[sdiy] Chorus question

[David Moylan]

It doesn't look like your VC clock is linear.  The center frequency is 
20k, upward deviation is to 40k, downward is to 10k.  For a linear clock 
you would expect equal frequency deviation.  A linear clock would 
probably provide an easier analysis, especially initially.

On 04/22/2014 09:17 AM, Tom Wiltshire wrote:
> Ok, here's a link to what I've got so far:
>
> http://www.electricdruid.net/ChorusStudy.html
>
> You can see the way a simple sine modulation gets bend out of shape by the changing delay.
>
> T.
>
> On 22 Apr 2014, at 17:05, Tom Wiltshire <tom at electricdruid.net> wrote:
>
>>
>> On 22 Apr 2014, at 16:19, Florian Anwander <fanwander at mnet-online.de> wrote:
>>
>>> Am 22.04.2014 17:07, schrieb Tim Ressel:
>>>> What is the best waveform to use on the LFO driving a chorus delay? Triangle? Sine? Other?
>>> Triangle
>>>
>>> Chorus is based on the detune effect that happens when you speed up / slow down a delay time. If you are using a sine, then the speed up will decrease around the peak and detune will get less there.
>>
>> I did some analysis of this recently(ish) using a BBD simulation and it isn't as simple as I first thought. I assumed that if I fed a simple signal like a VCO through a chorus unit with a sine LFO, I'd get a simple sine LFO vibrato. But you don't. Likewise triangle - in theory, you'd get a square LFO vibrato, with two different pitches, one for the rising slope, and one for the falling slope. But you don't.
>>
>> Because the samples that are being sped up/slowed down when you read them out were *also* read in at a (different) varying rate 10-20 msecs or so ago, you're detuning something that is detuned already. The net effect is that sine waves finish up noticeably non-sine. I think this depends on what fraction of the LFO period the signal is being delayed for - consider the case where the overall signal delay is exactly one LFO period; do you get any modulation? Perhaps then you get the shape you expect - I'm not sure yet. Since the effect of the LFO is to change the delay time, and therefore the portion of the LFO period that represents, it's a moot point anyway.
>>
>> Another effect to watch out for is that most chorus clocks use a linear (Hz/V) CV, so the depth of the effect (in semitones) varies as the LFO rises and falls. This can be corrected by using a Oct/V oscillator for the clock, but that alone doesn't sort everything out.
>>
>> I'd post a link to my workings-out, but I haven't got it straight in my head yet. I just wanted to wave a warning flag over this though, because I discovered that most of what I thought I knew was wrong (not for the first time!).
>>
>> Tom
>>
>>
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