26 Jul
2011
26 Jul
'11
8:33 a.m.
On Tue, Jul 26, 2011 at 12:17:18AM -0400, Jeremy Salwen wrote:
I'm interested in what you're saying about RMS
metering. Could you give me
specifics or point me to a reference? I can't find anything by googling.
What sort of low pass filter is used for audio metering, for example?
All sorts of, depends on the application. For a meter, the filter
determines how signal level variations are displayed. Simply put
how 'fast' the meter reacts to them, but there is more to it.
For example a second order filter will allow the meter to react
faster to a sudden level change while being less sensitive to
randon variations than a first order one with the same 'speed'.
Very few audio meters use true RMS, but you have the same issues
with e.g. meters that use absolute value instead, e.g. a VU. In
a 'real' VU, the filtering is done by the ballistics of the meter,
which are closely specified and quite different from those of normal
voltage or current meters. They are equivalent to a second order
lowpass filter, so if you want to emulate a VU in software you
have to implement that filter.
Just summing over N samples as in the LV2 proposal is a lowpass
filter as well, but not a very good one. It has zeros at all
multiples of sample_rate / N, and significant gain in between.
So for example if the sample rate is 48 kHz, and N is 480, the
meter will not show level variations with a frequency of 100 Hz,
200 Hz, etc, but it will be quite sensitive for others. And worse,
if N is the processing period, that means that the meter will show
different results for the same signal, depending on which Jack
period you happen to use. It doesn't affect steady signals of
course, but few musical signals have constant amplitude.
Ciao,
--
FA