On 07/27/2013 04:19 PM, Fons Adriaensen wrote: > The actual value used is found by measuring the > result, which is the only correct way.
OK. there's that :) How did you measure it? Did you plot the output of the filter for various input signals? or fit the coefficient to some data? > [..] Many thanks for the detailed explanation! FWIW, I took a somewhat empirical approach: feed 4 VU meters with the same input, video-tape it and step the video frame-by-frame: 1) hardware from a PreSonus TubePre 2) http://www.lsraudio.com/lvlmeter.html 3) http://www.pspaudioware.com/plugins/tools_and_meters/psp_2meters/ 4) Ardour 3.3-63 (jmeters-0.4.1 code) (I first sure jmeters and ardour3's use of the code show identical data on a linux-box. ..but I did not bother to compile jmeters on OSX where the tests run using AU effects for (2), (3) ) All meters are set to -18dBFS = 0dBu, 0dBu == 0VU (1) Has an incorrect deflection to start with. -18dbFS 1KHz sine wave calibrated to 0VU; then changing the volume by +-3dB deflects the meter by about -7dB, +4dB, besides it overshoots like hell. one down. (2) rises way to fast and falls off too slow. It also overshoots maybe 10%. On the upside it has a neat reflection of my non-existent living-room windows in the meter-glass which easily makes up for that :) see http://robin.linuxaudio.org/tmp/vu-011.webm ..another one out. (3) looks generally promising - but a simple sine-sweep 30second 20Hz-20kHz at -18dBFS shows some issue: Around 5kHz, The PSP2 meter is off by -2VU. see http://robin.linuxaudio.org/tmp/vu-010.webm The VU-meter specs say "[for a sine wave] the [meter] reading should not depart from the reading at 1kHz by more than 0.2 dB from 35 Hz to 10 kHz [..]" which leaves us with... Congrats Fons. Here's a series of various sine-waves and a kick-drum sample going into (3) and (4). Interestingly jmeters' algorithm is always a tad below the PSP meter. But that is somewhat consistent with the band-pass-filter-like artefact that the PSP meter shows at ~5kHz. ie. the PSP meter emphasizes lower frequencies: http://robin.linuxaudio.org/tmp/vu-008.webm ..and some Beethoven, just for the fun of it :) http://robin.linuxaudio.org/tmp/vu-001.webm > Why such a dinosaur meter in Ardour ? You'd be better off with > a DIN PPM or a K-meter Those already in there and working just fine. Cheers! robin _______________________________________________ Linux-audio-dev mailing list [email protected] http://lists.linuxaudio.org/listinfo/linux-audio-dev
