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

[Gene Stopp]

Dammit I hate hot keys :(

Unfinished email finished here....



Oh good some lively discussion! :)

One thing I should state is that now I realize there are two application
discussions going on at the same time:

1. DAC providing all pitch control - note, portamento, modulation, tuning,
all of it. Everything summed together on the digital side of the DAC and DAC
output going straight into the pitch source (i.e. VCO). This is where the
higher bit-count and accuracy is needed to avoid zipper noise and to provide
seamless continuous control. The output of a full-featured MIDI-CV converter
is an example. MIDI in, VCO control out.

2. DAC providing accurate note steps *only* for the western-scale
equal-tempered scale, with all modulations summing on the analog side after
the DAC. For full MIDI control, several DAC channels required such as an
octal 8-bit.

I do use both, but I've only *built* the second. The first method gives you
an easy way to control most commercially-built analog machines like
minimoogs etc. Great for a "black box" that has the flexibility and
compactness so you can move things around easily. The second method allows
you to give more patch flexibility on the analog side under digital control.
Example:

DAC Channel 1 controls note number (semitones)

DAC Channel 2 controls pitch bend, full-scale then attenuated, summed into
pitch CV

DAC Channel 3 controls modulation, full-scale then attenuated, controlling
VCA which is passing modulation signal into pitch CV

DAC Channel 4 controls aftertouch, full-scale then attenuated, controlling
VCA which is passing modulation signal into pitch CV or whatever

DAC Channel 5 controls velocity, full-scale then attenuated, controlling VCA
which is passing modulation signal into whatever or just without the VCA
controlling whatever

DAC Channel 6 controls portamento, full-scale then attenuated, controlling
VC lag circuit that note CV from channel 1 passes through

DAC Channel 7 controls some CC number, full-scale then attenuated,
controlling yet another VCA or whatever

DAC Channel 8 controls another CC number, full-scale then attenuated,
controlling yet another VCA or whatever

Please note the frequent "whatever"'s, this is on a modular synth so
hopefully there are a lot of voltage-controlled thingeys and modules to
choose from.

The first requires lots of design effort on the digital side, no hassles on
the analog side. The second requires more hardware, easier on the digital
side, more hassle on the analog side.

Example for #1 - JKJ CV-5 or similar

Example for #2 - Roland MPU-101 (not quite 8 channels...)

The discrete resistor DAC is good for a home-built single CV output from a
digital keyboard scanner. The multi-channel 8-bit DAC (a $30 part, BTW) is
good for modular hackers like me. The all-in-one DAC is good for small
synthesizers AND big modulars both.

It takes a while to set up the 8-channel system, and you need something
smart to drive it (I use a dedicated old 486 with home-brew code via the
parallel port). But after all the effort I can get some pretty drastic
effects, kind of Tomita-class sounds, from a MIDI keyboard. Gas music from
Jupiter I call it (there's one for Harry!).

- Gene


-----Original Message-----
From: jhaible at debitel.net [mailto:jhaible at debitel.net]
Sent: Thursday, September 19, 2002 7:30 AM
To: Gene Stopp; synth diy
Subject: RE: [sdiy] DAC selection in MIDI-CV Part #1



>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).

Right. I did the same thing, and it works ok. (If we have the time to
select resistors. It's fun if you do it once; not so much fun if you have
to do it all the time. Outright expensive if you have somebody else
doing it, i.e. if you buy 0.1% resistors. A real good DAC is less expensive
then.)

There's also the issue of the switch resistance. I've used a 4050 for the
clone of the OB-4-voice key assigner. You must perform a lot of
tricks to get the accuracy. Connect several gates in parallel for the
higher bits (to decrease the error from the driver output resistance),
and/or use a rather high resistance for the R2/2R ladder. 
But higher impedance also means slow settling time (assuming
constant stray capacitance), so you might get in trouble if you
want to demux a lot of channels in an acceptable time.

>Also I've used an octal 8-channel 8-bit linear DAC (AD7228 I believe)
driven
[...]
>Or maybe I picked a really good specimen for the DAC? 

Most likely, yes. In fact, *most* specimens should be better than worst case
data sheet errors. But what if you run into one that is not good enough?
As we're building this for a hobby, it's ok to just be lucky. (I guess the
few
who accidetially bought a real bad specimen are the few people who
swearcheap  8 bit DACs are not enough.)

If you are a manufacurer, IMO you cannot rely on this. Guess you
bought hundreds of cheap DACs which all are fine (because they
perform like *typical* specs, i.e. much better than guaranteed),
make a product, sell the product to happy customers, and then
you order a new batch of DACs, and you get specimens that
are just inside the worst case specs. Major disaster. Especially
if you have just soldered them in, and sent your Midi2CV boxes
out to your customers.

Testing is a very important factor in semiconductor pricing.
If you can test for yourself (selecting 0.1% resistors, or
just try and use (or in rare exception, discard) a cheap DAC),
you can save a lot of money. (You invest your own time,
which can be fun.) If you don't want to test for yourself
(because you plan to sell hundreds of items), your best
solution is just like Paul described.

>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.

Yes - because some of our garage workbench hacks are better than lab
quality. (;->)
That's big advantage of building single quantity stuff. We're in love with
what
we achieved, we love to adjust all these these trimpots to perfection.
In Production, however, you curse every extra trimpot you have to adjust.

JH.