[sdiy] A question about Chorus

[John Ames]

I'd never thought of sample aliasing that way before, but dang if that
isn't a good point.

On 8/31/13, rsdio at sounds.wa.com <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.
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>