On 1.8.2012. 18:13, robert bristow-johnson wrote:
Since we seem to be starting to run in circles let me try to restate my main
question as simple as possible: does using an IIR DC filter defeat the purpose
of using a (linear phase) FIR (anti-aliasing) LPF in the same signal chain?
If not, why? If yes, how can it be considering that this is exactly what ITU-R
and EBU recommend for "true peak" measurement?
i dunno about ITU-R or EBU (i s'pose i could click on those links you mention),
but the old analog meters had meter ballistics that were analog. IIR filters
most closely follow analog filters than do FIR, unless you make the FIR very
long.
The new loudness standards are all about declaring those old analog meters
exactly that, old, useless and obsolete :) You might want to skim through the
papers because pretty much all public broadcasting will have to behave according
to those regulations in most parts of the world... ;)
note that when R is very close to 1, the DC block filter is almost zero-phase
for frequencies high enough. the DC blocker does nothing to frequencies high
enough because it's just an HPF.
So, as I suspected in the first post, "the answer is in JOS" (i.e. it does not
touch content not-very-close-to-DC)...thanks ;)
This however is not the end of the (to me) unclear parts of the standard
proposal:
a) the standard also recommends an optional high frequency pre-emphasis shelving
IIR filter ("zero at 14.1 kHz, pole at 20 kHz") for the "true peak" measurements
- the question you are guessing is the same as for the DC filter: doesn't that
IIR filter defeat the purpose of using a linear phase FIR filter (and/or more
importantly, doesn't the non linear phase response of an IIR filter possibly
distort the very peaks we are trying to meassure)
b) as you can see on page 18 of
http://www.itu.int/dms_pubrec/itu-r/rec/bs/R-REC-BS.1770-2-201103-I!!PDF-E.pdf
they placed both the DC and pre-emphasis filters _after_ the upsampler...isn't
that just plain inefficient?
--
Domagoj Saric
Software Architect
www.LittleEndian.com
--
dupswapdrop -- the music-dsp mailing list and website:
subscription info, FAQ, source code archive, list archive, book reviews, dsp
links
http://music.columbia.edu/cmc/music-dsp
http://music.columbia.edu/mailman/listinfo/music-dsp
