[LAD] meterbridge 0.9.2 IEC-scale bug & fix

Fons Adriaensen fons at linuxaudio.org
Wed Sep 26 19:28:06 UTC 2012

On Wed, Sep 26, 2012 at 01:23:48PM -0400, Gene Heskett wrote:

> On Wednesday 26 September 2012 12:58:35 Fons Adriaensen did opine:
> > There is more to fix.
> > 
> > The meterbridge website still claims that those meters 'almost' conform
> > to some standards while in fact they even don't come close.
> > 
> > According to the standard, a VU measures the average of the absolute
> > value. For a steady input signal around 1 kHz, it must rise to 99% of
> > the real value in 300ms and overshoot it by 1 to 1.5% before falling
> > back to 100%. The one from meterbridge measures RMS and rises to the
> > final value in around 5.3ms, that is more than 50 times too fast (at
> > a sample rate of 48 kHz, and worse for higher sample rates).
> > 
> > According to the standards, a PPM or IEC meter must have a controlled
> > rise time of 5 or 10ms (depending on the standard). The one from
> > meterbridge indicates peak sample values instead.
> > 
> > Ciao,
> You are very correct, Fons.  However, as a BC engineer, I have observed 
> severe clipping of the waveforms because they exceeded the headroom of the 
> DA's, which in this case was _+28 dbm, while the true vu meter sitting on 
> that same line was reading -3dbm peaks.  For average loudness readings the 
> legal and pricey vu meter is fairly accurate, but it fails to detect the 
> transients in todays music, or even in tv's spoken dialog during a soap, 
> where the human ear as assaulted by the clipping cracklies the 70 year old 
> analog std meter simply doesn't detect.  To that end, the best production 
> audio boards are also equipt with the much faster LED setups that latch 
> such a peak condition excursion for at least 100 milliseconds to make it 
> obvious a peak has been exceeded.
> To that end, the operators under my control have always been instructed to 
> take it down a snudge until the overload LED at +16dbm only blinks at 5 
> second or more intervals.  Listener fatigue can be very apparent in much of 
> todays audio streams when the old 'vu' meter is peaking at _+4.
> And it shows in the ratings books when those instructions are being 
> ignored, folks might be fans of the program but something they only 'feel' 
> makes them channel hop looking for a more pleasant experience.  When 1 
> point in the ratings is as important as it is to a facilities cash flow, we 
> look for every conceivable advantage.  Bad audio hurts.

All very true. But that's no reason to claim that something that
just looks a bit like a VU meter but behaves *very* differently
actually is a VU meter. Same for the PPMs.

The code to this correctly has been available and GPL licensed for
4 or 5 years now (jmeters), so there's no excuse. 



A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)

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