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. Ciao, -- FA _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
