Olin Lathrop <olin_piclist@EMBEDINC.COM> wrote:
> Alessandro Queri wrote:
> > I'm wondering if someone of you did work on a multi-input ADC using
> > pic's integrated AD. I'd like to make a 4 channel vu-meter using
> > something like a 16f876, but I'm afraid about the ADC getting
> > multiplexed. In fact I guess the channels could interfere one to
> > another. I am also illiterate on the agument: do I have to read the
> > RMS data or just plain voltage on pin? Any suggesion is welcome.
>
> I'm assuming by "vu-meter" you mean you want to know the rough volume level
> of an audio signal smoothed out over a few 100 milliseconds or so.
>
> Low pass filter the output of a peak detector.

Actually, VU has a very precise definition:

   VU meter (volume unit) The term volume unit was adopted to refer to a
   special meter whose response closely related to the perceived loudness
   of the audio signal. It is a voltmeter with standardized dB calibration
   for measuring audio signal levels, and with attack and overshoot (needle
   ballistics) optimized for broadcast and sound recording. Jointly
   developed by Bell Labs, CBS and NBC, and put into use in May, 1939, VU
   meter characteristics are defined by ANSI specification "Volume
   Measurements of Electrical Speech and Program waves, " C16.5-1942 (which
   is now incorporated into IEC 60268-17). 0 VU is defined to be a level of
   +4 dBu for an applied sine wave. The VU meter has relatively slow
   response. It is driven from a full-wave averaging circuit defined to
   reach 99% full-scale deflection in 300 ms and overshoot not less than 1%
   and not more than 1.5%. Since a VU meter is optimized for perceived
   loudness it is not a good indicator of peak performance.

So, you need to measure the full-wave average voltage, not the peak (and
not the RMS), and you need a filter that gives you the time constant and
the precise amount of under-damping specified.

-- Dave Tweed

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