[sdiy] SVF High Pass Mode not Cutting Off

[Magnus Danielson]

On 11/01/2011 12:14 AM, Mattias Rickardsson wrote:
> ...but the number of overtones per octave increases when going upwards -
> it doubles every octave.
> +6 dB/oct, flattening the spectrum? :-)

It doesn't flatten the spectrum, but the power density of the spectrum.

Including that effect, a -6 dB/oct slope becomes flat and a -12 dB/oct 
slope has only a -6 dB/oct power density slope.

Never the less, the behaviour of the overtone strength works with or 
against the slope of the lowpass vs. the highpass. You just don't get as 
dramatic effect on the highpass which was my point.

Cheers,
Magnus

> /mr
>
> Den 31 okt 2011 22.34 skrev "Magnus Danielson"
> <magnus at rubidium.dyndns.org <mailto:magnus at rubidium.dyndns.org>>:
>
>     On 10/30/2011 05:17 AM, David G Dixon wrote:
>
>                 Thanks for the help, I tried lower integrator cap values
>                 (220pF
>                 instead of 470pF) but without any change. I have plenty of
>
>             Freq range.
>
>                 on the pot. A sawtooth waveform is "squashed" to a flat
>
>             line as Freq
>
>                 is turned up but a very sharp spike is left at the
>                 trailing edge of
>                 the saw no matter what the VCO freq is. Any thoughts?
>
>
>             Isn't that supposed to happen?
>
>
>         I don't think so, at least not to an annoying extent.  As I said
>         before, I'm
>         pretty sure that the problem here is imperfect cancellation of
>         the LP and
>         input signals at high cutoff.  The cure is definitely to tighten
>         up the
>         resistors on the input summer, and even perhaps to make one a
>         tad smaller
>         than the other and add a trimmer in series which can adjusted to
>         perfect
>         cancellation.  The problem is that even a few mV of bleedthrough
>         will be
>         quite audible.  I had this problem with the Korgo, and we fixed
>         it (mostly)
>         with 0.1% resistors.
>
>
>     The slope of signal overtones "goes the wrong way" typically, the -6
>     dB or -12 dB slope combines well with the -12 dB slope of the filter
>     to "close" quickly, a -6 dB slope with a -12 dB slope becomes -18 dB
>     slope. However, a -6 dB slope with a +12 dB slope (high-pass slope
>     increases 12 dB per octave) creates a +6 dB slope... which closes
>     slowly, so you will have to raise the filter frequency much higher
>     to loose the same amount of total signal power.
>
>     A -12 dB slope of overtones cancels with a +12 dB slope, but the
>     level is being controlled by separation of filter frequency from
>     fundamental frequency.
>
>     So... for equivalent effect the high-pass filter needs to be steeper
>     than the low-pass filter.
>
>     Cheers,
>     Magnus
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