MIDI-CV converters (was: Building my first synth...)

[gstopp at fibermux.com]

     On 2/6/96 johns at oei.com at ccrelayout wrote:
     
     >
     >I can't see any reason to go to all this trouble to build-build-build 
     >when you can simply buy a DAC card for your old PC and (viola) you 
     >have instant analog voltages.  Now your job is reduced to simply 
     >writing a program that will interpret the MIDI data and send the 
     >appropriate commands to the DAC card.
     >
     
     Now where's the fun in that??? :-)
     
     Seriously that is a viable option - I'm sure I've seen bunches of ads 
     for D/A cards in the back of various magazines alongside all the data 
     collection stuff that's out there now. Are there such cards with TTL 
     latched bits as well as voltages? Are the CV's accurate (and 
     noise-free) enough for pitch VCO use? Anybody out there know more?
     
     I just cruised through the Analog Devices databook and boy is there 
     some neat stuff in there. Of particular note (I think) are the quad 
     and octal 8-bit voltage-output D/A's. Heck there's even one with 16 in 
     it. These bad boys have all the latches built in - just stick 'em on 
     the databus, decode the address lines and the write pulse, blammo 
     instant memory-mapped analog outputs. Think I'll get me some samples. 
     I'll try to get prices while I'm at it.
     
     As for the driver on the PC, since the XT is not suitable for Windows 
     I would probably just let the program loop schedule all the I/O tasks. 
     Either that or write a driver in assembly and link it to the .EXE. 
     Everything would run under DOS.
     
     I've been thinking about some of the timing issues that would arise if 
     the serial port option were used. See I'm envisioning this box, 
     external to the PC, hooked up with some standard cable to a connector 
     that you'd find on the back of any PC. Ya need somewhere to put 
     thirty-two or sixty-four or some ungodly number of 1/4" jacks, right?
     
     As I said before serial would be the absolute least common denominator 
     between all machines - unfortunately it would be the slowest too. For 
     example imagine you make up a simple protocol, such as <address> 
     <data> <address> <data> etc., with the MSB set to "1" if it's an 
     address byte and "0" if it's a data byte. This would mean that to 
     latch in thirty-two bytes, each to it's own D/A, you would need two 
     writes for each D/A. At 9600 baud that would mean (assuming one stop 
     bit) a minimum of 66.6 milliseconds to update all of the D/A's. Is 
     this fast enough for music use? Maybe, since it's about the same speed 
     as MIDI itself. Notice that this does not include the gate latches, 
     and that the MSB will not be useable. Oh well, 7 bits covers ten 
     octaves. Probably the gate can be a toggle of succesive writes to the 
     same latch....
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Re[3]: MIDI-CV converters (was: Building my first synth...)
Author:  johns at oei.com at ccrelayout
Date:    2/6/96 10:31 AM
     
     Last year, I did just that.  I had an old 286 PC serving as a door 
     stop.  I needed a cheap CV for my Prophet so I bought a 2 channel PC 
     DAC card for $165 and wrote a cheap and dirty program to control it 
     with data from my MIDI card.  You can use a cheap parallel port card 
     as the gate signal (or you can use the 2nd channel of the DAC as 
     well).
     
     I think the convenience, reliablibity, and hideaway packaging of this 
     approach makes it worth the expense which is right in line with a 2 
     channel DIY or standalone CV.  But I must admit that if you want more 
     channels (more than 2), I think these cards get pricey.
     
     Or you could (very nicely) produce a PC card based MIDI-CV.  You would 
     write a Windows MIDI device driver that would receive MIDI messages 
     and control a DAC card that you would install in your Windows PC.  
     I'll bet this would be cheaper than buying a standalone unit.
     
     John Speth - johns at oei.com