[sdiy] Starting my ASM-1

[Gene Stopp]

Oooops! Sorry Ian, I was off by one issue! :) It's a sign of fading memory
over 40, I guess...

I did build the original resistor-string interface from EN 45 or 46 (okay I
won't try to be to exact this time) and it was quite prone to mis-latched
notes depending on contact condition. I never did try the modifications -
instead I jumped ahead an issue and did the digital interface, and I was so
happy I never looked back.

It's interesting to me how many commercial circuits use the gated sample &
hold interface architecture successfully. The Moog 951 is quite robust, as
is the ARP 2600 and Odyssey. The ARP Soloist stinks (needs cleaning all the
time), the Minimoog starts out good but then stinks after a while, and the
Micromoog/Multimoog HF oscillator thing seems to be pretty reliable. My
Minimoog has a digital interface tucked up underneath the bottom cover - I
was hesitant to hack into that and disturb the vintage quality of the
machine, but it really paid off because the keyboard never makes a mistake
anymore.

One of my projects has the digital design burned into an Altera EPLD. That
was an interesting achievement because the circuit as printed in EN 68
didn't work at first. I had to qualify a bunch of the timing signals since
the latch pulse trick in the original circuit apparently needs real (i.e.
non-ideal) gates in order to work properly. The "virtual" chips in the EPLD
created an infinitely thin pulse that didn't trigger the word latch - I
brought it out to a pin and looked with the scope and had to really mess
with the trigger just to find it.

Best Regards,

- Gene



-----Original Message-----
From: Ian Fritz [mailto:ijfritz at earthlink.net]
Sent: Friday, January 11, 2002 7:10 PM
To: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Starting my ASM-1


Regarding keyboard interfaces:

Information on the EN Newsletter may be found at:
http://electronotes.netfirms.com/
The "Builder's Guide and Preferred Circuits Collection" includes the 
original EN analog and digital keyboard interfaces.

The analog unit was quite fussy to get working properly, and I gave some of 
my modifications in EN #67. The modification for adding a non-retriggerable 
trigger consisted of the following:

1.)  A digital inverter driven by the keyboard Gate signal.

2.)  A 555 one-shot triggered by this inverter's output. The period of the 
one-shot is slightly longer than the "normal" trigger.

3.)  An AND gate combining the new 555 pulse and the original trigger.  The 
output trigger pulse from this gate only occurs immediately following a 
Gate leading edge, i.e., when all keys had to be up before the current 
keypress. The resulting trigger signal is accurately synchronized with the 
"normal" trigger, because of the use of the AND gate.

The digital scanning design (originally in EN #68) is a superior design. 
But if I were to do this now, I probably would use a microcontroller to 
scan the keys and drive a good D/A converter.

   Ian