ASM1 ADSR Idea

[Debby and Gene Stopp]

Sounds like a valid mod to me.... but let me give some history.

The main thing I had in mind when I designed the ASM-1 ADSR was simplicity,
using as many tricks and shortcuts as possible. First and foremost was the
idea of using the SPDT switch, rather than a bunch of SPST switches like
everyone else did (i.e. the 4053 rather than a 4016 or 4066). My thought
was, the cap was either charging up or charging down, so why not use on SPDT
switch to either attack or decay, and if decaying, either discharge down to
the sustain level or ground? Since the 4053 has three SPDT switches, and the
preceding algorithm only needs two, I could parallel two of them on the
attack/not attack to reduce the switch on-resistance for those critical
attack and decay times. This works great - I forget the actual rise/fall
times, but the time constants are real snappy.

The second trick was to use the voltage level of the buffered cap to reset
the S/R flip-flop built out of the dual 4-input NOR gate, just by relying on
the fact that CMOS switches at the V+/2 point (in theory). In other words,
an analog voltage (the final ADSR output) is used as a logic input to the
flip-flop to reset it. This is a text-book no-no, and the reason for the
existence of the "attack peak" trimpot. In practice, I find that some 4002's
don't really behave well so I usually socket those and swap around till
things work right (yeah I know, this would never fly in a production
environment). Specifically, sometimes I find that the ADSR will not
re-trigger properly if the decay time is set to a long value. Someday I
might scope this and see what is really happening.

The "sustain trim" trimpot is used to tweak the voltage that the decay phase
decays to - in other words, you can set it so that the EG attacks, then
"decays" *up* to the sustain voltage (assuming the sustain pot is set to
max). This is a trimpot because it is interactive with the "attack peak"
trimpot.

Another purpose I had in mind with this design was to be able to use this EG
as a precise pitch transient control. For example, you can truncate the
exponential attack so that it ramps up to some pitch, and actually reaches
that pitch after some short finite time (instead of never getting there,
like diode-based EG's). Then, if you want the pitch to stay at the sustain
level once attack is finished, you can tweak that to perfection too.

So - there were reasons behind the logic and the trimpots, albeit my own
reasons. If I were to change anything about this design, I'd probably loose
the 4002 and stick in a 555 as the set/reset flip/flop (in retrospect a much
better choice than the 4002). If you spend much time looking over commercial
synth schematics, I'm sure you've noticed how popular the 555/556 is in EG
circuits.....

BTW never try to drive the ASM-1 ADSR with bipolar gate waveforms! The input
comparator (LM358) is run off of ground and V+, and if the input goes below
a minus diode drop, the EG will falsely trigger because the chip will freak
out and disobey the +/- comparator rules. Use a blocking diode in modular
systems.

- Gene


-----Original Message-----
From: Steven Cook <steve at babcom.u-net.com>
To: Synth-Diy <synth-diy at mailhost.bpa.nl>
Date: Saturday, August 15, 1998 5:38 AM
Subject: ASM1 ADSR Idea


>Hi,
>
>With regard to the ASM1 ADSR, would the following modification work?
>
>Replace the 4002 dual 4 input NOR with a 4025 triple 3 input NOR. This
gives a
>spare gate which is wired as an invertor to replace U2. U2 is then used as
a
>comparator wired between the top of the sustain pot and the output of the
>ADSR. The sustain peak trimmer is replaced by a fixed resistor - say 47k -
to
>give a peak attack / sustain of about 10v.
>
>This would (If it works!) remove 2 trimmers and all setting up without any
>increase in parts count.
>
>Any problems?
>
>Steve.
>
>
>