• Miller Puckette via Pd-list pd-list@lists.iem.at [2020-03-04 16:49]:

vcf~ aims for an approximate peak gain of 1 (although this is _very_ approximate for low values of Q).

Thanks for the info Miller!

Measuring vcf~ with a Q of 5 and noise~ I do get different results depending on the center frequency. For 100Hz the filter output is 26dBRMS softer than its input signal, at 1000Hz it is 16dBRMS softer, and at 10000Hz it is 5dB softer. Raising the Q to 15 softenes all three levels accordingly.

Me, I use Q+1 to normalize filtered white noise.

Is that the way how you do it in vcf~'s code or how you would do it to normalize after bp~?

But theory would suggest the output power should be proportional to bandwidth (= f/Q) - with the bandwidth limited to Nyquist frequency - so one would divide by sqrt(min(f/Q, SR/2)) . I'll stick with multiplying by Q+1 for myself :)

I can't seem to get around the fact that sqrt(f/Q) changes with center frequency, and Q+1 does not. Is that part of the simplification?´

Shouldn't the bandwidth of the filter let the same signal enery pass regardless of center frequency?

Sorry if I skipped this part of my DSP classes...

thanks! P

Measuring vcf~ with a Q of 5 and noise~ I do get different results depending on the center frequency. For 100Hz the filter output is 26dBRMS softer than its input signal, at 1000Hz it is 16dBRMS softer, and at 10000Hz it is 5dB softer. Raising the Q to 15 softenes all three levels accordingly.

Aha - yes, my (Q+!) fix aims to allow changing Q without much affecting the perceived loudness, but doesn't account for varying the center frequency.

To loudness-balance filtered white noise, you'd want to aim to get the same signal power as the noise has in a one-bark-wide band. Above 500Hz this increases linearly with frequency. So you'd want a 10-dB increase in the signal for a 10x increase in center frequency. However, for frequencies below 500hz you'd want the result to be roughly independent of center frequency. So what you're seeing looks OK except that it should be corrected below 500 Hz.

OTOH if you want to balance with the whole of the white-spectrum noise (not just the slice that's in your local bark) then you have to go look at equal-loudness coutours (since in that case we're comparing loudnesses of sounds at different frequencies). At that point I just give up and use my ears :)

...

Me, I use Q+1 to normalize filtered white noise.

Is that the way how you do it in vcf~'s code or how you would do it to normalize after bp~?

That's how I normalize in the patch. Example (but ignore the Hilbert stuff):

http://msp.ucsd.edu/syllabi/171.20w/patches/7.e.graphing-resonant-filter.pd

...

But theory would suggest the output power should be proportional to bandwidth (= f/Q) - with the bandwidth limited to Nyquist frequency - so one would divide by sqrt(min(f/Q, SR/2)) . I'll stick with multiplying by Q+1 for myself :)

I can't seem to get around the fact that sqrt(f/Q) changes with center frequency, and Q+1 does not. Is that part of the simplification???

Shouldn't the bandwidth of the filter let the same signal enery pass regardless of center frequency?

Yeah, depends on whether you want to match power ( roughly dividing by sqrt(f/Q) for "reasonable" f/Q values) or "sound good" (as I think Q+1 does fairly well).

Also, the result for white noise isn't necessarily representative of what you get filtering real signals - all sorts of things happen then.

cheers Miller

Sorry if I skipped this part of my DSP classes...

thanks! P

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• Alexandre Torres Porres porres@gmail.com [2020-03-10 19:59]:

I've done a nice job plotting the frequency response of both bp~ and vcf~ and exposing their formulas,

Thank you! In what way will this help the makeup gain calculation?

best, P

Hi again,

On 04/03/2020 12:58, Claude Heiland-Allen wrote:

On 2020-03-04 11:18, Peter P. wrote:

...

I am trying to calculate a makeup gain factor for white noise sent into [bp~] and [vcf~] bandpass objects depending on the center frequency and the Q (width) so that it measures the same in dBRMS before and after the filter. I am currently measuring it but am wondering if b.) if there is an analytical solution to it already

For an un-normalized 2-pole bandpass filter as depicted at [1], I think the overall gain of the filter for white noise input is

sqrt ( integral from 0 to pi of |1/((exp(it) - (1-b)exp(iw))(exp(it) - (1-b)exp(-iw)))|^2 dt )

I tried it on Wolfram Alpha but it didn't succeed (time limit exceeded).  Probably there is no simple closed-form solution?

Then you need to factor in how bp~ [2] and vcf~ [3] convert f and q into w and b, as well as their custom gain factors.

I attached an example with simple numerical integration (using Pd dsp with bang to switch~), seems to work ok (but may be unreliable if q is too high), you could combine with automation for -batch and use that to generate data for surface fitting software?  I may try that later...

I think I made a mistake though: I used vcf~'s f q -> w b calculations, but now realized that maybe vcf~ has 1 (complex) pole and bp~ has 2 (a conjugate pair)?

As a bonus, the patch includes a normalized log-log spectrum graph calculated in DSP using tabsend~/tabreceive~ and order forcing.

[1] http://msp.ucsd.edu/techniques/latest/book-html/node143.html [2] https://github.com/pure-data/pure-data/blob/e2ef8158e83900734d263a3fda343a47... [3] https://github.com/pure-data/pure-data/blob/e2ef8158e83900734d263a3fda343a47...

Claude

https://mathr.co.uk