On 07/27/2013 04:19 PM, Fons Adriaensen wrote:
> The actual value used is found by measuring the
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
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.
(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 :)
..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:
..and some Beethoven, just for the fun of it :)
> Why such a dinosaur meter in Ardour ? You'd be better off with
Those already in there and working just fine.
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