lft
Registered: Jul 2007
Posts: 369 |
Avoiding the ADSR bug in the decay phase.
Random insight of the day:
The ADSR bug is well known: When we switch from a slow to a fast envelope rate, the envelope generator sometimes gets stuck for about 33 ms (1.67 PAL frames). This tends to happen when we have a slow release, and then try to trigger a note with a fast attack. The attack gets delayed unpredictably.
Some playroutines provide a hard restart feature to work around this problem.
But here's the insight: The same thing can and does happen when we switch from a slow attack to a fast decay, or from a slow decay to a fast release. So, if we use an ADSR setting of, say, 10 aa, we get a fast attack, and then we switch to an even faster decay. This will actually trigger the bug! The volume level could remain at the maximum for up to 33 ms. Furthermore, if we try to release the note too soon (within 1-2 frames), the release could be delayed unpredictably.
To avoid this, we have to use a decay rate that is greater than or equal to the attack rate.
On the other hand, sometimes this phenomenon is useful. If we want a slow attack and a fast release, and we know that we want to hold the note for more than two frames beyond the attack period, then this is exactly what we want. For instance, if we use f0 f2, we get a slow attack, followed by the ADSR bug (random delay of up to 33 ms). Then, as soon as we release the gate, we get an immediate response, because we already have the bug behind us.
To summarise:
If you want to reach the sustain level in the same amount of time for every note, or if you want to reach it as quickly as possible, ensure that D >= A.
If you want the release to start immediately when you turn off the gate bit, ensure that R >= D.
Advanced technique:
Suppose you want to reach the sustain level as quickly as possible, but you also want the release to start immediately. And you want a slow attack and a fast release.
A > R, R >= D, D >= A, does not compute.
But you can use e.g. 77 84 when you start the note, and then, after the note has reached the sustain phase, switch to an instrument with a 70 84 envelope. Then wait two more frames to get past the bug. Then, the release will be predictable and immediate. |
