info-mac@uw-beaver (info-mac) (07/10/84)
From: mark@harvard.ARPA (Mark Lentczner)
-=-
As far as I can gleen the DAC on the MAC is a bit of a kludge...
What follows is what I think is going on. I am currently involved
in Computer Music here at Harvard, and we like the idea of using
MACs for any number of tasks (if not sound generation) in our studio.
But the DAC, as you shall see, doesn't really rate up...
The DAC runs at 22khz , 8bits. The rate is fixed to the video rate
so, there is no changeing (i.e. it is done in hardware). Apple
keeps talking about the DAC being `linear phase modulation' DAC
(or some such silly wording). What i think this means is that at
the start of each video scan line the hardware fetches the corresponding
DAC byte from the sound buffer (thus the strange length of the sound
buffer, it is linked to the number of logical horizontal scan lines
(i.e. ignoring vertical blanking)). Then as the line is swept out,
when the pixel number being output matches the sound byte (i.e. pixel
139 matches the sound byte value of 139, although the mapping may
be two to one, thus pixel 139 matching sound byte value 69) then the
horizontal deflection is sampled by a sample and hold circut and the
output of that curcit is the `DAC' value, and is output to the sound
amplifier (which has three bits of control, i.e. seven amplitueds and
silence).
The problem with this `quick and dirty' dac is that the samples don't
come off it in a steady stream, the time of the sampleing is dependant
on the value being output. It varies with change in sample time: the
greater the difference (in magnitude) the greater the time skew (direction
is determined by the sign of the difference, positive makes the sample longer,
negative shorter). Of course on the average the samples are even. But,
if in sound `on the averaging' has the effect of filtering the sound.
I'm not entirely sure what this filter looks like, but i'm fairly sure
that it is a comb filter.
Well, if that is what's going on then it is great as far as home
computers are concerned, not the best for computer music. If anyone
knows better what's going on, please correct me. As I said, I'm
only inferring from Apple's descriptions, the visible architecture,
and the software that uses it.
-mark lentczner
electronic music studio
music department
harvard university
cambridge, ma 02138
lentczner@harvard
{allegra,genrad,ihnp4,ima,ucbvax}!harvard!lentczner