Vocoder concept - crazy idea really

frequencycentral · Joined: May 25, 2008 Posts: 186 Location: UK

Ok, so recently I've been thinking about building/designing a minimum vocoder. My original idea was to make it managable by using a minimum number of bands, maybe as few a 4. But today I had a crazy idea........instead of multiple fixed BPFs, multiple envelope followers and VCAs........how about a multiplexing vocoder? So, only two BPFs, one carrier and one modulator - but they are both voltage controlled and swept simultaneously through the same frequency ranges by an oscillator. The oscillator would have to be running pretty fast to get any sort of useable 'sample rate' though. Here's a block diagram:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

....all you have to decide is how fast the LFO needs to run to get to a workable resolution. In fact it won't be an LFO really, it'll have to be at least at the lower end of the audio range. Here's the rub - at those frequencies, the oscillator is certainly going to 'bleed' into the vocoder's audio path. I have used audio frequency oscillators as control voltages into VCFs on my modular system - the result is FM, which is not desirable here. So, any way to 'null' the oscillator? Maybe stick it through an inverter and mix the output into the audio path so that phase cancellation obliterates the bleed?

Hmmm, maybe it's actually a cross between a vocoder and a sample rate reducer? Anyway, if the challenges can be overcome it could be built with a couple of LM13700 and maybe a dozen opamps.

emdot_ambient · Joined: Nov 22, 2009 Posts: 667 Location: Frederick, MD

I have no clue what that would end up actually sounding like but...give it a try!

And remember, audio samples or it didn't happen Wink

Mongo1 · Joined: Aug 11, 2011 Posts: 411 Location: Raleigh NC

I'm not sure what that's going to sound like, but it's not going to be a vocoder.

You're sort of trying to do what is done in a DSP, processing multiple bands with a single signal chain. The thing is, a DSP can keep track of the context of each band separately. When it needs to sample the 1K band, it pulls out a bunch of context values, does the calculation, stores the output, and then stores the context for later. It then pulls the context info up for 2K (or whatever) and does it again. It can do that simply because it's fast enough and smart enough to keep everything separate.

Doing that with audio components won't work. There's no good way to keep all the information separate, so what you'll end up with is kind of a mush of information. I suspect it will sort of be a mix of the audio signal and the LFO frequency.

Sorry to pee in your chile.

Gary

Clack · Posted: Tue Oct 04, 2011 1:37 pm Post subject:

Its a good idea, I love minimizing

I imagine some kind of ringing would be impossible to avoid if the LFO is at audio rate, you cant subtract it because it will not be at the same frequency because modulation creates sidebands. Also I think a vocoder needs to have all bands on at the same time?

Would it not be possible to do it at above audio frequency? say 10x so it samples the highest possible sound 10 times in a cycle - the only problem is can BPF's sweep that fast? all sorts of phase problems?

Maybe look into switching filters? binary controlled filters? MHz switching between frequencies?

Why don't you look into making a channel as cheap as possible? FET VCA? CMOS inverter band pass filter?

fonik · Posted: Tue Oct 04, 2011 2:28 pm Post subject:

what about fixed CV values above audio rate? i.e. using a DAC?

[edit]: i don't care how you will call it! vocoder or not, that does not matter at all. try your idea and see where it gets you (and us, of course)....