Re: Frequency-response analyzer for Linux?

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On Fri, 17 Jul 2009 19:18:30 -0700
Ken Restivo <ken@xxxxxxxxxxx> wrote:

> On Fri, Jul 17, 2009 at 05:43:09PM -0700, Justin Smith wrote:
> > On Fri, Jul 17, 2009 at 4:47 PM, Ken Restivo<ken@xxxxxxxxxxx> wrote:
> > > On Tue, Jul 14, 2009 at 04:13:13PM -0700, Justin Smith wrote:
> > >> On Tue, Jul 14, 2009 at 3:56 PM, Fons
> > >> Adriaensen<fons@xxxxxxxxxxxxxxx> wrote:
> > >> > On Tue, Jul 14, 2009 at 03:25:39PM -0700, Justin Smith wrote:
> > >> >
> > >> >> Try running a white noise source through the filter, then
> > >> >> look at the spectral graph output in your favorite spectrum
> > >> >> analysis program. Since white noise should have a flat
> > >> >> spectrum, any peaks and valleys in the output are those of
> > >> >> the filter.
> > >> >
> > >> > For a more 'musical' analysis, use Japa. It has either
> > >> > a logarithmic or perceptual frequency scale. Pink noise
> > >> > source is built-in, and if you set the analyser response
> > >> > to 'Proportional' it will show a flat spectrum for pink
> > >> > noise. It can also show the difference between two spectra,
> > >> > e.g. input and output of a filter, and this produces much
> > >> > more stable displays when using a noise test signal.
> > >> >
> > >> > For the filter, you could try on of the four Moog VCF
> > >> > plugins I wrote years ago. Apart from the resonant
> > >> > lowpass they will also add some juicy distortion at
> > >> > high levels, as does the original Moog filter and
> > >> > probably all real analog Wah processors. Frequency
> > >> > control is logarithmic, which is probably what you
> > >> > want, and there are separate control inputs to
> > >> > set the static frequency and resonance and their
> > >> > modulation. One problem could be that the modulation
> > >> > inputs require audio rate signals (the filter were
> > >> > designed for use in AMS) but that can be changed
> > >> > easily if you want. If you want to play with them
> > >> > use AMS.
> > >> >
> > >> > But Japa and the filters to be found at
> > >> > http://www.kokkinizita.net/linuxaudio/downloads
> > >> >
> > >> > The filter plugins are in the MCP set.
> > >> >
> > >> > --
> > >> > FA
> > >>
> > >> I used japa years ago, but forgot its name, thanks for this
> > >> excellent piece of software, which I am excited to have
> > >> rediscovered.
> > >
> > >
> > > Mille grazie! I've got JAPA going on two platforms, but I've got
> > > to apologise for being a bit dense in figuring out how to use it.
> > >
> > > The UI looks a bit like an HP oscope, but, alas, i'm not much of
> > > an engineer, so I'm not sure to start. Is there a quick simple
> > > tutorial somewhere on how to use it? Anyone care to give me a few
> > > sentences of advice?
> > >
> > > I basically want to put white noise through some filters (prolly
> > > using JACK-RACK), and view the frequency response of it. Looks
> > > like JAPA already has a built-in white an pink noise generator,
> > > which is nice. What would I connect to what (using i.e. using
> > > jack_connect) and how would I set up the switches and control on
> > > JAPA to do this? A simple screenshot of the correct setup might
> > > be enough to get me going.
> > >
> > > Thanks again!
> > >
> > > -ken
> > >
> > 
> > Connect the pink output of japa to the input of the application with
> > the filter, connect the output of the app with the filter back into
> > japa, the curve you see in japa will be the frequency response of
> > the filter. Since japa can overlap displays, you can also try
> > putting some musical material through: one input straight into
> > japa, one through the filter first, and look at the compared
> > spectrums.
> > 
> > Another possibility would be routing the japa pink output to an
> > analog wah pedal, and comparing the frequency response of the
> > analog pedal coming back in to the digital wah emulator. Or you
> > could compare the spectral output of the real and digital wahs with
> > musical input.
> 
> 
> Thanks.
> 
> I did as you suggested, and it worked! But I'm confused by the
> results.
> 
> This is straigtht pink noise directly from JAPA's generator back into
> its input... but the frequency response appears not to be flat!
> http://restivo.org/misc/pink.png

Well, as Fons said, if you set the display to proportional you'll get a
flat response with pink noise.

> The white noise was similarly not flat:
> http://restivo.org/misc/white.png

White noise will be a straight line as well, but not flat

> So, in terms of viewing the response of the filter, do I need to A/B
> or A-B to get some kind of estimate of what the filter is doing?
> 
> -ken

Seems like there are a lot of things you can do, a lot of ways to
handle that, storing in X or Y, etc.

Regards,
Philipp
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