Re: [linux-audio-dev] ableton live

Frank Barknecht wrote: I have already tried several basic algorithms for time stretching. You can basically split all the schemes into 2 different methods : the time domain 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 samples(a)44.1khz ) - 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.