linuxaudio.org

On Wed, Sep 26, 2012 at 12:08:00PM +0100, Chris Cannam wrote:
> On 11 June 2012 18:47, Robin Gareus  wrote:

quoted text
> > 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
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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