[sdiy] Analog bandwidth

[Tom Wiltshire]

But then the question is (and the OP's original point was) "How high do you need to go?"

It is an interesting question. Most analog synths *don't* push a lot of energy into the high end, with roll-off caps set fairly conservatively, and 4-pole filters that only open to 18-20KHz as a maximum. Although an analogue ramp might theoretically have energy out to dog-bending frequencies, it certainly won't after it's been through a 4-pole filter and past several op-amps with a 6dB roll-off.

Nonetheless, modelling those synths might be best done with some judicious oversampling, even if just because sometimes that makes life easier. For example, Richie mentioned the digital SVF. The simple implementation of this starts to become unstable at around 1/6th of the sampling frequency. At 48KHz this is obviously a serious problem, but oversampling to 192KHz makes it not an issue any longer. There are other solutions, but they'll cost you processor cycles too, and just repeating the same simple calculations a few times might be easier.

As often, it's a trade-off. You can do more complicated processing at lower sample rates to avoid the problems associated with limited bandwidth, or you can increase the bandwidth and simplify the processing, but have to do more of it. Which of those is more efficient is a difficult thing to judge before having actually written it!

T.

On 21 Feb 2014, at 19:09, cheater00 . <cheater00 at gmail.com> wrote:

> Hello,
> signals which are inaudible are very important when performing
> outright ring modulation, pitch shifting (including chorus), and when
> doing things that push supersonic information down into the audio
> band: amplitude modulation (every single processing stage in a synth),
> multiplication, non-linearities. Saying that it doesn't matter because
> you can't hear above 20 kHz is a cop out.
> 
> Cheers,
> D.
> 
> On Fri, Feb 21, 2014 at 6:56 AM, David G Dixon <dixon at mail.ubc.ca> wrote:
>>> I think that many old synths were also designed with the
>>> knowledge in mind that we can only perceive sounds up to
>>> 20kHz.
>> 
>> I can only perceive sounds up to 13kHz.  That's why my whole world sounds
>> full of warm analog goodness!
>> 
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