Noise when certain AMP envelope shape used and sustain=0 and note held

[williamfields]

Describe the bug
If sustain is set to 0, and the decay part of the AMP envelope shape is set in the middle (the default setting), and you trigger a note and hold it down, a low-level noise is generated. See attached for a demonstration.

Please let us know your surge version
This information is all in the menu/about screen

• Surge Version: 1.6.2.1
• Plugin type (VST2, 3 or AU): VST2
• Bits (32/64): 32

To Reproduce

1. Set sustain to 0.
2. Make sure decay part of AMP envelope shape to the middle. (default setting)
3. Trigger a note and hold it down.

Expected behavior
Volume should go to 0. No noise.

Screenshots
See above.

Desktop (please complete the following information):

• OS: Windows 10
• Host: REAPER
• Version: 6.03

Additional context
Add any other context about the problem here.

[baconpaul]

Thanks. For the upcoming 1.6.5 release I fixed quite a few things in the analog mode, including the sustain target being wrong. I'll give this a try, but it might also be fixed in the nightly already.

[baconpaul]

Oh sorry this is the digital envelope not the analog.

Well much easier to debug for sure. Will peek.

[baconpaul]

OK I now know why this happens but am not quite sure what to do

surge/src/common/dsp/AdsrEnvelope.h

Line 275 in d4e5017

l_lo = phase - 2 * sx * rate + rate * rate;

that code which is the quadratic version of the decay in digital mode has the problem that when phase -> 0 lo = hi as opposed to lo and hi bracketing the sustain. I think the answer is to make the low have a - rate * rate but I am still debugging.

But yeah this is a real bug in the implementation when sustain is zero.

[baconpaul]

OK this is fixed. Will be corrected in 1.6.5 coming in the next few weeks; and should be fixed in the nightly on our website in an hour or so. Thanks for the super accurate report. Really helped me find the problem quickly.

[williamfields]

Amazing! Thank you!