[sdiy] MIDI VOL PEDAL with no micro??

[rsdio at sounds.wa.com]

It's possible to create ADC using a DAC and a comparator, working one  
bit at a time. It's called successive approximation, and starts with  
the msb. You would probably use a shift register to hold the value as  
it's calculated. Many early synths saved money on (then) expensive  
ADC chips by reusing the DAC, but it really made things run a lot  
slower when they used the CPU to implement the loop.

It's possible to create a DAC using a resistor ladder, which is  
reasonable when you only need 7 bits of resolution. However, you  
might get a lot noise unless you have one or two extra bits of  
resolution, because the value will jump around all the time and fill  
up the MIDI stream. The Prophet 5 rev 2 had a 7-bit resistor ladder  
DAC, with hand-tuned resistances! There are notes in the Service  
Manual explaining that you should mess with any extra resistors that  
might have been added to improve the accuracy.

You'll need a 7-bit latch to hold the previous ADC value, because you  
only want to transmit a new MIDI message when the value changes.

Then, as Roman explained, you'll need to send a 3-byte MIDI message  
with start and stop bits. He recommended a 30-bit wide shift  
register. I'd recommend a normal shift register, or 8 to 10 bits,  
with a counter to step through the 3 bytes and reuse the shift  
register. You might be able to generate the start and stop bits with  
logic, rather than actually loading them into the shift register like  
the data bits. But Roman's idea might end up needing fewer parts.

I think folks have covered everything, but I wanted to add those  
tricks above for creating your own ADC and DAC from CMOS and linear  
analog parts (like the comparator).

Brian Willoughby
Sound Consulting


On Feb 8, 2014, at 11:16, Dan Snazelle wrote:
> is it possible to create a MIDI volume pedal with nothing but an  
> optocoupler, opamps, cmos and passives?
>
> i ask because i have a friend who wants to make one himself
>
> id just use an AVR with an opto and a pot based pedal.... but i  
> thought it might be fun to go the other route
>
> are CC messages complex? i wouldnt dare try this with SYSEX but i  
> thought a CC pedal might be within the realm of possibility!