[sdiy] Weird Vocoder project

[Magnus Danielson]

From: Tom Wiltshire <tom at electricdruid.net>
Subject: Re: [sdiy] Weird Vocoder project
Date: Mon, 28 May 2007 14:57:37 +0100
Message-ID: <B734C988-417F-4F53-9319-29C4CA5CBFC0 at electricdruid.net>

> 
> On 28 May 2007, at 09:29, JH. wrote:
> 
> >> Any opinions out there? Is this
> >> totally insane? Thanx in advance.
> >> Rig
> >
> > I don't know about this particular EQ, but _normally_ the filters  
> > in EQs
> > aren't steep enough for the "typical" vocoder effects.
> > May give _other_, possible interesting results, but I wouldn't be  
> > overly
> > optimistic.
> 
> A 31-band EQ is going to have 1/3 octave bands, which will require  
> fairly high Q filters. So if you _can_ build a vocoder from a graphic  
> EQ, I'd say you've started with the best one for the job, given that  
> you're unlikely to find one with narrower bands than that.

A typical vocoder analyzer channel will use a dual-resonance (4-pole) bandpass
filter where as a graphical EQ uses typically a single-resonance (2-pole)
bandpass filter. The dual-resonance peaks are spread such that you acheive an
approximation of a flat pass-band with a little responce-dip inbetween. The
slopes at the resonances are steeper than the bandpass roll-offs would allow,
and since the non-resonant gain is set to damping, those slopes is considerably
below the detection level. Those, response-wise they are quite different
animals.

> For the VCAs, I'd personally be inclined to use SSM 2024 Quad VCA  
> chips, one of the few SSMs that is still in production. In fact,  
> Juergen used these chips in his interpolating scanner circuit. 31  
> VCAs is only 8 chips, and the associated circuitry isn't complicated.

If you make a 30 band solution you have the input and output VCA for the
compander solution.

Cheers,
Magnus