[LAD] meterbridge 0.9.2 IEC-scale bug & fix

[Fons Adriaensen]

On Wed, Sep 26, 2012 at 12:08:00PM +0100, Chris Cannam wrote:

> On 11 June 2012 18:47, Robin Gareus <robin at gareus.org> wrote:
> > Anyway, some of you who package or copied code from meterbridge may be
> > interested in this as well.
> > [...]
> > //   def = (db + 60.0f) * 0.5f + 5.0f;  //  5.0  ..  10.0 // bug? v0.9.2
> >      def = (db + 60.0f) * 0.5f + 2.5f;  //  2.5  ..   7.5 // fix!
> 
> You're quite right -- I just double-checked the standard and your fix
> is the right one. I have cargo-culted this code into a couple of other
> places myself without ever seeing this mistake, so I'll have to fix
> those.

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,    

-- 
FA

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