[LAD] audio meters nostalgia

Fons Adriaensen fons at kokkinizita.net
Sat Apr 12 14:56:45 UTC 2008

The first (beta) release of Jmeters is available at

Jmeters is a Jack multichannel audio level meter app.
It looks very similar to meterbridge since it uses the
same pixmaps. 

This first release offers VU, PPM and stereo versions of the
same. The stereo versions have two indicator needles on the
same scale. I've never seen a twin VU, but stereo (or M/S)
PPMs *do* exist. Later releases will add bargraph meters,
digital peak indicators for the analog ones, and a stereo
correlation meter.

The main difference to meterbridge is that Jmeters has the
correct ballistics for both the VU and the PPM.

The VU meter measures the average of the absolute value of
the signal, 'average' meaning a second order filter that
reaches 99% in 300ms and overshoots between 1.0 and 1.5%. 
It is calibrated to indicate 0dB for a sine wave at -10dB 
w.r.t. digital full scale (which is +/-1 peak in this case).

The particular VU scale used is not entirely linear and 
starts at -20dB (it's one designed for a passive VU meter
with a diode bridge), so the meter will not move at all
for inputs below that level. Later versions may use a
VU scale designed for an active meter which doesn't have
this threshold.

The PPM is a pseudo-peak meter. It will indicate 80% of
the steady-state value for a 10ms burst, and fall by 24db
in 2.8s. Each scale division (1..7) represents 4dB. It is
calibrated to indicate '7' (+12 dB on the EBU scale) for
0dB FS.

For speech and music with distinct short peaks the PPM
will usually indicate higher. For music with continuous
long notes, and for heavily compressed signals the VU
indicates higher. Both meters require some 'getting used
to' in order to read them correctly.

You can modify the calibration by using the -g(ain) option, 
but it should normally not be necessary.

Enjoy !


Laboratorio di Acustica ed Elettroacustica
Parma, Italia

Lascia la spina, cogli la rosa.

More information about the Linux-audio-dev mailing list