Friday, July 14, 2017

MIDI Interface - Test #1

I am conscious that this project is taking a long time. As you can see I'm going through another rush of activity whilst the weather is warm and I can sit comfortably in my drafty garage but how long it will last, I don't know. In the last post I said I would pack the boards carefully into the rack enclosure before making a start on the MIDI interface at long last. Well, last night I thought again. Now I have my new bench power-supply I decided I would just check if the j-Omega MTP8 I bought back in (checks blog) 2009 was powering up okay.

I'm pleased to report that when I set-up a 15V supply the MTP8's power LED came on. At this point I couldn' t resist trying a simple test with an LED. And it worked.

The MIDI keyboard I'm using is my trusty old Yamaha (of course) PSS-580. A lovely example of the Portasound breed with a programmable 2-operator FM synth and useful MIDI spec. It says it's a Workstation and it really is. The fact that it's nearer in age to the SS-30 than to now is slightly amazing to me though. It is possible to record sequences on this keyboard too so for testing it will be possible to set up a simple note on, note off for each key which I can play at will. I could do this from my phone or iPad with a MIDI interface too. I might re-think and do that but for now I like having a keyboard and I can easily use it for a tuning reference.

Here's a fun demo I found on YouTube of some of the more extreme possibilities of the synth section.

Handily, the keyboard is exactly the same size as the SS-30 - 49 keys from C1->C5 - so it's ideal for this project. And although I still have a soft spot for this keyboard  - it was my first ever synth - it's not in use in my studio these days. I have a Volca FM performing FM duties, so it's not really required.
Something else I will need to do is check the range of the keys used by the MTP8. I connected output number 32 (of 64) and that mapped to G3. If output 1 was C1. C2 would be on 13 and C3 on 25, C4 on 37 and C5 on 49. G3 would then be on 32 so it is mapped exactly as I would have wanted it to. Great!

With the MTP8 working and the SS-30 ready and working I still had time to take the obvious next step -  try to control the SS-30 from MIDI! At long last.

First though I had to find something to handle the switching. As you may remember from this post, on how to switch the -7V keys to ground, there are a few options but the obvious choice is some sort of opto-coupler/opto-isolator. I thought I must have one somewhere and remembered that I have a CNY17 chip. Originally this was used for a failed attempt at a DIY MIDI CV converter. And then it was stolen for a Gameboy MIDI interface. For MIDI it's always recommended to use an opto-isolator and on the MTP8 there's a similar chip for the same purpose. The CNY17 is a passive component of the type I expect to use so I de-soldered it from the GB interface and with a bit of wiring and a couple of resistors on the bread board I could switch the LED on/off through the opto-coupler.

It was the moment of truth. I knew it should work, but could I play one note of the SS-30 via MIDI? Yes, of course  :-)

So there you have it. All I need to do now is make another 48 similar circuits and it's job done!

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