Re: [LAD] Calculate R M S (Alfs Kurmis)

On Sat, Feb 19, 2011 at 09:16:32AM +1300, Jeff McClintock wrote:

> > From: Fons Adriaensen <fons@email-addr-hidden>
> > On Fri, Feb 18, 2011 at 07:36:44AM +1300, Jeff McClintock wrote:
> >
> > > With a RMS VU Meter you measure a 1KHz tone as a reference.
> >
> > A contradiction... A VU does not measure RMS, whatever does measure
> > RMS is not a VU.
>
> Isn't a VU Meter a standard root-mean-square function followed by a 300ms
> integration to give it some 'weight'? ...calibrated against a 1kHz tone?

No. VU meters were used in the times when audio equipment was using
tubes (valves) so they could not use complex electronic processing,
at most an amplifier stage to drive a bridge rectifier and a passive
moving coil meter. So, ignoring the rectifier threshold, the current
driving the meter would be the absolute value of the signal, not the
square of it. The meter itself is equivalent to a second order lowpass
filter, and its response was quite strictly specified. For a steady
signal, it should rise to 99% of the final value in 300ms, and overshoot
it by 1 to 1.5% before falling back to the real value. The overshoot
isn't a detail - it has quite a marked effect on the response.
Many software 'VU' meters use a first order lowpass - this doesn't
even come close to the response of a the real thing.

Ciao,

-- 
FA
_______________________________________________
Linux-audio-dev mailing list
Linux-audio-dev@email-addr-hidden
http://lists.linuxaudio.org/listinfo/linux-audio-dev