[sdiy] A question about Chorus

[rsdio at sounds.wa.com]

You're probably right, at least on some points. I certainly didn't  
mean to imply that FM, RM, and sampling are all exactly the same in  
every respect.

Digital sampling does introduce quantization error, which is not part  
of FM or RM.

I disagree with your characterization of a BBD as having analog  
quantization. A BBD works by storing a voltage in a capacitor, and  
there is nothing about that circuit which forces quantization. A  
capacitor can hold any possible voltage (within its maximum limit),  
only quantized at the Coulomb level when you consider discrete  
electrons. However, any analog circuit is quantized at the discrete  
electron level, so a BBD is no more quantized than a single-pole low- 
pass filter. There is certainly error when reading the voltage out of  
a capacitor stage in a BBD, and this error is compounded 256 or 1024  
or 4096 times as the signal passes through the long brigade of  
buckets. However, this signal error is not a quantization error.  
There is no mapping of values into a smaller set, nor is there rounding.

There is utility is looking at the ideal sampling process, as if  
quantization did not occur. In that case, sampling is basically the  
same as ring modulation, at least in terms of what happens in the  
frequency domain and not the amplitude (quantization). Analog  
sampling is closer to RM than digital sampling.

Also, within small ranges, phase modulation and frequency modulation  
overlap. Of course, they do not overlap over the full range of  
possible depths of modulation.

Brian


On Aug 31, 2013, at 15:13, Richard Wentk wrote:
> 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