FM vs PM in GEN, unwanted artifacts & clicks

Bumping this thread, in hope someone with more DSP knowledge than me passes by

After searching in the forum i found an old thread about the very same issue:
https://cycling74.com/forums/cycle-cos-pm-vs-fm

This was 10 years ago with MSP objects but with GEN you'll get the exact same result.
I understand it has to do with sudden phase increments causing a click in the carrier but it's very annoying, because you can't implement FM feedback correctly without phase modulation.

Makes me wonder how the people who designed the old Yamaha hardwar dealt with this, or even more recent software synths (FM8 & al.)...

Hey Lionel. I looked at your patch and took the liberty to remodel it. I didn't really have the time to look at exactly what was causing the clicks but I suspect it had to do with the way the Phase Mod algo was implemented.

Here are a few guidelines on how to avoid clicks in Max from my own modest experience :
1) Always use an audio smoother if you are using control values to change parameters
2) For percussive sounds, the max envelope generators (curve, line) are perfect. However, for synth leads and pads, I tends to add a slide object just after the curve~ or line~. Usually i use the gen~ native version of the object with a value between 0.5 and 1.5 milliseconds - don't forget to translate to samples - for in2 and in3.
3) Don't be surprised if waveforms other than triangle and sine produce clicks => an audio smoother or a slide object can also help in this case

Here's the patch, tell me what you think ;)

Hi Quasar

thanks a lot for answering, appreciated.
Nice rework of the patch, i know i didn't put enough time into it cause it was meant for testing but your UI makes it a lot more usable.

I'm well aware of the problems when using control rate or even signal rate stuff, so i've already been trying out various smoothing options, either onepole LP like you do, smoothstep~ in MSP or slide in GEN. After some experiments with the curve~ & PM combo, i'd say you need about 400 sps. of sliding to get rid of audible zipper noise, which is about 10 ms worth of latency at 44 khz. That seemed a lot to me if i was going for sharp transients with a very steep envelope, but then i might be wrong.

I'm no DSP expert but i understand the difference between the 2 algos has to do with integrating vs. differentiating (ie frequency vs phase), and with quick changes in phase causing a discontinuity in the waveform. I've been trying various workarounds but as soon as you touch to the phase angle in one way or the other, you'll get clicks. So i wonder how professional programmers dealt with it in available software / hardware.
Do you think there's some param smoothing going on under the hood, or maybe some magic DSP trick only known to a happy few ?

Best regards

10ms of smoothing for the slide in and out values does seem quite high.

If you look at my version of the patch I'm only using 1.25 ms of smoothing for the gen slide object and, to my hears at least - it's the perfect balance of percussiveness. Also, the sliding time is definitely not equal to latency. Slide should actually be seen as a sort of filter which smoothes the edges by filtering them in a logarithmic way.

Another thing, usually in FM Synths, the index envelope is in some way related to an amplitude envelope. I think in your first patch there was no amp envelope at all which made the phase changes introduced by the curve envelope's transient even more noticeable I think.

Hi

i looked at your patch again and the main difference is in the amp envelope, indeed.
There's no real difference in the phase mod. implementation. If i remove or modify the amp envelope, the clicks are back.

It was part of my intention to dissociate both envelopes in order to get evolving timbres independantly of amp., but maybe i was expecting too much of this method.

Thanks again

Hi

re-upping this thread because i finally found a solution (it was given to me by someone from the kvr forum)

You just need to latch the modulator to the carrier's zero-crossings, which is very easy to do in GEN.

Just in case someone stumbles upon the same problem, here is a modified patch with various options to compare the outputs (FM, PM without declicking, PM with declicking).

Thanks a lot ! Thought about zero-crossing in granular contexts but never in this one.

Hey @lionel I didn't need the latch because im just doing 2 free running oscillators (cheers for uploading btw :D) but I liked the way you patched FM better than the built in examples. Just curious... im assuming those gen patches called 'firlp3p' are a 3 pole low pass filter, whats the cut off set to?

My patch is 2 oscillators with wavefolders that FM & AM each other and you can cross fade between each osc. Sounds pretty kewl :P very loosely based on an instruo Cs-L

Hi

which examples are you refering to ? AFAIK there's only the simple FM patch in the tutorial section, which is strictly MSP. In my opinion GEN is the way to go to implement proper phase modulation. Also worth mentioning is the modfm patch from the GEN example files (very inspiring one).

These firlp3p aren't of my own making, you would have to ask Quasar (see above). I guess you could compute the actual cutoff from the filter coefficients, but i don't think it's so important. The way i see it, these things work as moving averages, they're meant to smooth out sharp discontinuites and to avoid part of the nasty aliasing in the feedback path.

Free running oscs are cool for some applications, not so much for others. Sometimes precise phase control between the operators is key. That's where GEN comes in handy.
You can make a decent square wave using a pair of suitably dephased operators in a 2:1 ratio, see here for example:
https://www.youtube.com/watch?v=q3fQDePuaS8