[sdiy] Decode between Decay phase and Sustain?

[Tim Daugard]

Sorry, I fixed the typos, and spell check is now on.

: Technically there is no separate state in the sustain mode of a
: exponential ADSR. Sustain sets the voltage to which the decay
: is falling towards. Decay mode is valid until the release is
: activated. Sustain isn't an event in time, but a controllable level.

It depends on the design. My ADSR sets a decay time (and a decay
rate). The ADSR uses a 556 with one timer used for attack time and one
timer used for decay time. A signal from the timer tells the module
that it is in the decay time.

This makes for a very flexible ADSR:

During the Attack time:
    The attack timer control determines the time for the attack.
    The attack level control determines the ramp of the attack.
        If the ramp is fast, the attack flat tops.
        If the ramp is slow, the attack doesn't reach the peak.
    The Attack LED is lit.

Durring the Decay time:
    The decay timer control determines the time for the attack.
    The decay level control determines the discharge current.
        Decay discharges the timing cap from wherever the attack
          ended
        The Decay can theoritically go to 0
    The Decay LED is Lit.

After the Decay Period ends:
    The sustain circuit charges or discharges the timing cap
    The sustain LED is Lit.

This ADS(R) was built to be used with my bass, the release is a fixed,
reasonably quick time. The control input to the ADSR is a level
comparator so that it can be used with the signal from an envelope
follower (ENF). When the input signal is high enough the attack phase
triggers When the ENF signal drops the cycle ends. The comparator can
chop the signal at any point -attack, decay or release. If the note on
signal ends during the attack or decay phases, the timmers are  reset,
the phase ends and the release phase starts immediately.

This means that playing stacato, I can quick cycle the ADSR. If I hold
a note, the ADSR can go through the full cycle. The tone from the bass
can sustain for a long time, so setting the threshold for the note on
can run the release portion of the envelope while the sound is still
there.

(These notes are from a review of the schematic, my word proccesor
hasn't been fully installed yet.)

Tim Daugard
AG4GZ 30.4078N 86.6227W Alt: 12 feet above MSL
http://home.sprintmail.com/~daugard/synth.htm