Several analysis parameters are set before analysis begins. An STFT can be viewed as a sequence of DFTs or as a bank of bandpass filters equally spaced from 0 Hz to the Nyquist frequency. So for each window, time and spectrum have been frozen. it can be pictured as isolated spectral lines).
a limited block of sample values) and discrete in the frequency domain (i.e. A DFT analysis is considered to be discrete in time (i.e. While the mathematics of the STFT algorithm are beyond the scope of this article, it is important to note that the STFT procedure applies an FFT or fast Fourier transform (a math 'trick,' but highly efficient form of the discrete Fourier transform ( DFT)) to the windowed samples one window at a time. Mentioned earlier in this text was Fourier's concept that all sound can be broken down into its constituent sine waves (their frequency, their amplitude and their phase relationship). Click here to see diagrams of various windowing functions.Įach window is then subjected to a form of spectral analysis called an STFT, or short-time Fourier transform. Proper windowing, along with adding some 0's to the end of the sample frame (called zero-padding), helps minimize the phenomenon of spectral leakage, whereby the energy at a particular frequency band actually spills into adjoining bands. It is well worth the effort to try each windowing function to observe the effect on your final sound. Common window shapes for phase vocoding, most being bell-shaped, are Hamming, Kaiser, Blackman and von Hann (Hanning).
The shape of these windows will have an effect on the weighting of the resultant analysis, and ultimately the character of the recreated sound. The first part of the process involves dividing a digital sample input signal into numerous time segments or blocks of samples and then multiplying those blocks by an envelope called a windowing function, pictured without the often used overlaps on the left. The initial phase vocoding analysis component entails a process whereby a highly granular time domain sequence of digital samples is converted into a less granular time and also frequency band representation of sound. And (perhaps sadly) the ubiquitous modern-day auto-tune algorithm is also based upon phase vocoding technology.
#Buffer size windows motu digital performer software#
Most recently as of this writing, a C/C++ library and SDK made by Zynaptiq called ZTX is at the top of the precision pitch/speed change game and is incorporated into many popular audio software titles such as MAX and Digital Performer. Miller Puckette, creator of MAX and Pd, might be credited with developing the first real-time phase vocoder. These took advantage of the much higher computational speed available at the time. More practical phase vocoders were developed in the 1970's and 80's, perhaps most notably Mark Dolson's in 1983. Unlike the classic studio technique of changing the speed of an audio tape, where time change is linked to pitch change (the slower the tape, the lower the pitch and vice versa), phase vocoding was a welcome tool for composers when it came along with the advent of digitized audio files and analysis algorithms as early as 1966 (Flanagan and Golden, Bell Labs). Often a combination of the two (alterations of both pitch and time) is used. Other possible uses include compressing a sound in time without changing the original pitch or changing pitch without changing original time. The classic use of a phase vocoder is to stretch the original sound out in time without changing its original pitch. Unlike the similarly-named channel vocoder, due to the spectral analysis phase, pv is a purely digital technique. Phase vocoding (pv) is a form of synthesis by analysis in that a digital sound file or stored buffer is analyzed and is then resynthesized by the phase vocoder.
0 kommentar(er)
