On Tue, Jul 14, 2009 at 03:25:39PM -0700, Justin Smith wrote: 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 Io lo dico sempre: l'Italia è troppo stretta e lunga. _______________________________________________ Linux-audio-user mailing list Linux-audio-user(a)lists.linuxaudio.org http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user

On Tue, Jul 14, 2009 at 04:13:13PM -0700, Justin Smith wrote: 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

On Fri, Jul 17, 2009 at 05:43:09PM -0700, Justin Smith wrote: 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

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

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

On Fri, Jul 17, 2009 at 07:18:30PM -0700, Ken Restivo wrote: To get a flat trace for pink noise, you have to set the 'Resp' (response) control at the bottom to 'Prop' (proportional). To understand what's happening here you need to grok the following. A spectrum analyser uses a set of bandpass filters to measure signal levels at different frequencies (the implementation maybe different, but the basic idea is set of filters). There are two well-known types: - A 'linear scale' analyser uses a set of filters with a constant _difference_ of center frequency between adjacent filters, and all filters have the same bandwidth. This is what you get using a simple FFT, as used for example in Jaaa. - A 'logarithmic scale' analyser uses a set of filters with a constant _ratio_ of center frequency between adjacent filters, and the bandwidth of each filter is proportional to center frequency. This is the case e.g. for a 1/3 octave analyser. Now assume that you design each analyser so it will indicate the correct level for a sine tone at the exact center frequency of one of the filters. Then the two types of analyser will react differently to noise. The 'linear' one will indicate a flat spectrum for white noise, and a slope of -3dB/octave for pink. The 'logarithmic' analyser will indicate a flat spectrum for pink noise, and a slope of +3dB/oct for white. The analyser used in Japa is neither linear nor logarithmic. It uses a 'warped' frequency scale that is adapted to human hearing. You can see the scale if you set the 'Scale' control (bottom) to 'Warp'. With this setting, each filter corresponds to the same width on the screen. The 'Warp' control (right) will select on of three 'warp factors'. The first and default one called 'Bark' corresponds closely to the so-called 'Bark scale' used in psycho-acoustics (there's no relation to dog sounds). Since Japa is neither 'lin' nor 'log', it will normally not display a flat spectrum for either white or pink noise. Setting the 'Resp' control to 'Prop' will adjust the filter gains so you get a flat trace for pink noise. This setting is for _noise_ measurements only, it will produce wrong results for sine tones. For these use the 'Flat' setting. Measuring FR using noise can be difficult since noise by its very nature is random, and you have to select a slow analyser response to get any accurate readings. Japa allows to use a trick to make this more practical: Connect the pink noise out to the app to be measured, _and_ to one of Japa's own inputs. Connect the output of the tested app to one of the other Japa inputs. Now select the app's out on channel A, and the noise loopback on channel B, and set the display to 'A/B'. The resulting trace is the difference (in dB) of the two signals and it will move a lot less since most of the randomness cancels. Since we are using a relative measurement, the 'Resp' setting doesn't matter in this case.

Probably it's way overkill but I think I'd start with Jamin. It has good visuals, built in equalizers and limiters. you could play with sounds, learn what you think works. just my vies, Mark