MIDI input circuit to trigger 'analog synth voices'

[renegade-system-generator]

Sincere apologies for any ignorance I'm about to display, I'm more an artist than an engineer. If I am building a synthesizer which has multiple relaxation transistor oscillators all going into one output, running off of 9v, how I would I look into combining a MIDI input so you could control them like synth voices with a musical keyboard. I have looked into building the MIID input circuit with an optocoupler and I think I understand that I need some kind of microcontroller (arduino/teensy?) to convert the digital signals into something that triggers the oscillators? Does this make any sense? Would the MIDI input circuit work off the 9V? Would I have to decrease the voltage going to the circuit?
I've looked on youtube and online quite extensively but I need to converse with someone that can help.

Thanks

 

[dr pepper]

An arduino is the right thing to do that, midi usually goes through an opto isolator so the voltages are not critical, 9v should be fine.
Google arduino midi keyboard interface or something on those lines and see what you get, there are lots of midi projects for the arduino.

 

[renegade-system-generator]

OK thank you for clarification on the power. so I have looked at alot of arduino material but as I'm looking to sell these projects I'd prefer to understand the fundamental of their platform and build it myself and get it working that way. If their were 6 oscillators and I wanted 6 voice polyphony, Midi din input, not usb midi and 9v Vcc using some kind of arduino based dac would this be plausible?
Thanks again

 

[rjenkinsgb]

You need something with a serial port that can be set to run at 31250 baud (bits per second).
I've used various PIC MCUs for MIDI gadgets, but you can use an arduino or whatever you prefer.

This shows the electrical parts for MIDI output and input:

https://cdn.sparkfun.com/assets/learn_tutorials/4/0/8/signaling-one.png

And this is a list of MIDI "messages" - the most relevant ones would be note on and note off.

Summary of MIDI 1.0 Messages – MIDI.org

www.midi.org

The first "command" byte always has the high bit set, the data that follows has the high bit clear, so you can resynchronise at any point in the data of something gets corrupted or out of step.

A table of note numbers to interpret the message data you recieve:

https://newt.phys.unsw.edu.au/jw/graphics/notes.GIF

 

[renegade-system-generator]

OK cool thx for that, done some reading and understand a little better about the signals that MIDI outputs -> then to a PIC then to a -> DAC then some kind of voltages come out which will 'affect the oscillators'. Unsure at that point at the moment as the oscillators have an extremely basic CV input using a vactrol. Would this it make it easier or harder for results, Im looking for experimentation/interesting results not flawless operation.
Understanding what these oscillators need to be told in order for them to sing when a keyboard is played challenging me

 

[wkrug]

When Your filters and oscillators are working with Control Voltage ( CV ), You can try to use a MiDi to CV Converter.

e.g. Look here

 

[rjenkinsgb]

the oscillators have an extremely basic CV input using a vactrol.

Some PICs have multiple PWM (pulse width modulation) channels. Using those at high frequencies with some simple R-C filtering you can easily create variable voltages, possibly directly or otherwise with just with an opamp for buffering.

Some also have built-in DACs

Do the oscillators have a gate signal as well? Another output pin could be used for that.

A PIC16F1779 looks to be a possible part to base it on, it's available in a 40 pin dual-in-line package so easy to use in stripboard or protoboard setup, plus it has eight, 10 bit DACs [I did not know they did one with that many!], plus ADCs, PWMs and plenty of general I/O pins for gating, indication or general use (and UART for the serial MIDI in, plus all the common peripherals).

They look to be around 3.00 each.

Datasheet here: https://ww1.microchip.com/downloads/en/DeviceDoc/40001819B.pdf

You would need a PICKIT programmer / debugger to work with that.

 

[Nigel Goodwin]

A quick google finds this:

Soundtronics Projects :: MIDI Ultimate Synth

The MIDI Ultimate project is a fully-featured analogue synth with a performance comparable to many commercially available instruments The MIDI Ultimate is an evolution of the musicfromouterspace com MFOS Sound Lab Ultimate design by the late Ray Wilson who is credited for much of the design

www.soundtronics.co.uk

Might be worth buying the EPE back issues covering it, and 'borrowing' some techniques from that.

 

[dr pepper]

It depends what kind of control you want, if you just need note on note off, then you could arrange a simple inhibit for each oscillator, using a digital output pimn from the processor and maybe a transistor.
However if you want to control volume and maybe accent or whatever then yes you'll need analog outputs from the processor, and that might well mean also redesigning your oscillators.
The Epe synth looks like its pretty comprehensive.

 

[renegade-system-generator]

I appreciate all of this thanks alot people. Although the EPE synth is crazy out of my league, the visuals on how it 'works' proved useful f so thanks for that Nigel.

After further research, I do realize I'm looking at a MIDI to CV type scenario like you said wkrug; i.e. a modular synthesizer. At the moment I'm still playing with basic analog ways of controlling the oscillators, using a trimpot to kind of - permanently tune an oscillator etc and setting up interactions that way. Like I say its more an art project than a Prophet V but might as well get in that big beautiful deep end.
The microcontroller stuff is where I'm treading water. I've found a couple articles on these specific 'midi2cv' chips I can experiment with easily and the code is quite rudimentary for a pseudo-intellectual like myself but apparently the guy that sells them has dissolved off planet earth and I can't find them anywhere. Im assuming I'm going to have to experiment with arduino or teensy so I can learn. Do I need analog input or output or is it both ways?

I've learnt that 1V is 1 octave or 1volt per octave which is fascinating (for me) and ye Dr Pepper I'm literally looking for basic note on note off, as basic as it gets but with some hopefully familiar operation for musicians i.e you plug in a keyboard and you hear the chord you're playing. Could you elaborate on the oscillator 'inhibit'? Does velocity fall under that or is that more complicated? The CV thats coming out of the oscillator is just pitch.

Thanks
.

 

[Nigel Goodwin]

I've learnt that 1V is 1 octave or 1volt per octave which is fascinating (for me) and ye Dr Pepper I'm literally looking for basic note on note off, as basic as it gets but with some hopefully familiar operation for musicians i.e you plug in a keyboard and you hear the chord you're playing.

1 volt per octave was the standard used for analogue synths, and is fairly important so that things can work together.

A classic example would be using a slow sawtooth oscillator to sweep both an oscillator and a low pass filter together, in order to generate a swept sine wave.

If you google for 'midi to cv', or 'arduino midi to cv' there are many examples, such as this one:

MIDI To CV/Gate The Easy Way

Let’s say you’ve got a modular synthesizer. You’re probably a pretty cool person. But all your cool laptop DJ friends keep showing off their MIDI-controlled hardware, and you&#821…

hackaday.com

Or a PIC one:

Midi to CV Gate Converter

PIC16F628A based Midi to CV converter with an additional CC output. Here is my version of an Midi/CV converter, a low cost, low part count design with an additional CC output, 8 bit resolution and …

obsoletetechnology.wordpress.com

 

[renegade-system-generator]

Ah beautiful thanks Nigel

 

[dr pepper]

Note on note off probably can be arranged with a transistor that electrically stops/starts the oscillator from working, a lot of oscillators can be modded to get this to work, the more knowledgeable electronics or music enthusisast would be able to do this.
Velocity and pitch or any kind of continuously variable control is a lot more complicated and requires analog control from the processor.
This makes the software and hardware much more complicated, and existing analog oscillator designs may not lend themselves to such a mod.
If you want to connect say a pot or analog voltage from a synth to a micro then you'd use analogue to digital conversion, and if you wanted the micro to output a voltage then you'd use digital to analog conversion, most micro's have the former, not so many the latter, well not good enough for quality sound.

 

[renegade-system-generator]

I've been following this guys videos [ https://www.youtube.com/channel/UC0Hu_QzAeI-gEg1s0pCeV4w ] he's building a full polyphonic analog synth and had a couple of videos on his midi to cv circuits and exponential vco things. Im replicating the Arduino with their atmega chip and a few components and into some kind of 12 bit dac situation and start experimenting