On Sat, Oct 24, 2009 at 12:16:58PM +0100, Folderol wrote:
> Very interesting information. It's fascinating how complicated something
> like a 'simple' metering system is!
Well, it's not really complicated...
'Traditional' VU meters had no active electronics
at all, just a bridge rectifier and a resistor. The
dynamic response was determined only by the mechanics
of the meter. This means that a 'real' mechanical VU
was not just a V or A meter with a different scale,
it had to be engineered to have the right response.
Now such a moving coil meter is just a classic mass +
spring + damping system, which corresponds to a second
order low pass filter. So to get the correct response
from a software VU you take the absolute value of the
signal and apply the low pass filter. For a VU the
filter was defined in the time domain: with a step
input It should reach 99% of the final value in 300ms,
overshoot to between 101% and 101.5%, and finally
come to rest at 100%. The overshoot means sub-critical
damping, so the filter is 'ringing' (but not much).
The -3dB frequency for a VU filter is around 2Hz.
The filter used in jkmeter is similar, but has exactly
critical damping, it does not overshoot, and the -3dB
frequency is around 1Hz. The other difference is that
the input to the filter is not the absolute value of
the signal but the square of it. This means you can't
direclty compare the 'speed'. For short signals (less
than 125 ms) the K meter is faster than a VU would be,
for longer ones it is a bit slower, and it also falls
back slower than a VU. If anyone is interested in the
exact time and frequency domain curves just ask off-
list.
Ciao,
-- FA _______________________________________________ Linux-audio-user mailing list Linux-audio-user@email-addr-hidden http://lists.linuxaudio.org/mailman/listinfo/linux-audio-userReceived on Sat Oct 24 20:15:01 2009
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