DAC bit res.

[gstopp at fibermux.com]

     Hey - not a dumb question at all! Probably a matter of some 
     controversy, which means it's a pretty good question. I'm sure there's 
     several good opinions on this and no true "best solution". On that 
     note (ha ha) here's my take:
     
     It really depends on system architecture IMHO. For example, you can 
     use a single high bit-count hi-res hi-accuracy DAC (expensive), with 
     multiplexed S/H's hanging off of it. Or you can use the same scheme 
     with a lower bit-count DAC (cheap), and just split up your parameters 
     into fine and course control. If you want the CPU (or whatever) to 
     calculate everything, like pitch bend and glide and intonation and all 
     that microtonal stuff, you'll need at least a 14-bit DAC at first 
     guess. If you go the split-parameter approach you can get away with an 
     8-bit DAC. If you're just going to use equal temperment, and your 
     basic pitch control is all the digital side will control, and all of 
     your fine tuning and pitch bend and all that extra stuff will be in 
     the analog domain, then a 7-bit DAC will give you the same 10-octave 
     range as the MIDI note 0-127 range. To make a 7-bit DAC, an easy 
     technique is to use 8-bit DACs and ground the LSB.
     
     To minimize CPU overhead you can use a bunch of latched DACs rather 
     than the single DAC/analog mux. This means that each DAC can be used 
     to store the parameter, and the logic will not be required to cycle 
     through continuously to update analog S/H's. This approach is also 
     totally immune to drift and offset problems. There are a bunch of 
     chips from Analog Devices that can do this, like quad and octal 
     latched 8-bit DACs. They're not cheap, but the reduction in circuit 
     complexity will be drastic, and for some people this is worth a lot of 
     money.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: DAC bit res.
Author:  "Duane R Balvage" <dbalvage at ptdcs2.intel.com> at ccrelayout
Date:    2/27/97 12:45 PM
     
    - Another dumb question, guys!
     
     How many bits of resolution for frequency control would you use?
    8? 10? 12? More? I am *trying* to figure out the necessary accuracy for a 
    polyphonic synth with ANALOG oscillators! My guess is you don't want the 
    tuning to drift more than a few cents in either direction. Any help in this 
    matter will be GREATLY appreciated!