DAC King

[Paul Schreiber]

> A lot of people have brought up glide curcuits, etc., but the high
bitcount
> DAC seems glanced over.  What are the pros and cons of that?  The only
> immediate con I can imagine is that anything beyonf 7 (8?) bits is out of
> MIDI resolution.  I would propose that solved by using two MIDI signals, a
> "course" resolution and a "fine" resolution.  What am I missing?

Since this is my favorire "beat it to death" topic:

A DAC has 2 important, and *different* specs: resolution and accuracy (also
called linearity).

Here's how to figure out "how many bits do I need"

a) decide on the *range* of the DAC output. Is it 0 to 100V? -5 to +5V?

b) The DAC can only "hit" certain voltages. The "dividing line" between
voltages is
the number of bits, raised to the power of 2. Examples:

4-bit DAC can "hit" only 16 different voltages
8-bit DAC can "hit" only 256 different voltages
12-bit DAC "hits" 2048 different voltages
16-bit DAC "hits" 65535 different voltages (!!)

and most times, 1 "hit" is 0.000000V so that one is "wasted"!!

c) now, you have to *fully understand* what it is you are controlling. The
cutoff on a VCF
can be "looser" to hit than a VCO. To properly hit a VCO CV at 1V/Oct, you
need to "hit"
the proper voltage within about 600uV (remember, 1 note is only 83.3mV).

So, if we want a DAC to control a VCO over 6 octaves, we need how many
bits?? Anyone??

8-bits? 6/256 = 23.4mv/step err no
12-bits? 6/2048 = 3.9mv/step  err no
14-bits? 6/16384 = 366uV/step OK!

But, you also have to keep in mind the DACs *accuracy* to hit that voltage.
This is spec'd as a
"fraction of a LSB (Least Significant Bit)", of the output per step. An
*average* DAC has 1/2LSB.
This means the ACTUAL OUTPUT VOLTAGE is +- this error. If you have a 14-bit
DAC with 1/2LSB
linearity, the output voltage in the above example can range from 549uV to
183uV. That's quite
a range! Generally, this error is *constant over the whole range*, meaning
if you increment step-to-step
the outout changes *the same amount* per step. Every DAC you buy will step
differently!

So *really good* DACS can spec 1/8th LSB. Some *really sorry-assed* DACS
will spec 1LSB. Which
is another way of saying the last bit is garbage.

Lastly: a DAC output is a *system engineering exercise*. You need a STABLE
reference, and a low drift
output stage. Hooking a 14-bit DAC to a TL072 buffer is useless. Hooking a
12-bit DAC to a voltage
reference made from a Zener diode is useless (temp drift + noise!).

It *does not matter 1 bit* ((PUN ALERT!!)) how many bits MIDI can resolve:
it DOES matter what the DAC CAN RESOLVE!!


Paul "DAC King" Schreiber
www.synthtech.com/motm   <<lack of DACs