Time Frequency Processing
Direct FFT/IFFT Approach
Andrew Nelder
0422391
Colin MacIntosh
0423916
University of Victoria
ELEC 484 – Audio Signal Processing
Abstract
Vocoding technology has been around since
the 1930s, where it was used to send
securely encoded messages between two
points. It can also be used to apply effects
to the transmitted sounds, such as
robotization and whisperization. These are
not the only uses for vocoders, but they will
be investigated in this project as well.
Uses for phase vocoding are constantly
changing as more uses for it appear in
today’s world. For these new uses,
particularly digital telecommunications,
efficiency is a necessity, and as it stands, the
standard Linear Predictive Coding (LPC)
vocoders are no longer efficient enough.
This project will develop an LPC, experiment
with the sound effects and will describe the
procedures necessary to improve on
efficiency and quality while maintaining
intelligibility.
project is to produce a phase vocoder using
the Fast Fourier Transform and Inverse Fast
Fourier Transform Method described in the
DAFX: Digital Audio Effects Textbook. Upon
obtaining a basic working model, focused
research can be produced to yield a variety
of audio effects on theoretical constraints.
1 INTRODUCTION
Due to the large increase in the number of
commerical communications technologies,
research into phase vocoder technology is
becoming more valuable. Understanding
the fundamentals of producing a phase
vocoder is necessary to produce a system
constrained by limited processing and
computing power. The objective of this
Figure 1: Block Diagram of Phase Vocoder Using STFT
Figure 1 displays the steps necessary to
implementa a phase vocoder. These steps
describe the theory behind the Direct Fast
Fourier Transform and Inverse Fourier
Transform method of phase vocoding.
Effectively, small packets of data (bins) are
broken down in windows. The Fast Fourier
Transform is applied to the signal in each of
these windows. Following the Fast Fourier
Transform signal manipulation occurs in the
frequency-domain to produce a variety of
audio effects. After these manipaulations
are applied, the altered signal is then
remodulated to the time-domain using the
Inverse Fast Fourier Transform. These
altered packets of data are reconstructed to
form a new modulated signal.
2 BACKGROUND
Improved Phase Vocoder Time-Scale
Modification of Audio, by Jean Laroche and
Mark Dolson, addresses the problem of
“phasiness” present in time-scaled outputs
of phase vocoders. Despite having many
commercial applications, the phasiness
created by vocoders has posed a major
barrier to vocoders being used more
frequently. Another artifact that vocoding
leaves behind is “transient smearing.” This
undesired effect causes the modified sound
to have less “bite”, and is usually prevalent
with sounds such as a piano attack. The
paper focuses on two things: understanding
the problem; what it is that is causing such
artifacts, and also presents two new
solutions to this problem, as well as
reviewing two previously proposed ones.
In New Phase-Vocoder Techniques for PitchShifting, Harmonizing and Other Exotic
Effects, Laroche and Dolson present and
examine two new “phase-vocoder-based
techniques” that, when applied to a signal,
allow the user to apply more effects than
are usually used with vocoders. Pitchshifting, chorusing, harmonizing and partial
stretching are amongst the new effects
examined in this paper. Laroche and Dolson
present the standard techniques used and
comment on some of the more common
drawbacks to using these methods. New
methods to achieve the effects mentioned
previously are offered.
Phase-Vocoder – About This Phasiness
Business, by Jean Laroche and Mark Dolson
describes methodologies that may be
utilized to reduce the “phasiness” of signals
passing through a vocoder. Unfortunately
due to the non-linear phase relationships
inherent to most windowing methods, it
can be extremely difficult to reduce these
effects.
Their methodologies describe
completely discarding any physical phase
calculations and present a rather surprising
alternative:
using
fine-tuned
approximations to calculate a series of
theoretical phases. These methods are
unfortunately very computer intensive and
can sometimes yield a decrease in
performance; however, it may prove fruitful
so the paper itself has not been discarded.
Chris Duxbury, Mike Davies, and Mark
Sandler present “method using temporal
information of musical audio” to improve
the time-scaling effects whilst preserving
pitch in their paper Improved Time-Scaling
of Musical Audio Using Phase Locking at
Transients. Duxbury, Davies and Sandler
Under ideal situations, this project will fully
incorporate a completely self-contained
vocoder test application. The finalized
objective will be to produce a piece of
MatLab software that can perform the
vocoding algorithm based on the Fast
Fourier Transform and Inverse Fourier
Transform Method.
4 TIMELINE
21/06
01/07
Write a project proposal detailing the steps
required to complete the project.
Task 3: Initial MatLab Design
3 IDEAL RESULTS
11/06
Task 2: Proposal
11/07
21/07
Task 1
Task 2
Task 3
Task 4
Task 5
The project was split into several smaller
tasks that allow for manageability. The
figure above details the timeline for the
expected (red), worst case (green), and best
case (violet) scenarios.
Task 1: Initial Research
Conduct a thorough research of journal
articles
for
information
on
the
implementation and uses of phase
vocoders.
Begin drafting a MatLab implementation of
the theoretical phase vocoders and
experimenting with effects.
Task 4: MatLab Debugging
Debug the initial MatLab designs and
remove any fragments and errors in output.
Task 5: Finalize Implementation
Finalize the project implementation by
producing seamless effects and optimized
code.
5 DATA COLLECTION
We will be using MatLab to implement the
phase vocoder. Sources include articles
from audio engineering publications such as
IEEE Transactions on Acoustics, Speech, and
Signal Processing, and Audio Engineering
Society monthly journals.
6 BIBLIOGRAPHY
[1] Laroche and M. Dolson. Improved phase
vocoder time-scale modification of audio.
IEEE Trans. on Speech and Audio Processing
7(3): 323-332, 1999.
[2] J. Laroch and M. Dolson New phasevocoder techniques for real-time pitch
shifitng, choruing, harmonizing and other
exotic audio modifications. Journal of the
Audio Engineering Society, 47(11):928-936,
1999.
[3] M. R. Portnoff. Implementation of the
digital phase vocoder using the fast fourier
transform. IEEE Transactions on Acoustics,
Speech, and Signal Processing, 24(3):243248, June 1976
[4] Z. Settel and C. Lippe. Real-time musical
applications
using
the
FFT-based
resynthesis. In Proc. Int. Computer Music
Conference (ICMC), 1994
[5] Laroche, J; Dolson, M: Phase Vocoder:
About this Phasiness Business. IEEE Trans.
on Speech and Audio Processing 19-22 Oct,
1997.
[6] De Götzen, Amalia ; Bernardi, Nicola &
Arfib, Daniel: Traditional Implementations
of a Phase Vocoder: The Tricks of the Trade
Proceedings of the COST G-6 Conference on
Digital Audio Effects December 7-9,2000