Why does my modulation disappear?

Feb 18, 2012 at 2:43pm

Why does my modulation disappear?

Hi,

Just messing with gen~ and I tried to make a cross fm patch – pretty simple but when I put it at certain settings the modulation disappears. I am not sure why. Would somebody be able to explain why it happens. There is an example below.

– Pasted Max Patch, click to expand. –
#61857
Feb 22, 2012 at 3:24am

Feedback FM is very hard to predict… but it may be that at low frequencies the interaction between the oscillators never totally gets over the hump so to speak. Hmm, not a very good explanation, sorry… but the point I’m trying to make is that this is an expected(-ish) result of the mathematics – there’s nothing wrong with your patch… :-)

#223370
Feb 22, 2012 at 5:33pm

Looking at the first-order bessel function for a simple (not feedback) FM pair, you can see some negative amplitudes:

https://ccrma.stanford.edu/software/snd/snd/fm.html

I’m guessing this could have something to do with it (the phase relations of the partials).

Some things to investigate:
- Is it the range around 0? (maybe try some sort of inverted clip in a neighborhood around 0?)
- Is it negative frequencies? (use abs to rectify the modulators)
- What about exponential feedback FM, where there’s no through-zero?
- Could you look for low output via some sort of envelope following and goose it back into life by injecting energy into it temporarily? (This is probably by far the weirdest, but could be pretty interesting)

Both of those solutions will change the spectrum, but since you’re interested in feedback FM, I’m assuming a little mayhem is okay…

#223371
Feb 22, 2012 at 10:11pm

thanks Graham – good to know I am not completely missing the point.

Peter the sound I had in my mind was cross FM like on an analogue synth. But of course that is not thru zero so I didn’t think about the phase cancellation coming back up.

Thanks for answering – now to turn them into wavetables for even more mayhem ;)

#223372

You must be logged in to reply to this topic.