Final
Project: Implementing a Channel Vocoder
Xiao Lu
2618 C Programming
May 5, 2016
Description
The project is an implementation of a classic channel vocoder instrument. A vocoder
is a tool to, in some way, combine two kinds of sounds together by using signal
processing techniques. The most common way of processing vocals with a vocoder is
to use a synthesizer part as the carrier signal and the voice as a modulator; this creates
the sort of filtered sound you hear in the vocals of Herbie Hancocks I Thought It Was
You and more recently, the bridge of Daft Punks Get Lucky.
Specifically, the program reads in two WAV files, a carrier and a modulator, and then
writes the resulting signal into a new WAV file as the output. Although the program is
used in a non-realtime way, it aims to simulate a real-time process. Therefore, the
overall framework is generally designed as a real-time program in which I employed
an input buffer and an output buffer with a size of N (e.g. 512 or 1024) frames. Also
there are parameters defined in the code that can significantly improve the effect,
such as number of bands and hop size (related to overlapping).
Program Flow
I wrote all of the code in files test.c and vocode.c, with corresponded header files. The
code of fft.c was mainly borrowed from convolve.c that was used in the previous
assignment; what I did was re-organizing the code and writing a .h file for it.
In file main.c:
The main() function parses the command line; sets up and open the audio
object WAV file; opens the modulator and carrier WAV files, reads them into
buffers in memory and does necessary error checking; calls the vocoder
function where the core algorithm is performed; after that, it writes the output
buffer into a new WAV file; and free used buffers.
A number of important parameters are defined in the header file including
NUM_BAND which is the number of bands to use, BUFFER_SIZE that is the size
of a buffer and a window, and HOP_SIZE which means how many frames the
buffer will hop.
In file fft.c:
Two datatypes are defined in fft.c, real and complex, which are in essence
floating points data.
The fft and ifft function was originally written by the author of convolve.c,
which require the input buffer, the length, and a scratch buffer in order to
complete a recursive process.
In file vocode.c:
In this file, a set of buffers for window, modulator, carrier and output are
initialized and allocated with memory; windowing and FFTs are performed
on both the modulator and carrier; the magnitudes of the two resulting
arrays from FFTs are calculated by the magnitude function I wrote; then, for
each filterbank, a ratio of x_mod_mag to x_car_mag is obtained as a gain value
that is then used to calculate the output values; after getting the whole
spectrum of the output, an IFFT is performed to re-construct the timedomain signal; finally a normalization process is applied by calculating RMS
values of input and output and then scaling the output.
In addition, in the outer loop, the increment of each count is set to the hop
size; the filterbanks are linearly arranged across half of the spectrum (only
the content below the Nyquist frequency is useful); the window type is
flexible but usually a Hanning or Hamming window is a good choice.
I developed and ran my project on Mac OS X, but it will also compile and run on
Windows/Cygwin.
My Code and Libraries
I wrote the code described above, and used the following libraries:
Sndfile
I obtained the libraries by using brew
brew install sndfile
Computational Complexity
The critical part contributing to the complexity of the program is in the vocode
function, where a number of buffers are accessed, a series of mathematics are
involved, and there are also FFTs being called.
For FFTs, the Big-O complexity is Log-Linear Time (N*log2(N)). For most of the
computations in the code, the Big-O complexity is Linear Time.
The non-realtime program works fairly fast and stable, so it is highly possible to work
in real-time (which is also my future work) as well.
Comilation and Linking
The makefile shows how to compile and link my program for the OS X platform.
Command Line
The generic command line to run the program is:
./vocoder modulator.wav carrier.wav [output.wav]
A specific command line is
./vocoder mod_testing.wav car_synth.wav output.wav
