[sdiy] A question about Chorus

ASSI Stromeko at nexgo.de
Sun Sep 1 09:25:48 CEST 2013


On Tuesday 27 August 2013, 20:01:20, Tom Wiltshire wrote:
> If I've got a classic chorus effect, with a delay modulated by an LFO,
> what exactly is going on?

A delay is a phase shift and a modulated delay then becomes phase 
modulation.
 
> The output from the delay line is a "vibrato" effect, e.g. the input
> signal is frequency-modulated (or the phase-modulated version anyway).

Yes.

> So can I assume that the standard FM equations apply, if I ignore the
> overall delay in the signal?

The system is only realizable when the average delay is constant and the 
instantaneous delay is always positive.  A constant phase offset doesn't 
change the result of phase modulation.  It does make a difference when you 
try to mix the original signal back in.

> I'm not especially interest in the fact that
> it's a sampled system. That's just standard nyquist stuff. I'm not
> expecting discrete maths here. Assume it's analog.
> 
> The reason I ask is that it just occurred to me that if a chorus is
> actually doing FM, then you ought to be able to mimic the effect of
> multiple delay lines by using more complex LFO waveforms.

Forget FM, think PM, it's what the whole DX line is actually doing anyway.

> Each new sine
> wave in the modulating signal will generate a pair of sidebands in the
> output, won't it? Two sines modulating one delay would be equivalent to
> two delay lines set up for the same delay time, with two sine LFOs
> modulating them. The point being that that's considerably easier to
> implement, saving you input and output filters and a delay clock and
> delay line chip.
> 
> I suppose that for a genuinely complex sounding chorus, the fact that
> there are several different delay times also helps, but it seemed to me
> that the need for more delay lines could be reduced by using richer
> modulation waveforms than the usual sine/triangle.

That general line of thinking is correct and is known as "complex LFO" 
modulation (a lot of chorus plugins use it, at least optionally).  The devil 
is in the details.  Summing arbitrary sines leads to a signal that has a 
high peak-to-average ratio (sometimes known as "crest factor") unless you 
can control the phase relationship between them.  A high crest factor 
necessitates extra headroom (in your case a longer delay line) and thus 
inefficient resource utilization.  This doesn't cost you much of anything 
for a typical contemporary digital realization, but is wasteful for analog 
implementations.  Also, you're not interested in the modulated signal alone, 
but the mix between it and the original.  Here, the fact that the average 
delay is the same for all modulation products will be showing since your 
input is not a stationary signal.


Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

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