[sdiy] MIDI VOL PEDAL with no micro??

[rsdio at sounds.wa.com]

Yep. The Rev 3 switched to a quality 16-bit DAC chip! (although only  
14 bits were really used)
As an aside, folks revere the Rev 2 because it has SSM chips instead  
of CEM, and supposedly sounds phatter. But nobody ever mentions that  
the resolution of the controls is really only 6-bit (64 steps per  
knob turn) except for frequency, which is 7-bit. What's really  
strange (to me) is that the Rev 2 used a "real" 8-bit ADC, but had a  
7-bit R2R (800k 1%, 400k, 200k, 100k, 50k, 25k, 12.4k 1%) DAC!


I forgot to add below that, along with the 7-bit latch, you'd also  
need 7-bit binary compare functionality to detect changes in the  
Volume Pedal that require a new MIDI message. You could build this  
with with 7) 2-input XOR gates followed by a 7-input NOR gate (which  
doesn't exist, you'd need multiple OR gates, cascaded into a NOR).  
You could also use a logic subtractor to compare the old and new ADC  
values, then look for zero on the output, for which you'd need a 7- 
input NOR equivalent.

Another feature would be a counter that would resend a MIDI Volume  
message if a long time passed without a change, just in case the  
receiving synth missed the previous message (due to a cable being  
attached too late).

By the way, the whole no-micro MIDI Volume pedal could partially be  
done with an FPGA chip rather than discrete logic or CPU, but there  
aren't many affordable FPGA chips with linear analog comparators  
inside, and you'd need a development tool to program the FPGA.

Brian Willoughby
Sound Consulting


On Feb 9, 2014, at 01:52, Dan Snazelle wrote:
> 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
>
>> 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 not 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, of 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).
>>
>>> 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!