Surge: Blip at start of notes with keytrack modulating wavetable morph

[fgimian]

Bug Description:
When assigning Keytrack -> Wavetable Morph, higher notes have a clearly audible blip at the start of them (seemingly due to interpolation). However other synths like Hive, Zebra and Serum don't do this, they start at the appropriate wavetable position based on the note without the little blip.

Surge Version

• Version: 1.7.1.5270cfb
• Plugin Type: VST3
• Bitness: 64-bit

Reproduction Steps:
Steps to reproduce the behavior:

1. Set Oscillator 1 to WT and choose the Basic / Sine wavetable
2. Click the Keytrack target and assign it to the max amount against Oscillator 1 Morph
3. Play notes above middle C, especially audible 1 and 2 octaves above middle C
4. Note that the start of the note has a little blip

Expected Behavior:
Notes should start at the appropriate wavetable position without this blip.

Computer Information (please complete the following information):

• OS: Windows 10
• Host: Cubase Pro
• Version: 10.5.20

Additional Information:
Here's a sample preset based on the instructions above
Wavetable Problem.zip

And here's a sample using the wavetable which I was using to produce my sound
SE Wavetable Problem.zip

The goal is to obtain a sound similar to this (rendered using u-he Hive with the same wavetable and some processing).

Solo Voice Demo.zip

[baconpaul]

Alright! I have written a retest which does this and, as expected, yes, the keytrack modulator is blipping from 0 the the value across the block on the note inception. I think I can fix it from here.

@fgimian I used your patch as test data, and would like to add it to our test patches in the repo (they are not loadable in the synth only the regrets engine). If this is a problem please let me know.

Wonder what the fix will turn out to be. Must be an order of modulation thing.

[baconpaul]

Yeah I've found a fix to this. It's a real bug and it is due to a special treatment of keytrack in switch_toggled.

[baconpaul]

OK pushing a fix. Good find! Subtle bug.

[fgimian]

Thank you so much @baconpaul 😄