Audio Mod for the layman.

[dsetto]

Hello. I've searched trying to understand what Audio Mod does on the Prophet '08.

Does anyone want to try to explain what's happening when Audio Mod is used on a Prophet '08?

Thank you.


Audio Mod: 0...127 – Controls the amount of audio from Oscillator 1 used to modulate the filter cutoff frequency. For filter-only audio, set OSCILLATOR MIX to 127, OSCILLATOR 2 SHAPE to Off, and OSCILLATOR 1 SHAPE to the desired waveshape. This is useful for bell-like FM sounds. A wide range of sounds can also be made using AUDIO MOD with the oscillators routed normally through the filter.

[chysn]

Imagine using an LFO to modulate the filter. You probably understand that, and have a pretty good idea how that sounds.

With Audio Mod, it's just like that, except it's using the output of the first oscillator to modulate the filter.

Hints for playing around with this: Try all the waveforms for Oscillator 1. Try Audio Mod while turning the Mix entirely to Oscillator 2. Try slowly modulating pulse width for Oscillator 1. Try slowly modulating pitch for Oscillator 1. Try modulating pulse with or pitch at audio rates. There are a lot of wild timbres lurking in there.

[dsetto]

Thank you very much chysn, the LFO analogy is very helpful.  I don't have a Prophet yet, but I am very curious, and often I get lost on tangents, searching the web. (Found a great & historically interesting discussion on this in KVR, 2005.)

It reminds me of the FM concept. Osc 1 modulating the filter, analogous to a modulator oscillator modulating a carrier oscillator. Except, filter alone, not self-oscillating, is like a silent ghost?

I'm trying to wrap my head around:
A filter applied to a sounding Osc 1, whose waveshape is concurrently modulating said filter. Like oscillator feedback, one step removed?

I wonder about ...
1. I have read the filter can self-oscillate at audio rates. If you apply Osc 1 (sans volume, osc 2 shape off) to the self resonating filter, is this primitive FM?
2. Can audible self-oscillating filter be at a low pitch, like C3? (as opposed to only high zones, like C6.) I'd assume yes.

[chysn]

I'm trying to wrap my head around:
A filter applied to a sounding Osc 1, whose waveshape is concurrently modulating said filter. Like oscillator feedback, one step removed?

Not really like feedback, because the filtered signal is never fed back to the oscillator. It's a one-way trip.

I wonder about ...
1. I have read the filter can self-oscillate at audio rates. If you apply Osc 1 (sans volume, osc 2 shape off) to the self resonating filter, is this primitive FM?

It's not primitive FM, it's FM. You can get the same sort of bell-like textures. You'll never get great tracking across the keyboard, buy you can play melody lines if you set the filter's keyboard tracking carefully.

You can also get FM using LFOs at audio rates on these synths, but that's a different topic.

2. Can audible self-oscillating filter be at a low pitch, like C3? (as opposed to only high zones, like C6.) I'd assume yes.

Oh, yes. Just play around with the cutoff frequency until you get the pitch you want. Again, keyboard tracking is limited.

I've spent many many hours working on FM synthesis for my old Mopho Keyboard. It's kind of fun, but you hit the wall of capabilities pretty hard. Audio Mod's best use (in my opinion) is adding a little bit of sparkle to a sound.

[dsetto]

Thank you very much for the detailed reply, Chysn.