One way of stretching the length of a signal without affecting the pitch is to build a phase vocoder after Flanagan, Golden, and Portnoff.
- compute the instantaneous frequency/amplitude relationship of the signal using the STFT, which is the discrete Fourier transform of a short, overlapping and smoothly windowed block of samples;
- apply some processing to the Fourier transform magnitudes and phases (like resampling the FFT blocks); and
- perform an inverse STFT by taking the inverse Fourier transform on each chunk and adding the resulting waveform chunks.
The phase vocoder handles sinusoid components well, but early implementations introduced considerable smearing on transient ("beat") waveforms at all non-integer compression/expansion rates, which renders the results phasey and diffuse. Recent improvements allow better quality results at all compression/expansion ratios but a residual smearing effect still remains.
The phase vocoder technique can also be used to perform pitch shifting, chorusing, timbre manipulation, harmonizing, and other unusual modifications, all of which can be changed as a function of time.
Read more about this topic: Audio Timescale-pitch Modification
Other articles related to "phase vocoder":
... British composer Trevor Wishart used phase vocoder analyses and transformations of a human voice as the basis for his composition VOX 5 (part of his larger VOX Cycle) ... American composer Roger Reynolds uses the phase vocoder to perform time-stretching of flute sounds ... commercial music production, is based on the phase vocoder principle ...
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