On Sat, Jul 18, 2009 at 01:46:59PM +0200, Fons Adriaensen wrote: > On Fri, Jul 17, 2009 at 07:18:30PM -0700, Ken Restivo wrote: > > > The white noise was similarly not flat: > > http://restivo.org/misc/white.png > > 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. > Awesome! That's an excellent tutorial, and your instructions worked perfectly. I did the following: japa -J & sleep 10 jack-rack -s testing & sleep 10 jack_connect jack_rack_testing:out_1 japa:in_1 jack_connect japa:pink jack_rack_testing:in_1 jack_connect japa:pink japa:in_2 And it turns out that your wonderful Moog filter indeed looks pretty close to characteristics I need for the Wah pedal. http://restivo.org/misc/japa-moog.png Thanks for writing a great tool, a great tutorial, and a great filter! -ken _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
