[Fpga-synth] Envelope - linear attack & expo decay/release?

[Eric Brombaugh]

Scott Nordlund wrote:
> I've checked a couple datasheets for analog envelope generator chips (CEM 3310, 3312 and SSM 2056).  For the attack stage they seem to use a sort of "truncated rising exponential decay", where an RC circuit charges to about 77% of the charging voltage before the circuit switches to the decay stage.  I'm not sure how this would compare sonically to a linear attack.  It might be best to play around with some software-generated envelope curves before putting much work into it.

That sounds like a strategy to ensure that the attack curve never 
flattens out before going to decay. Seems pretty practical, and for fast 
attacks it's probably sonically indistinguishable from linear slope.

> Anyway, if you wanted to implement something more than an ADSR, this might be a nice way to do it, since no additional logic is required for rising vs. falling stages.

Interesting thought. My plan was to go for linear or expo by muxing out 
the (current - destination) value with a constant roughly equal to the 
max value. Doesn't take much more logic.

Eric