Multi-rate digital filter for audio sample-rate conversion

A method for providing a sample-rate conversion (“SRC”) filter on an input stream of sampled data provided at a first rate, to produce an output stream of data at a final rate that is R/S times said first rate, comprising the steps of:

• operating on said input stream of sampled data with a Σ-Δ modulator filter routine to produce a first stream of intermediate data at an oversampled rate of between 64 and 384 times said first rate;
• operating on said first stream of intermediate data with a SINC filter routine having a fourth order to produce a second stream of intermediate data downsampled to a rate of sixteen times said first rate;
• operating on said second stream of intermediate data with a simplified interpolation filter routine, having a substantially small number of operations to calculate the coefficients thereof, by applying a cascaded FIR digital interpolation filter having r filters in cascade relation, where r is an integer between 1 and 5, having coefficients that are all ones, having an order of r(S−1), and that provides a third stream of intermediate data at a rate 16 times said final rate;
• operating on said third stream of intermediate data with a decimating filter routine to produce said output stream of data.

A sample-rate conversion (“SRC”) filter receiving an SRC input and outputting an SRC output, comprising:

• a first, low-order interpolation filter receiving said SRC input;
• a second, low-order interpolation filter receiving the output of said first interpolation filter, said first interpolation filter and said second interpolation filter being cascaded;
• a third, interpolation filter receiving the output of said second interpolation filter, and having a substantially higher order than said first interpolation filter and said second interpolation filter, and having a substantially small number of operations to calculate the coefficients thereof,
• a first decimating filter receiving the output of said third interpolation filter;
• wherein an optimized indexing procedure that automatically determines a subset of operations required to determine said SRC filter output is used to reduce computations involved in implementing said filters.