of critical discrete input and output sample rate frequency conversion pairs;
• (c) defining a single-stage, finite impulse response (FIR) low-pass filter, the filter being configured so as to be capable of substantially eliminating undesired signal components appearing in a frequency band occurring above a predetermined normalized cutoff frequency .function..sub.C-NORM ;
• (d) calculating an interpolation (L) and a decimation (M) factor based on the .function..sub.INPUT and the .function..sub.OUTPUT sample rate frequencies and the .function..sub.C-NORM of the filter;
• (e) generating an intermediate signal having an intermediate sample rate frequency .function..sub.INTERMEDIATE by increasing the sample rate .function..sub.INPUT of the input signal by inserting L-1 zero values between each of the existing samples in the input digital signal
• (f) filtering the intermediate signal with the FIR low-pass filter, wherein the filter operates on every Lth data point and skips the intervening L-1 zero data points;
• and
• (g) decreasing the sample rate frequency of the filtered intermediate signal by a factor of M to produce the output signal.

A computer-readable medium having computer-executable instructions for performing steps comprising:

• receiving a digital input signal sampled at a first sample rate frequency .function..sub.INPUT ;
• receiving a desired second sampling frequency .function..sub.OUTPUT for a digital output signal;
• receiving a predefined single-stage, finite impulse response (FIR) low-pass filter, the filter being configured so as to be capable of substantially eliminating from a signal undesired signal components appearing in a frequency band occurring above a predetermined normalized cutoff frequency .function..sub.C-FORM, wherein .function..sub.C-NORM is calculated based on a plurality of critical discrete input and output sample rate frequency conversion pairs, and wherein the normalized cutoff frequency provides a zero pitch error for any conversion of an input signal having an .function..sub.INPUT sample rate to an output signal having an .function..sub.OUTPUT sample rate, the .function..sub.INPUT and the .function..sub.OUTPUT sample rates being selected from one of the plurality of critical discrete input and output sample rate frequency conversion pairs;
• generating an intermediate digital signal by inserting L-1 zero values between each of the existing samples in the input digital signal, wherein L is an interpolation factor based upon the values of the .function..sub.INPUT and the .function..sub.OUTPUT sample rate frequencies and the .function..sub.C-NORM of the filter;
• filtering the intermediate signal with the FIR low-pass filter, wherein the filter operates on every Lth data point and skips the intervening L-1 zero data points;
• and
• extracting from the filtered intermediate signal M-1 out of every M data samples in the filtered intermediate signal, wherein M is a decimation factor based upon the values of the .function..sub.INPUT and the .function..sub.OUTPUT sample rate frequencies and the .function..sub.C-NORM of the filter.