[sdiy] MIDI VOL PEDAL with no micro??

[Dan Snazelle]

wow

very interesting reading

i have made R2R ladders before for home-made AVR projects so i know it can work pretty well but I found it surprising that the expensive Prophet 5 used one!!

now i want to find a rev2 manual !!

thanks




Sent from my iPhone

> On Feb 9, 2014, at 2:59 AM, rsdio at sounds.wa.com wrote:
> 
> 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!
> 
>