[sdiy] Chorus question

[David Moylan]

Still, I think getting a handle on the x axis warping would be easier 
with a linear frequency deviation graph.

2014 11:44 PM, Tom Wiltshire wrote:
> Sorry David, but that can't be it.
>
> The distortion in the sine wave is in the X direction, not the Y direction as you'd expect if it was caused by plotting pitch not frequency. What you're suggesting would affect both rising and falling portions of the sine wave equally, which is not the case here.
> The distortion is caused by the fact that it's not only the sample rate that changes as the signal goes through the BBD, but also the overall delay time. This is why the wave is bent in the X direction.
>
> T.
>
> On 22 Apr 2014, at 20:03, David Moylan <dave at westphila.net> wrote:
>
>> I see now that you're aware that you're using an expo clock, but my guess is that's what causes the warping of your output frequency graph.  I would try plotting a linear clock for comparison, also perhaps you could look at the output frequency in terms of pitch instead of frequency, since the use of an exponential clock was to affect equal pitch deviation.
>>
>> 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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