[sdiy] A question about Chorus

Richard Wentk richard at wentk.com
Sun Sep 1 00:13:36 CEST 2013


Nope. 

Sampling is the same as passing a signal through a sample and hold. This is not the same as ring-mod with a square wave or FM, no matter what frequencies you use, because the sampling introduces quantisation errors and resulting noise. 

Ring-modulation is amplitude modulation - i.e. instantaneous four-quadrant signal multiplication. 

FM is (more or less) phase modulation. If you do wide FM on a general audio signal, you have to do it +/- an average delay.

Just because they all add extra frequencies *does not* mean they're the same process. 

You can of course use a delay line for phase mod. That's why it's so easy to use delay lines for phasing and flanging. With a faster modulation rate and wider depth this does indeed turn into FM. 

The others aren't FM. 

BBDs are complex things. The sound of a BBD is a mix of analog quantisation slop and noise, sample aliasing, the analog filtering around the BBD, other non-linear distortions - I suspect these play a big part in the sound, as does the fact that the BBD cells are slightly leaky - and the 'pure' theoretical output of a delay effect.

I suspect there's also some frequency drift in most modulation oscillators, which adds some further detail. As does the fact that modulation sine waves are probably not all that perfect, so dm/dt is going to be more complex than a plain cos curve. 

Richard

On 31 Aug 2013, at 22:39, rsdio at sounds.wa.com wrote:

> This is a great topic.
> 
> I've never really thought of it in terms of FM, but a modulated delay such as a chorus is similar.
> 
> FM, ring modulation, and sampling are all fundamentally the same process. Each replaces the original frequencies with their sum and difference frequencies. With simple sine waves on input, you only have two frequencies, A and B, on input and two new frequencies, A+B and A-B, on output. Replacing either sine wave, or both, with a more complex signal adds more frequencies. Sampling is the product of a square wave modulation and a general audio signal, although we generally want to avoid aliasing and thus the audio signal is band-limited to be entirely below the Nyquist frequency, and the output is also band-limited. But it's basically the same thing. Note that the infinite harmonics of the ideal square wave mean that sampled audio has an infinite number of aliases, unless filtered.
> 
> A modulated delay line shifts the pitch of the original so that the output is slightly above or slightly below the input. The original frequencies are not present in the output of the BBD, but there is a mixing circuit to combine the original with the shifted frequencies to allow for detuning and phasing.
> 
> Tom mentioned that he wanted to ignore the fact that the chorus is a sampled system, but I don't think it's possible to dismiss that. If you want to use a BBD with modulation to achieve FM type signals, then you'll run into the aliasing of the sampled system very quickly. It might be possible to increase the sample rate, which will also reduce the delay, and minimize the bandwidth limitations, but I believe that a BBD can only run so fast due to the switching speed of the FETs.
> 
> For an analog system, I'd think that Ring Modulation would be an easier way to get FM-type sounds than using an analog BBD delay line with a modulated sample clock. You might need some log-linear conversion in there somewhere.
> 
> Brian Willoughby
> Sound Consulting
> 
> p.s. Ever notice that badly aliased audio sounds like its ring-modulated? That's because it's the same thing.
> 
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