View Full Version : GK3 MIDI Pickup out to Propeller out to MIDI

11-05-2010, 02:49 PM

I have been looking for a pitch to midi unit and they are all really expensive. I figured if I bought the GK3 MIDI Pickup and use the propeller chip to decode the audio signal coming from the polyphonic pickup to send out a midi signal it would be a lot cheaper that spending $800. I don't know where to begin. The concept is pretty easy to grasp but executing seem difficult. If someone could give me some direction or help I would really appreciate it.

Thank you

11-08-2010, 10:20 PM
I'm actually really interested in this as well, even if it is way over my head. At the least I would like to know how to get the fundamental frequency so that a propeller guitar tuner could be made, or perhaps to record tabs on the fly if it were quick enough.

Bob Lawrence (VE1RLL)
11-09-2010, 01:45 AM

The prop can do it no problem. The pickup you have, has 6 hex coils in it. think about each coil as being on a channel. Design one channel and get it working, then copy that circuit for the other 5 channels for a total of 6(one per string). I would use a cog per string + 1 for the serial object. A cog per string is not necessary but I believe it make coding easier and allows for more flexibility per channel should you need it.

A very basic Example
Channel 1:

From the MIDI pickup you'll need take the output from a coil and put it through a low noise preamp. The input level (threshold) should be adjustable via a variable resistor (pot). From the pre-amp you need to go through a A to D (not delta sigma as per the prog Edu book, use a chip based one such as the Microchip line) converter to convert the audio to digital and feed it to a propeller input pin. Once it's inside you have to write software to figure out the pitch of the note. Once you have the pitch you can select a midi note from a lookup table in the software. You can then use the serial object (setup for MIDI) to transmit MIDI note on-off, note number etc and send it to an external sound module (there is one available that's prop-based)

I may add more to this later because I just thinking out-loud at this point. :)

You could do it all in spin as a proof of concept and later you could update a few modules to asm if required, for speed.

For the code you need:( list is not any particular order) ADC conversion finished? , Pitch detection, is detected value relevant?, threshold value, Peak values, increase velocity Averages, Freq to MINI note lookup table , note on ,note off, midi transmit, etc.

then hardware you wold have a few LED's for Power, signal input LED for each channel, TX MIDI, etc.........

11-16-2010, 02:09 PM
I bought a GVOX guitar. It has the hex pick up and an amplifier for each pick-up on the bridge. The amplifiers volume can be adjusted. How do I get started in programming the propeller chip to recognize pitch detection and converting it to MIDI. For example: if pitch A is 440hz the MIDI out would have a channel number, note number and a velocity signal. This could be sent to a rack mount sound module or a virtual instrument in the computer.

Thank you