[sdiy] A question about Chorus

[John Ames]

There's no fundamental difference between an audio-frequency signal
and a low-frequency signal, other than the limitations of our ears, so
there's no reason modulating a signal's frequency with a low-frequency
modulator would *not* produce FM-style sidebands (at the carrier
frequency plus or minus multiples of the modulator frequency.) I would
guess that the reasons we don't really hear them the same way are that
(A) the modulation depth is typically quite low, so the sidebands are
very quiet, and (B) with a low modulator frequency, the sidebands
would be very closely spaced around the carrier frequency, which
(combined with the rapid sideband roll-off thanks to the low
modulation depth) would mean that there's very little in the way of
audible sidebands by the time you get out to even the second harmonic
of an audio-frequency carrier. On synths where you can crank up the
vibrato depth good and high, you actually can start to hear
ring-mod-style effects which would be the first couple sidebands
produced by the frequency modulation.

On 8/31/13, Mattias Rickardsson <mr at analogue.org> wrote:
> Isn't the modulation frequency way too low to let the system be seen
> as a kind of FM synthesis with sidebands and all?
>
> But I agree it would be a neat trick to fake more delay lines. :-)
>
> /mr
>
>
> On 27 August 2013 21:01, Tom Wiltshire <tom at electricdruid.net> wrote:
>> Hi All,
>>
>> If I've got a classic chorus effect, with a delay modulated by an LFO,
>> what exactly is going on?
>>
>> The output from the delay line is a "vibrato" effect, e.g. the input
>> signal is frequency-modulated (or the phase-modulated version anyway).
>>
>> So can I assume that the standard FM equations apply, if I ignore the
>> overall delay in the signal? 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. 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.
>>
>> Thanks,
>> Tom
>>
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