Re: [LAD] vu-meter DSP

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
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Received on Sun Jul 28 04:15:02 2013