Auto-tuning for Electric Guitars using Digital Signal Processing Pat Hurney, 4ECE 22 nd January 2009

Contents System Overview Project Background Progress to Date Remaining Work

System Overview Signal from Piezo Saddles sent to 6-channel interface Altered Signal sent to Guitarists Rig One signal sent in to VST for DSP

Project Background Tuning problems are a major problem for professional and amateur guitarists. They can be very difficult to correct in live situations and a slightly ‘out-of-tune’ signal can cause an entire performance to sound very ‘out-of-tune’. This system aims to help eliminate these problems with the help of Virtual Studio Technology (VST) while also providing extra benefits such as open- tunings to guitarists.

Progress to Date Pitch Extraction Pitch Manipulation VST Development

Pitch Extraction Fast Fourier Transform used to acquire Magnitude Spectrum of signal. The peaks in the spectrum are then used to approximate the pitch of the signal. Filtering used to determine appropriate resolutions for FFT based on where fundamental lies. Low frequency fundamental requires higher resolution while higher frequencies can use much lower resolutions. Additional filtering also used to filter out any frequencies above 8kHz. The Piezo saddles output a large amount of harmonics (~7-8 harmonics). The high frequencies that are being filtered out cause very little audible difference to the average listener.

Pitch Manipulation A Phase Vocoder has been implemented to manipulate the pitch. A MATLAB implementation of this was found in the book ‘DAFX – Udo Zolzer’. This is being modified for the projects needs. This works by first time-stretching the signal to an appropriate length. The signal is then re-sampled to the appropriate pitch. Large pitch shifts with the use of this have resulted in undesired sounds so far. Pitch shifts of a few semi-tones have been acceptable in tests so far.

VST Development A VST is used to implement the effect in real time. The VST will consist of two modes for ease of use for guitarists. The first mode asks the user to strum the guitar to obtain the frequencies and pitch offset of the open-strings. The second mode will allow the user to play normally while the system applies pitch shifts to each note played. This will be run from a Digital Audio Workstation (DAW). Numerous DAWs are available, the most likely candidate for the DAW at this time is REAPER. This is a free recording application that is compatible with VSTs and allows multi-channel recording. ASIO4ALL will be used for the ASIO driver. This was chosen as it is a very low-latency driver that is also compatible with REAPER.

Remaining Work More research has to be put in to the phase vocoder that has been implemented. It also has to be modified to be able to pitch shift in increments other then semi-tones. The pitch algorithms are currently all written in MATLAB, these need to be re-written in C++ for compatibility with the VST SDK. The VST also has to be written in C++. The core classes will be implemented by building upon existing classes included with the VST SDK.

Timeline Development of core VST functionality - 30 th Jan Additional research on effects of large pitch shifts on piezo pickup signals with a phase vocoder – 6 th Feb Combination of pitch extraction and manipulation algorithms – 20 th Feb Embedding of effects in the VST – 27 th Feb Add additional functionality to VST for open tunings and calculating pitch drift when VST is operating in second mode – 6 th -10 th March

Questions?