Pitch-Preserved digital audio playback synchronized to asynchronous clock

A method for synchronizing an audio stream to an asynchronous clock, said method comprising the steps of:

• extracting a current analysis time from the variable rate asynchronous clock;
• accessing a current input block of the audio output stream corresponding to the current analysis time;
• setting a phase vocoder input hop size equal to the difference between a current analysis time and an immediately previous analysis time divided by the sampling rate;
• utilizing a phase vocoder to synthesize a current output block of said audio output stream, with the analysis time of the phase vocoder set to the current analysis time;
• overlapping the current output block with a previous output block separated by a fixed output hop size.

A system for synchronizing an audio stream to an asynchronous clock, said system comprising:

• clock extraction circuit which receives an asynchronous clock signal and generates a current analysis time specifying a portion of the audio stream synchronized to the asynchronous clock,
• an audio store, coupled to said clock extraction circuit, for storing an audio signals in digital format and for providing a current portion of the audio signal specified by the current analysis time,
• a processor, coupled to said audio store to receive said current portion, with said processor for:
• performing an FFT on the current block of the audio input stream to generate a set of frequency bins;
• performing an inverse FFT on said frequency bins to generate a current output block of the audio output stream;
• setting an input phase vocoder input hop size equal the difference between the current analysis time and an immediately previous analysis time divided by the sampling rate;
• adjusting the phase of current output block relative to a previous output block based on input hop size;
• overlapping the current output block with a previous output block separated by a fixed output hop size;
• and
• an audio output unit that contains a Digital to Analog Converter (DAC) and a DAC sample clock for providing a constant DAC clock rate, with the audio output unit coupled to said processor to receive said current output block and rendering the current output block at the DAC clock rate.

A computer program product comprising:

• a computer readable storage structure embodying computer readable program code for causing a computer to implement synchronizing an audio stream to an asynchronous clock when executed by a computer, with said program code comprising:
• program code for causing the computer to extract a current analysis time from the variable rate asynchronous clock;
• program code for causing the computer to access a current input block of the audio output stream corresponding to the current analysis time;
• program code for causing the computer to set an input phase vocoder input hop size equal the difference between the current analysis time and an immediately previous analysis time;
• program code for causing the computer to perform an FFT on the current block of the audio input stream to generate a set of frequency bins;
• program code for causing the computer to perform an inverse FFT on said frequency bins to generate a current output block of the audio output stream;
• program code for causing the computer to overlap the current output block with a previous output block separated by a fixed output hop size.