Can somebody explain Vectrex ADSR envelopes to me?
 
I know what ADSR envelopes are, for example, on the C=64 you would set the 
Attack, Decay and Release values as time (ie: 1 Millisecond) and the Sustain 
value as a Volume (ie: Loud or Soft). When you triggered the sound, it would
ramp up to full volume in the time specified by Attack then ramp down to the 
Sustain volume in the time specified by Decay. The note would be held at 
Sustain (potentially indefinately) until you told the C=64 to begin Release, 
at which point it would ramp down the volume in the time specified by 
Release. (So you determine the length of the note by how long you have the 
computer hold the note at Sustain).
 
And that's how you make sound of the C=64 (Well, you also set the frequency, 
obviously, as well as waveform (Square, Triangle, Sawtooth, Noise)). That's 
the last machine I played around with ADSR envelopes...
 
Now, on the Vectrex, ADSR envelopes take the form:
 
ADSR1:    fdb $4488,$ccff,$ffff,$ffff,$ffff,$ffff,$ffff,$cc44
 
From what I've played around with, these values seen to represent volumes 
from 0 to maximum volume with each word being 1/8 of the total length of the 
note.
 
So, you could actually make notes more complicated than a "normal" ADSR 
envelope (ie: ramp up, ramp down, ramp up, ramp down, sustain, ramp down..)
 
That's fine, but, how is the "time" sliced up? According to word, byte or 
nybble?
 
For example, does $00f0 mean 0 volume for 2/32 of the note, then full volume 
for 1/32 of the note then back to 0 volume for 1/32 of the note?
 
Or, does $00f0 mean 0 volume for 1/16 of the note then almost full volume 
($f0) for 1/16 of the note? Or, does it mean almost half volume ($00f0) for 
1/8 of the note?
 
Some of the ADSR waveforms in ROM look like each nybble is a time slice, 
while others look like each byte. However, it could also be by word or even 
double word (2 words each for Attack, Decay, Sustain, Release).
 
Or, am I misreading this completely and these values actually have some other 
meaning (Time instead of volume)?
 
Any help would be appreciated!!
 















>Can somebody explain Vectrex ADSR envelopes to me?
> 

No, not really :-(!

I can make some (more or less) clever remarks!

Indeed the table consists of nibbles.

Actually only 31, but you can not store 31 nibbles, so they took
32, the last is just for fun.

It is not exactly a ADSR table as you described, it is just a table
of volumes.
Internally a counter is count down with each step of the sound
routine (usually at each wait_recal).
This is, as we know, about 20ms or 30,000 cycles.

The sound routine keeps track of the length of the note
and looks the volumes for the notes up in the ADSR table.
(the corresponding nibble)

Lets have an example (our beloved Vectrex Frogger :-)):

           FDB     $FEE8,$FEB6 ;ADSR and twang address tables
           FCB     2,12               
...
FEE8   FDB, $EEFF,$FFEE,$EEDD,$CCBB,$AA99,$8888,$8888,$8888
...

The sound routine will remember the addresses of the 'fade'=='ADSR' and
twang table, will remember the startaddress of the music score,
gets the note (here the 2) and stores it to PSG channel A.

It will also set up the other channels according to the data found (here nothing).

Than it looks for the note's length and stores the length.

It loads the first nibble '$E' of the ADSR table and stores it to the PSG
Volume of Channel A, the only active channel. (only a 4 bit value)

If there were other active channels, they also would recieve '$E' as volume
information. (BTW the non active channels get nibble 31 '$8' here as volume
information)

(than it does the VIBE stuff)

Than off we go...

Next round... 
Looks up the next volume, another '$E'... and stores it...
the counter is count down from 11 to 10 (oops, forgot to mention the countdown above... well, it did countdown in the above section from 12 to 11- here we go).

Next round... 
looks up '$F'... stores it... count down...

Next rounds... 

looks up '$D' stores it count down to 0, initiates next note
(but playing the last 'fade' of this note)

There can be at most $1f = 31 different such FADE's.
If the note is played longer it will linger on the nibble 31 volume.

OK...
That's all for the clever remarks!

Malban

PS
I hope this is correct, I just figured it looking at the ROM...