Compander DA & Korg ms20

[WeAreAs1 at aol.com]

In a message dated 10/6/99 8:33:29 AM, you wrote:

<<I just learned about compander DA convertors
(logarithmic) in class (oh, so much fun) so i had
plenty of time to think. Now could this be used, in
combination with a linear AD to build an easy Expo
convertor? Anyone ever did this? 
This would be nice to put in my endwork project...>>

It's probably not a good idea, for a couple of reasons:  First, you'd have no 
control over the actual shape of the logarithmic curve - it would be 
hard-wired into the DAC.  Second, you would not have control over the step 
size (the size of output steps between bit transistions).  There's no 
garantee that the steps would be in the right place for the output voltages 
that you desired.

This brings up another point:  Companding DAC's exist mainly because of the 
need for getting greater effective resolution out of a DAC with fewer bits.  
The typical companding DAC is an eight-bit DAC.  The step sizes of the bits 
are arranged such that the steps are very small at the low end (LSB) and very 
large at the high end (MSB) of the DAC's range.  This gives greater 
resolution where it is needed most; for the very low level signals.  For 
audio, this is great - an 8-bit companding DAC can simulate the apparent 
resolution of around 10 to 12 bits, in practice.  They are used to good 
effect in some of the old 8-bit digital drum machines, such as the LinnDrum 
and the Oberheim DMX and DX.  I imagine that they are also utilized in 
telephone systems, as well.

Unfortunately, for use in a digital expo converter, you would need 
sufficiently high resolution in all ranges of the DAC - so you could 
calibrate it to put out the correct, *exact* voltages for the MSB's as well 
as the high-resolution LSB's.

This might work, if you could find a 16-bit companding DAC - but I don't 
think they exist.  Nevertheless, it's probably best to relegate this function 
to old-fashioned analog methods, and use linear DACs for everything except 
low-bit-resolution audio.

Michael Bacich