[sdiy] Filter slopes

[Guy McCusker]

On Mon, 11 Jun 2018 at 10:35, Tom Wiltshire <tom at electricdruid.net> wrote:

>
> > On 11 Jun 2018, at 09:32, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> >
> > For a practical implementation hunt out the Oberheim multimode filter
> circuit:
> >
> http://www.synfo.nl/servicemanuals/Oberheim/OBERHEIM_MATRIX-12_SERVICE_MANUAL.pdf
> >
> > See page 4.
>
> While the Oberheim is definitely the original source of this idea


I don’t know the dates of the various discoveries of this idea but Bernie
Hutchins wrote an article on how to create various responses by mixing the
stages of a four pole low pass in Electronotes 85 (1978). That predates the
Oberheim implementations that I’m aware of but there could be more.

It might be worth adding that although the asymptotic slopes are always a
multiple of 6dB/Oct, the slope near to the cutoff frequencies can be
different, depending on the details of the integrator stages for instance,
and this may have an impact on the synthesis of acoustic sounds. Certainly
that was what the marketing copy of the Serge variable slope filter
implied...

There’s quite a bit of material on variable slope filters and some stuff
about their use for acoustic sounds in Electronotes too, I think around
issues 109-120 but I don’t have them to hand right now.
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