System and method for generating fractional length delay lines in a digital signal processing system

A method of delaying digitally sampled data by a smoothly varying non-integer number of delay periods, comprising the steps of:

• storing digitized data in a sampled data delay line having a multiplicity of integer positions at which data is stored;
• filtering data from the delay line with first and second allpass filters, each having an associated read pointer for reading data at a corresponding integer position of the delay line, each allpass filter generating an associated output in accordance with a filter coefficient assigned thereto and the data read from the delay line;
• assigning to each allpass filter's read pointer a respective delay line position and assigning to each allpass filter a respective filter coefficient value, wherein the delay line position and filter coefficient assigned to each allpass filter together corresponding to a respective effective delay line position for each allpass filter;
• and
• smoothly crossfading between the outputs of the first and second allpass filters so as to generate an output signal that corresponds to a delay line read position that smoothly transitions from a first effective delay line position to a second effective delay line position.

A signal processing system, comprising:

• a data processor;
• memory coupled to the data processor for storing data structures and instructions to be executed by the data processor;
• the data structures stored in the memory including a sampled data delay line having a multiplicity of integer positions at which data is stored;
• the instructions stored in the memory including:
• filtering instructions implementing two allpass filters, each having an associated read pointer for reading data at a corresponding integer position of the delay line, each allpass filter generating an associated output in accordance with a filter coefficient assigned thereto and the data read from the delay line;
• control instructions for assigning to each allpass filter's read pointer a respective delay line position and assigning to each allpass filter a respective filter coefficient value, wherein the delay line position and filter coefficient assigned to each allpass filter together corresponding to a respective effective delay line position for each allpass filter;
• and
• crossfade instructions that generate an output signal that smoothly crossfades between the outputs of the first and second allpass filters, the output signal corresponding to a delay line read position that smoothly transitions from a first effective delay line position to a second effective delay line position.