Module #004: MIDI to CV prototype

We now return to our program, already in progress...

I've got a VCO, VCA, and ADSR Enveloper. But I don't have a CV/Gate keyboard, or any other suitable inputs. It seems like there ought to be some open source MIDI solutions.

I don't know which microcontroller is best for this, so I started with the ATTiny84, per something Jan Ostman said in his article. This turned out to be a fools errand, as the ATTiny84 has a hard time decoding the MIDI byte stream. So I've gone over to the ATTiny85, which is what Jan originally used in his project, and what Kevin has used in his updated code.

I had this working on breadboard, but found that the performance was linked to noise on the MIDI serial input. As the next step, I've drawn up a design for Stripboard. It's pin compatible with Kevin's code, and includes a header for easy re-programming. I've included a pad for PB0, even though Kevin's code doesn't use this, and I've also included a SPDT switch to toggle between summing the Pitchbend CV with the Note CV, or to send it to a separate jack. The Pitchbend code isn't implemented in Kevin's sketch, but looking at Jan's, you should be able to get the idea.

I hope to approach these fine folks soon about potentially open sourcing the code and putting it on GitHub.

It's currently working well enough that I can send single note events from my Alesis Q88 with reasonable stability. Some events are dropped. This is more pronounced as more data is written to the incoming MIDI stream.

Update 11/17: It's working very well on stripboard, compared to the breadboard setup i had before. Not dropping many events at all, and the few that were dropped, i think would be improved by further shielding. I've uploaded a new version that includes a couple of jumpers that were missing from the previous layout. They are light blue in this drawing.

My build is not tracking perfectly, as indicated by a clip-on guitar tuner on the monitor output. However, i am not sure if this is the PWM CV out, or the unmatched transistors in the exponential converter section of the VCO. The journey continues.