This is a simple jack program to visually measure the harmonic distortion in AD/DA converters, outboard units, plugins and other kinds of stuff.
I originally worte it to test my dither routines in jackd's alsa driver.
Just run it as "demeter" it doesn't take any arguments. By default it connects to alsa_pcm:out_1 and alsa_pcm:out_1, if you want to change that, hack the source or run a jack patchbay app ;)
It should be realtime friendly-ish, but don't look to it for an shinig example of how to write a jack app, its a quick hack.
What you see
The little control window lets you vary the amplitude of the test signal and the rate the display is updated.
The graph window shows the level of the input signal at different frequencies.
The amplitude is show a the height of the black line, in decibels, relative to 1.0f, the nominal jack 0dB level. The frequency is shown in kHz as across the x axis.
The small peaks you see jumping around near the bottom of the display is the noise floor. Ideally it should be fairly level across the frequncy, but more likly it will vary.
If you can see more stable protruding peaks equally regually from the signal peak then you are seeing harmonic distortion, probably caused by clipping or overdiving. If you are testing your AD covnerters then you are diving them too hard, turn down the trim until they disappear.
- Steve Harris <steve@plugin.org.uk>, 2002-09-08