Sample rate converter with automatic anti-aliasing filter
AssigneeMicrosoft Corporation
InventorThomas Craig Savell
Life of this patent
Estimating the term…
Abstract
The subject disclosure is directed towards dynamically computing anti-aliasing filter coefficients for sample rate conversion in digital audio. In one aspect, for each input-to-output sampling rate ratio (pitch) obtained, anti-aliasing filter coefficients are interpolated based upon the pitch (e.g., using the fractional part of the ratio) from two filters (coefficient sets) selected based upon the pitch (e.g., using the integer part of the ratio). The interpolation provides for fine-grained cutoff frequencies, and by re-computation for each pitch, smooth anti-aliasing with dynamically changing ratios.
Claims20
A method comprising, obtaining a pitch corresponding to an input audio sampling rate of an input sample and an output audio rate for a sample rate converted sample, selecting one filter comprising one set of coefficient values based upon the pitch, and another filter comprising another set of coefficient values based upon the pitch, obtaining a resultant set of pitch-based coefficient values, including by interpolating between the one set of coefficient values corresponding to the one filter and the other set of coefficient values corresponding to the other filter, and filtering data corresponding to the input sample to provide for anti-aliasing of the sampling rate converted sample for output using the resultant set of pitch-based coefficient values.
In a computing environment, a system comprising, a sample rate conversion component configured to convert an input sample into audio output based upon a specified pitch value, the sample rate conversion component coupled to a coefficient lookup table set and further configured to obtain a pitch value and automatically read a plurality of sets of filtering coefficients from the coefficient lookup table set based upon at least part of the pitch value, and automatically interpolate the sets of filtering coefficients based upon at least part of the pitch value into anti-aliasing filtering coefficients for use by a digital audio filter.
One or more computer-readable media having computer-executable instructions, which when executed perform steps, comprising,
- (a) accessing audio data corresponding to an input sample;
- (b) obtaining as a current pitch value a value corresponding to a ratio of an input sampling rate of the input sample to an output sampling rate for outputting a converted output of the input sample;
- (c) interpolating between filter coefficient sets based upon the current pitch value to obtain a filtering coefficient set;
- (d) performing anti-aliasing filtering using the filtering coefficient set;
- (e) outputting audio data corresponding to the converted output after anti-aliasing filtering;
- (f) obtaining as the current pitch value a changed pitch value corresponding to a changed ratio of the input sampling rate of the input sample to the output sampling rate for outputting a converted output of the input sample, and
- (g) returning to step (c) for a plurality of iterations, each iteration having the changed pitch value obtained at step (f) as the current pitch value.
Dependencies are inferred from the claim text.