I have already tried several basic algorithms for time stretching. You can basically split all the schemes into 2 different methods : the time domainHallo, Adrian Gschwend hat gesagt: // Adrian Gschwend wrote:
But as far as I know timestreching algorithms are 1. not easy to
implement and 2. not open source if they sound good :)
I'm quite sure, that Live uses a granular approach. If you timestretch far away from the original, it gets obvious. There are several open source granular synths. For incorporating into another software, I would recommend SndObj which I use in the syncgrain~ external for Pd.
SndObj lives at: http://www.may.ie/academic/music/musictec/SndObj/main.html
ciao
and the spectral domain ( using a phase vocoder ).
In time domain, there are several algorithms :
- the OLA method, which is straight forward, and is just to move blocks of memory into buffer. Sounds really bad.
- SOLA method : use correlation to connect different frames ( grains is not accurate, I think, as grains are normally blocks of a few ms, and the
frames used in time domain technique have typically 2048 [EMAIL PROTECTED] )
- WSOLA method : quite the same, but more efficient.
A good reference is "Time scale Modification algorithm based on the subband Time domain techniques for broad band signal Applications", from Roland K.C. Tan and Amerson H.J.Lin , int J Audio Eng Sic, vol48, number 5 5000 May.
Phase vocoder :
- the basic approach ( see http://www.musicdsp.org : I posted a matlab script. As the script only uses FFT, it must be easy to port it to scilab or octave of you don't have access to matlab ).
- all the different approaches are derived from the basic one. The main problem is that a phase vocoder has to make a compromise between frequency resolution and time resolution. The idea is basically to adapat the frame size to the "harmonic vs transient" content ( a bit like in MP3, for exemple, where the frame size of MDCT is adapted ).
The fundation article is this one : "time-scale modifications of speech based on short time Fourier analysis" IEEE Trans Acoust, Speech, Signal Processing vol 29(3), pp 374-390, 1981. From R. Portnoff
More recent techniques ( with improvements in efficiency and quality ) :
J Laroche and Mark Dolson "New Phase Vocoder Techniques for Real Time Pitch Shifting, chorusing..." in JAES, vol47, No11, 1999. It describes the phase locking scheme.
I may be give you even more recent articles, as one of my collegue did his thesis about it. I just have to find out if it has already been published.
P.S : the time stretching in the new Windows Media Player is really good. I don't know which technique it uses.
