hey there - just to offer my armchair two cents on the subject...

i am definitely no electrical engineer  but to me Q, resonance and bandwidth are basically the same thing. this is commonly found in parametric EQs - there's not a lot of difference functionally between a fully parametric one band EQ (with gain/cut, frequency center and Q) as compared to something like a bandpass filter on a synth, although synth filtering is generally covering a much wider range to achieve a lot more sculpting possibilities, whereas mixer parametric bands are more limited in scope. 

the difference to me between a bandpass filter as traditionally applied in synthesis and a resonant bandpass filter is the additional setting of resonance basically. this acts as a bandwidth control which makes the peak more and more pronounced. in a synth a standard bandpass filter would usually have a single control for frequency, while the bandwidth and gain would be preset. adding a resonance/bandwidth control narrows the range of the band and eventually at high settings it can self oscillate. but the main issue is i believe the same mathematical /signal principle is involved, it's just that at low settings it sounds more subtle and high settings more dramatic.

however since synth filter design is a bit of alchemy, i personally wouldn't be surprised to see unusual methods applied in the process to create a more robust sound, like ganging up bands, or possibly offsetting the resonant peak as compared to the center frequency. i'm totally guessing but i'm curious what things synth companies do or did to create their warm, fat sounds that still sounded different from maker to maker - even later on when they were using chips like the Curtis filters.

scott

On Tue, Jan 13, 2015 at 3:02 PM, Tilo Kremer <pd@dadacafe.org> wrote:
Hello,

On 13.01.2015 04:20, Martin Peach wrote:
> I was looking at circuit diagrams for analog synthesizers recently and
> noticed that the "resonance" control is nothing more than feeding some
> fraction of the output back to the input. With more feedback oscillation
> occurs at the cutoff frequency for any type of filter, highpass, bandpass
> or lowpass.
>
> Martin

when i asked about the difference between the meaning of 'Q' in
filters regarding a parametric EQ on a mixing console and the
'Q'/Resonance knob on [analogue synthesizer] filters, i was given an
explanation along the lines of 'Q on a parametric eq will adjust the
bandwith of the filter while the Q / resonance knob on the other one
will adjust the amount of the feedback [back into its input]. Both
however will have a similar impact:
Reducing the available bandwith will lead to similar results as
increasing the amount of "data" in the system.'

hope that isn't too simplified, if it is, i recommend watching Aaron
Lantermans lectures on filters on the web.

hth,
Tee


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