DAC resolution

[jhaible]

> Full scale is 256 notes - that would be +/- quarter of a semitone - that
> probably would be noticable - I must have been lucky :-)
> I have measured the output voltages of for the first 12 notes on an 8 bit
> DAC, and the accuracy of each 0.083mv step was better than I could
measure
> with a 3.5 digit mm.

The biggest problem is not the accuracy of "some" step. You can have fairly
accurate steps thruout the whole DAC range, but the *one* step where the
MSB switches (and consequently all the other bits are switching in the
other
direction) will kill your performance. Small difference of two large
numbers ...
(This is just the most obvious example. There are many other sources of
error.)

What you actually need for a 5 Octave range is a precise 6-bit DAC.
(For precise semitones; no bend !)
Us DIY builders can build such a thing by selecting a handful resistors and
connecting them to the output of CMOS buffers. Just take care for the ON-
resistance of the buffers (connect several ones in parallel for the higher
bits)
and for a stable reference voltage. The Oberheim 4-Voice circuit uses this
method (schemos are at Synthfool.).
For commercial products selecting resistors would be too expensive.
So they have to use commercial DAC chips. 
If you buy a DAC IC, chances are that if you buy an N-Bit DAC, you
have N-x "good" bits. That is, you want 6 bits precisely, buy
a 12-bit off-the-shelf DAC and you'll be fine. There should be 8-bit
DACs that are good enough, too, but I guess that would be selected
ones. So you can pay the DAC manufacturer to do the selection, and
pay him for that, or you can buy a 12+ bit unselected one that might
even be less expensive. Mind you, the cheap unselected 12-bit might
be one of the rejects from selecting accurate 12-bit DACs (;->)
That's the choice you have. For my own stuff I prefer 6 bit discrete
and selecting the resistors myself. For a commercial product,
higher number of bits is the way to go, and you even get resolution
(not accuracy!) for bend and modulation for free.

JH.