AutoTune is a rudimentary pitch corrector. I wanted to create a project that would involve frequency-domain processing in MATLAB, and this project taught be a lot about the FFT and using a phase vocoder.
Auto-tuning is an audio processing effect that can allow sounds to be perfectly tuned despite being oringally off-pitch. The auto-tuner determines the desired frequency in equal temperament, and shifts the frequency by a calculated ratio.
Ensure that pitchShift.m can be called from auotune.m.
AutoTune takes 3 inputs: The input sound file, its sampling frequency, and a window length.
Example usage:
[X, fs] = audioread('input.wav');
Y = autotune(X, fs, 1024);
soundsc(Y, fs);
Any input is compressed to mono using mean. Any stereo output will be average between the two channels, and output that is already mono will be unaffected.
nextpow2 rounds the window length to the nearest power of 2 for simplicity of the FFT. A Hamming window is generated with the size of the new window length.
For each window, the frequency is calculated using max. Note that even if the frequency selected with this method is a harmonic and not the fundamental frequency, both are out of tune by a constant ratio.
The frequency in equal temperament (A=440Hz) is found with the following function:
nearest = 2^(round(12*log2(freq/440))/12) * 400
I tried several different implementations for the pitch shifting, but eventually had the best results with one I found here.
The pitch shifter splits the window its given into frames with 75% overlap, performs an FFT, adjusts the phase, and performs an IFFT. It then does an overlap-add, and resamples with linear interpolation.
This was a fun project that taught be a lot about the FFT, and frequency domain processing in general. The phase vocoder proved difficult to implement: I spent a lot of time trying my own implementation from scratch, but could not come up with one that I thought satisfactory. However, implementing the one linked above, as well as learning the math behind it, proved a rewarding experience, and I am now much more confident in concepts related to frequency domain processing.
Some sound outputs can be quite distorted. This is a result of phase non-linearities when combining windows after they have been individually pitch-shifted. Solving these phase non-linearities would make this project fully functional, but unfortunately I did not have time over the course of this project to implement a meaningful solution.
Overall I'm satisfied with my work; it's enjoyable to give funny screams or non-musical sounds as input and hear the auto-tuned result. Given the scope and amount of time dedicated, I think I created a reasonable prototype that I learned a lot from.