Moog Music Forum

Hello,

I have my DFAM running 1 step in the sequence, with the fist step velocity full, pitch turned all the way down, and all other steps with pitch off/velocity off. I have the VCA decay and all other knobs turned off, including centering certain knobs to '0'. All effects are 'disabled'.

I expect to hear a consistent repeated beat with a slight 'click' at the start of the beat when the VCA turns on. Some beats do not have the click, and some beats do. It appears that the VCA level is higher on some beats. I can see this on the level monitor in Logic Pro. I can also hear it, but it is barely noticeable.

To reproduce the issue, I play a 1 step sequence at a high tempo, I hear a general oscillation, as though an LFO is slightly modifying the VCA. I have included a link to a wav file in my modwiggler post which demonstrates the phantom oscillation. ( Uploading wav or sound files to this group is banned )

https://www.modwiggler.com/forum/viewtopic.php?t=259789

The oscillation also appears when the wav file is viewed graphically. There is no LFO on the DFAM, and tweaking the knobs does not fix the issue.

Can anyone else check their DFAM to see if this is common to other devices?

Any help is appreciated!
Best,
Lou

Hi Lou, welcome to the forum!

Here is my guess about the moving click amplitude.

Facts:
1- When the VCA opens, it will let the oscillator signal through
2- On most of the analogue synths, oscillators are free running, i.e. they are just oscillating whether the VCA let them through or not. An exception I know is the Minitaur which is capable of restarting the oscillator phase when a note is triggered and consequenctly when the vCA opens.
3- The VCA attack time is 1ms (fast mode) or 100ms (slow mode)

Now the math!

About the variations in click amplitude: you said the oscillator is running all the way down. I think it is lower than 20Hz, but if we assume it is 20Hz, it means it will execute one oscillation in 0.05s, i.e. 50 times slower as the VCA opening (in fast mode).
It means that each time the VCA opens, i function of "when" it does in the oscillation cycle, the signal it let through can be of various aplitudes. The click is due to the high speed transition between no audio signal and a high amplitude audio signal. The larger the delta, the louder the click, hence hearing clicks with different amplitudes.

About the "phantom oscillation": I think it's a stroboscopic effect Let me try to explain that shortly: the oscillator is running low. Imagine the sequencer is running at the exact same pace. The VCA would open each time at the same "place" in the wave form (let's say the higest amplitude point for simplification sake)and you would hear all clicks with the same amplitude. Now imagine you slow down the sequencer just a bit. When the VCA opens for the secon time, it will let through the signal not at the same place as the previous time, but just before, i.e. with an amplitude a bit lower that the time before. The third time: same story... and so on. When it gets to the lowest amplitude point, it will get higher and higher again, etc. If you change the oscillator frequency (pitch) or the sequencer pace, you will notice this phantom oscillation rate changes too, as well as its apparent "direction".
So all in all, it will look like it oscillates slowly in amplitude.

I've just been looking for a video to explain that better than me and I've found a coolissimo one!! Here!

This is my guess.
Hope this helps
Sorry if the MW topic answered taht already, I've just read only a part of it.

Thanks stiiiiiiive for the awesome explanation. I think it describes exactly the behavior I'm seeing. I plan on connecting my oscilloscope to see this in action.