Post on 21-Dec-2015
Effects in frequency domain
Stefania SerafinMusic InformaticsFall 2004
Overview
Time versus frequency domain effects The phase vocoder The STFT Time stretching Pitch shifting Cross synthesis
Overview
Until now we examined effects based on delays and working in time domain.
What happens if we want to work in the frequency domain?
In this case we talk about SPECTRAL MODELS and SPECTRAL TRANSFORMATIONS
The phase vocoder
A phase vocoder represents an ensemble of techniques which take a sound in the time domain,calculate the Fourier transform, perform some manipulations in frequency domain and then reconstruct the sound in time domain.
Analysis/synthesis steps
ANALYSIS
TRANSFORMATIONS
SYNTHESIS
INPUT
OUTPUT
Analysis synthesis steps
Analysis: STFT
STFT
Time-domain audio signal is separated into successive short frames.
Each frame is multiplied by a window function to smooth transitions.
Resulting windows are passed on to FFT module. The FFT process then extracts spectral information on the signal.
Spectral transformations
Pitch shifting Time stretching Morphing (Cross-synthesis)
Pitch shifting
How would you perform a pitch shift in time domain?
What are the drawbacks of doing it?
Solution: move to frequency domain In frequency domain 1) Calculate the spectrum 2) Shift it upwards or downwards 3) Go back to time domain
Time stretching
Time-scaling is an application that allows signal length to be stretched or shortened without affecting the frequencies of its components.
1) Calculate spectrum 2)Reduce/increase time information 3) Go back to time domain
Morphing
It is the combination of two signals
In frequency domain:
1) Calculate spectrum of signal A
2) Calculate spectrum of signal B
3) C = A B
STFT resynthesis
STFT resynthesis is the final stage of the phase vocoder process. It is based on recreating a signal in time domain from the modified spectral components of the input signal.
The third stage implements an exact inverse of the process that was used in STFT analysis of the signal. Modified spectral components of the input signal are passed through the Inverse Fourier Transform to recreate a set of time-domain frames.
Application
Farinelli
Application (ctn.)
Karaoke
In Max/MSP
Check the examples Folder. Convolution and Phase vocoder example.