[sdiy] Analog bandwidth

[Andrew Simper]

If you bypass the filter and amp section of an analog synth and
generate a 15 khz saw / square then there are plenty of harmonics up
over 20 khz, but that doesn't really have much impact on design
decisions for analog modelling synths since having such high frequency
harmonics justs causes problems for audio quality and cpu usage.

Using band limited linear phase steps (BLEPs) you can keep the
aliasing inaudible for synthesis of basic waveforms (saw, pulse,
triangle, parabola) at a sample rate of 44.1 khz, but if you are going
to introduce any non-linear elements in the signal chain, or if you
support modulation of the oscillator pitch or filter cutoff then you
are better off processing at 88.2 khz minimum. This means any nyquist
problems are eased for IIR filters, and your BLEP oscillator can use a
shorter corrective grain FIR. If you implement a self oscillating
filter like the MS20 mk1 then the harmonics generated mean you will
need around x4 to x8 oversampling of 44.1 khz to prevent audible
aliasing when the cutoff is up around 20 khz, which people do all the
time with a filter.

-Andy


On 21 February 2014 07:35, Donald Tillman <don at till.com> wrote:
>
> On Feb 20, 2014, at 10:58 AM, rburnett at richieburnett.co.uk wrote:
>
>>> Obviously in theory there's no need to have much happening above 20k.
>>> But I'm wondering how true that is in real circuits.
>>
>> There is energy above 20kHz but not much.  Waveforms like Saw, Square and PWM inherent have 1/f rolloff (-6dB/oct)
>
>
> Uh...
>
> While the level of the harmonics of saws and squares roll off at 1/n, the number of harmonics doubles each octave.  So overall the power per octave is theoretically roughly constant.
>
> In practice it's a piece of audio equipment and not radio equipment.
>
>   -- Don
>
> --
> Don Tillman
> Palo Alto, California
> don at till.com
> http://www.till.com
> 650 888-9632
>
>
>
>
>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy