[sdiy] SVF High Pass Mode not Cutting Off

[Mattias Rickardsson]

True.

On the other hand though, our hearing is used to acoustic sounds where
high overtones are already quite weak, and also get dampened further
by acoustic (HF dampening by distance, reflections, obstruction) and
psychoacoustic side-effects (masking, and dramatically higher hearing
threshold towards HF). This makes the raw sawtooth and raw square
waves sound as sharp as they do compared to real objects. I personally
experience these raw tones much more as treble than as midrange or
bass... maybe almost like a further 6dB/oct slope... triangle waves in
comparison are almost more in line with the "body" of natural sounds.
Ergo: in sound synthesis, we often start with signals that *already*
have a high-pass shelving character - which makes further high-pass
filtering appear more dramatic and further lowpass less dramatic.

Speaking of acoustic habits and psychoacoustic side-effects, lowpass
filtering also appears more natural to our ears, whereas a highpass
dampening the fundamental and lower overtones in comparison feels
very, very strange indeed. :-)

/mr

On 1 November 2011 08:49, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 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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>
>