[sdiy] MIDM-CV DACs, Part #3 (last)

[phillip m gallo]

jorgen,

Remember Groucho's phrase "who are you going to believe, me or your own
eye's?".  12 "Effective Bits" drives a VCO very well.  Theoretically, an
"ideal" 12 Bit DAC, being used to generate 0-10V  would provide  409.6 steps
per 8ve, 34 steps per semi-tone (or a little less than 3 cents).  If you
generate 0-5V you double this theoretical resolution. The discussion to date
argues that any deviation from ideal linearity impacts this theoretical
response.  My specific argument is that if you are aware of the cause of
this deviation you can often perform compensatory actions that decrease the
deviation from ideal.

Your specific observations about drift and the repeatability of settings is
my favorite arguments for DAC derived control voltages (ideal sample and
hold).  This is why i argued last week against advising someone to build an
analog keyboard when a scanned key/DAC combo is just as easy to make (micro
or no micro), can provide a stable and repeatable voltage source, easily
provides additional control (velocity), and allows flexible Gate and Trigger
response (i.e. delayed as a function of velocity for instance).

regards,
p






-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of
jorgen.bergfors at idg.se
Sent: Wednesday, September 18, 2002 11:39 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] MIDM-CV DACs, Part #3 (last)



Although this theoretical discussion probably is correct, in reality a 12
bit DAC is more than enough.
I have built the Oakley MidiDAC, which I believe is based on a 12 bit DAC.
I have measured every single midi note value with a 4 1/2 digit DVM. The
accurracy is way better than you could dream of tuning a piano or guitar,
for instance.
Also the the drift is neglible. I tuned my VCOs against a digital synth.
Then I turned everything off The next day, after a warm-up period, the VCOs
were still perfectly in tune with the digital synth (more than a second
"beating time"). No knobs were touched in the meantime.
Actually, this is my normal way of working. I just turn everything off, and
continue working on the same project another day. I never have to retune,
as long as I'm using the same patch. I retune only when I do the final
recording, just to make sure.

/Jorgen

http://hem.bredband.net/bersyn/


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