[sdiy] A question about Chorus

[Scott Nordlund]

uhh... well that wasn't really the way I intended the message to look, sorry.

----------------------------------------
> From: gsn10 at hotmail.com
> To: tom at electricdruid.net; synth-diy at dropmix.xs4all.nl
> Date: Tue, 27 Aug 2013 19:08:15 -0400
> Subject: Re: [sdiy] A question about Chorus
>
>> From: tom at electricdruid.net
>> Date: Tue, 27 Aug 2013 20:01:20 +0100
>> To: synth-diy at dropmix.xs4all.nl
>> Subject: [sdiy] A question about Chorus
>>
>> 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
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
> Two things to note here: first, analog BBD delays (fixed delay length, variable sample rate) aren't quite the same as DSP delays (fixed sample rate, variable read pointer). Consider that it's the difference between a tape machine with a fixed playback head and varying tape speed versus a tape machine with a moving playback head and fixed tape speed. In the static case of course they're equivalent, but it's not the same when modulation is applied. In the DSP case, the pitch change is proportional to the derivative of read pointer position, but in the BBD case, it's more complicated (in a way that I don't really intuitively understand) because there's a lag between the input and output. You can easily notice this if you have a long digital delay that changes the delay time by varying the sample rate. The pitch is changed even after you stop turning the knob, because the sample rate was changing as the signal was being recorded. This can be accurately emulated in DSP if you also
> use a modulated delay write.
>
> Anyway it's the mixing of multiple detuned signals from parallel signal paths that produces the chorus effect, rather than the modulation (vibrato) itself. What you're describing seems more equivalent to several modulated delays in series, which isn't the same as several detuned signals. It might be an interesting effect (and I think I've seen a variety of patents describing similar things), but it's no replacement for a chorus with multiple BBDs or delay taps.
>
> Also remember that the classic string ensemble chorus does use complex modulation (most often 0.6 Hz sine + 6 Hz sine), but still uses 3 BBDs (with the modulation for each offset by 120 degrees).
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy