What is the Most Efficient Way to Program an ADSR Envelope?

[UNIXcoffee928]

An ADSR Envelope is a method of describing a sound's envelope characteristics, through the manipulation of four elements:

 

1. Attack

2. Decay

3. Sustain

4. Release

 

 

This is the common way that sounds are described on synthesizers.

 

I've been thinking about different ways to implement ADSR envelopes on the Atari (in Atari BASIC & Assembly Language), and no matter what, it seems to end up being a lot of nested loops.

 

- Does anyone have a good algorithm for this already?

- Is there a good mathematical formula that would simplify things?

- Are there any good assembly tricks using timers to help to do the ADSR?

 

I'd like to make each element of the ADSR envelope variable & tweakable via the paddles.

 

 

Just figured that I'd ask...

[Rybags]

There's a number of ways... table-based might be one, but possibly costly in memory use.

 

How about something like:

 

Attack- use an 8-bit accumulator, each step it gets added to the volume value. Upper 4 bits of volume value get converted into actual volume level (via shifting or table-lookup).

Addition continues until peak target value is reached.

 

For more precision, a 16 bit level could be used... upper 4 bits decide actual volume as before. Or maybe 12 bits, means upper 4 bits are readily ORed with the distortion value.

 

Decay - same as Attack, except subtracting backwards toward a target Sustain level.

 

Sustain - easy... just a selectable number of iterations where nothing changes.

 

Release - as A/D, target level either zero or some specific volume level.

 

 

Or, partially table based. You could pack values together, so maybe something like:

 

Target volume level in bits 7-4. Attack "speed" in bits 3-0. Speed value used in a table lookup which gives 16 possible rates.

 

Or, fully table based. The entire envelope is defined.

Could also be done in a packed fashion where volume and duration of each step is contained within a single byte.

Edited February 10, 2009 by Rybags