[sdiy] DAC selection in MIDI-CV Part #1

[harrybissell]

Guess I'm between devil and deep blue sea then...  ;^P

I agree with Paul... non-linearity is of supreme importance in a DAC...
but I agree with Gene that often, for a monophonic synth, a decent
8 bit DAC can give performance that well... you can't hear anything
wrong with.

I'd say to look fot a very linear part, and use whatever DAC suits
your needs and budget.

Many early monosynths had 1% resistors in the keyboards, if you were
lucky they were hand selected.

I had the early PAiA 2720 with the nasty trimmer per key, and a 'super trimmer'
at the octaves to get a shitty response over three octaves. That used 5% parts.
I drove (well, had MOM drive me...) fifty miles to get some 2% carbon comp
replacements, the tightest tolerance available in consumer distribution at the
time.
Made the keyboard linear, still needed the trim pots.

The day I got a string of 1% resistors I could REALLY hear the difference, and
was satisfied.  Now I'd buy 1% resistors and match them to .1% and be truly
amazed... OTOH I doubt I could hear this last difference, although I could for
sure if I left one oscillator on drone, and keyed the other...

H^) harry

Gene Stopp wrote:

> Permit me to play the devil's advocate for the purposes of furthering
> discussion about homebuilt DAC-based pitch CV sources...
>
> I've found that the hand-matched metal film resistor R-2R ladder gives good
> results on monophonic keyboard interfaces. Start with a bunch of 2% or 1%
> and narrow it down with a DVM to as close as you can get. Six bits is all
> I've ever used (5-octave keyboards). What would be the math to find the
> deviation in cents for 0.1% matching on such a DAC? I suppose you would use
> worst-case values at opposite extremes on the LSB side...
>
> Also I've used an octal 8-channel 8-bit linear DAC (AD7228 I believe) driven
> from a PC parallel port to give 8 CV's plus 8 gates. I drove the Vref from a
> low power voltage regulator (78L05 with lifted ground? can't remember
> exactly) set to 10-point-whatever volts, and grounded the DAC LSB input so
> that the top 7 bits were the 128 MIDI semitones. The 8th bit I used for the
> gate. 0x7F and below is note off, 0x80 and above is note on. This also gives
> good results. If the pitches of the VCO's are sloppy, then I sure can't tell
> the difference. The thing works just fine. Of course this may say something
> about my pitch perception :) Or maybe some detune sounds good? Or sounds
> good to *me*? Or maybe I picked a really good specimen for the DAC? Back
> when I built this I don't think I did any math at all at the design phase -
> I was just in a solder-happy stage in my life. At least now I have another
> cool box. I use it to drive my ENS-74, usually patched so CV 1 is the pitch
> and the other seven are various bend/mod depth/velocity/CC/whatevers. The
> seven extra gates have a threshold setting in my program so that you can set
> the trip points. It gets pretty whacky. Which is a good thing.
>
> BTW I prefer not to use triggers. I like the way you can retrigger *if you
> want* with a gate-only controller. Or you can let the attacks die out when
> you play legato. You can phrase your VCF cutoff without using a pedal or a
> knob. However I have done the pitch-CV differentiator trigger on a 5-octave
> minimoog conversion I did for a friend once, as a switchable option. I wish
> I could buy that machine now :(  What I'm trying to say I guess is that both
> single and multi-trigger modes have their uses. I suppose if you're playing
> live and you need every note to trigger (synthetic vibes solo?) you would
> prefer to have the interface do the triggering for you.
>
> Anyway not to belittle any efforts to obtain subjectively perfect pitch
> accuracy on anybody's DAC - especially on a product offered as a kit or
> finished unit. In that case the math and accuracy are much appreciated and
> even demanded. Lab quality to me is a big selling point. It's just that the
> garage workbench hacks can certainly produce useable machines, good enough
> for rock and roll.
>
> - Gene
>
> -----Original Message-----
> From: phillip m gallo [mailto:philgallo at attglobal.net]
> Sent: Wednesday, September 18, 2002 12:01 PM
> To: synth diy; Paul Schreiber
> Subject: RE: [sdiy] DAC selection in MIDI-CV Part #1
>
> Amen, i say unto you.  Paul has pretty much nailed the issue.
>
> When you are going to use a DAC to drive an analog synth you have a lot of
> issues to deal with.  If you peruse the WEB offerings for MIDI2CV you will
> find most folks just output a voltage equiv. to the 7 bit MIDI key code.
> This allows one to "sum" other linear modulation, fz offset, etc.. sources
> to allow "detailed" control of an oscillator.
>
> This is also why general "Sample and Hold" output structures are suitable
> for use they need only have 7 bit linearity and total error which is fairly
> easy to achieve.  (An aside, some WEB Midi2CV avoid or less than perfectly
> handle the critical voltage reference which impacts drift and tuning
> stability ).
>
> If you are going to use a DAC for exclusive control of a VCO then you are in
> the "parts per million" environment that you had better be prepared for.
> Paul already indicated a "proposed spec" of 1% or 1-cent.
> If you accept this spec you now have a target for analyzing you required
> accuracy.
>
> One really good way to achieve this accuracy is to use multiple 8-12 bit
> DAC's (with stable references) summed to achieve your final output control
> voltage.  One Dac emit's the 7 Bit MIDI Key equiv. voltage.  Another DAC
> provides a modulation output, or fz offset.  The number of DAC's you "sum"
> is a function of how much accuracy and range you require.  You also can some
> them in a "weighted" structure.
>
> Note that Paul's comment regarding your op-amps drift and accuracy still
> applies.  This is an arena where you get a lot of help from manufacturers
> and allows a greater "pool" of parts to choose from and even provides an
> (pin for pin) upgrade path for those who trade accuracy and stability for
> now but want a future capability.
>
> I have implemented the multi-dac scenario quite a bit for digital control of
> VCO's.  The software isn't really very tricky and even allows some
> interesting opportunities for a multi-tasking approach.
>
> regards,
> p
>
> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Paul Schreiber
> Sent: Wednesday, September 18, 2002 10:10 AM
> To: Seb Francis; Micke Askernäs
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] DAC selection in MIDI-CV Part #1
>
> I'm designing one at the moment (16 channels + 4 gates).  It will be based
> around a PIC and
> MAX525BCPP 12bit D/As.  These Quad-D/As are relatively cheap, but accurate
> enough (+/- 1 LSB) for
> pitch control over a full 10V scale.
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>
> No, they are NOT! (speaking as Maxim's DAC FAE).
>
> Although I've discussed this in the past several times (check archives), I'm
> doing some DAC apps
> now (on the older MAX502 parallel 12-bit DAC), it's time for a ADC/DAC
> refresher course :)
>
> Here is a DAC FAQ.....
>
> a) What errors are in DACs, and what errors can I control?
>
> There are MANY sources of error in DACs and their 2 main "outside"
> components: the voltage
> reference and the buffer amp. You *must* do a full "system level study"
> because all the errors
> add up, and you have to address ALL of them.
>
> The first thing to address is simple but often overlooked: what voltage
> accuracy do I need. This
> has NOTHING AT ALL to do with the number of bits in the DAC! This has to do
> with the VCOs. What
> ideal voltage does the VCO need to receive in order to be on pitch (assuming
> the VCO is
> 'perfect')?
>
> At 1V/octave, a full step is 1/12 or 83.333mv. If we desire 1% of that
> (pretty much the limit of
> pitch perception), then the voltage fed to the VCO out of the DAC system can
> not have an error of
> more than 833uv. That's 833 MILLIONTHS of a volt. It doesn't matter if we
> are in the 0-1V octave
> or a 9-10V octave, the DAC system MUST be able to 'hit' a voltage within
> 833uv.
>
> Setting the DAC chip aside, let's look just at the op amp buffer (this may
> be inside the DAC chip
> in some parts). The critical spec here is the op amp's Output Offset Voltage
> Drift spec. We
> really don't care what the initial offset voltage is (it's a constant), but,
> it had better not
> drift around over temp or the VCOs drift. The best op amps have drifts of
> 5uv/C. or better. Be
> sure yours does (like the MAX427 or MXL1013, used extensively in MOTM, see
> www.maxim-ic.com for
> datasheets and FREE samples).
>
> The next thing to worry about is the temp drift of the reference. The
> reference may be internal
> or external to the DAC. As a rule, external references are *much* more
> accurate and stable. There
> are again 2 specs of concern: accuracy and drift. Accuracy is part of the
> overall "voltage
> offset", as the output of the DAC is some fraction of the voltage reference
> (DACs work by
> precision dividing down the reference). Drift is in PPM (Parts Per Million)
> per degree C. A 5V
> reference with 5PPM drifts 1uv/C. Choose a reference that has 50PPM or
> better (the same as a
> really good 0.1% resistor). Get 10PPM and be done with it :)
>
> Now it's time to look at DAC errors (Part #2).
>
> Paul S.
>
> ---
> Incoming mail is certified Virus Free.
> Checked by AVG anti-virus system (http://www.grisoft.com).
> Version: 6.0.389 / Virus Database: 220 - Release Date: 9/16/02
>
> ---
> Outgoing mail is certified Virus Free.
> Checked by AVG anti-virus system (http://www.grisoft.com).
> Version: 6.0.389 / Virus Database: 220 - Release Date: 9/16/02