Re: [LAD] vu-meter DSP

On 07/27/2013 04:19 PM, Fons Adriaensen wrote: 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 Those already in there and working just fine. Cheers! robin