[sdiy] Chorus question

[Tim Ressel]

I remember an article in old Polyphony where they claimed a rectified sine was the best for sweeping a bbd-based phlanger. I wonder if that is also true for chorus?

 
Tim Ressel
Circuit Abbey
503-750-9331
timr at circuitabbey.com



----- Original Message -----
From: Tom Wiltshire <tom at electricdruid.net>
To: Florian Anwander <fanwander at mnet-online.de>
Cc: synth-diy <Synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, April 22, 2014 9:05 AM
Subject: Re: [sdiy] Chorus question


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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