[LAD] vu-meter DSP

[Robin Gareus]

Hi Fons,

Would you mind explaining how the vumeterdsp.cc from jmeters works?

In particular how you arrive at the filter-constant:

 _w = 11.1f / fsamp;

With the DSP using a 2nd order low-pass filter, and since VU should have
an integration-time of 300ms, I'd have though it should rather be:

 _w = (1.0 - exp(-2.0 * M_PI / 0.3 / fsamp)) / 2.0;

for large values this can be approximated by _w = 10.468 / fsamp;

What am I missing?

-=-=-=-

I suppose the gain is arbitrary, seeing as it is mapped to a GUI element
without any numeric display, anyway. But is there something special about:

  _g = 1.5f * 1.571f;

1.5 * M_PI/2 ?? ie 90 degree deflection? But why the 1.5?

-=-=-=-

The context of all this is re-using your code to implement a VU meter in
Ardour3. For reference, the source-code is at
https://github.com/Ardour/ardour/blob/master/libs/ardour/vumeterdsp.cc

Some experienced beta-testers contested the ballistics of the meter and
I'm trying to get to the bottom of it...

Since this might be of general interest, I'm CCing LAD.

thanks in advance,
robin