5ms moving-average doesn't sound very right for it cuts off anything below
200Hz, no matter how much one upsamples it. However it is probably "just
fine" to subtract a DC measured 500ms ago from the current waveform because
the DC shouldn't change much in that time or it can't be DC. This
subtraction you can write as a anti-DC filter which is FIR with a big delay
on the unwanted part but no delay on the wanted part. There might be some DC
left - but probably significantly suppressed - depending on how DC-ish it
is.
Xue
-----Original Message-----
From: Domagoj Saric
Sent: Tuesday, July 31, 2012 9:45 AM
To: music-dsp@music.columbia.edu
Subject: Re: [music-dsp] DC blocking (again :)
On 30.7.2012. 20:51, robert bristow-johnson wrote:
i didn't have anything to do with the subtract-the-moving-average DC block
filter.
I apologize...at least I attributed too much rather than too little ;)
if you can put up with delay (which is what you must for a causal and
linear-phase filter), you can subtract the moving-average from the sample
in the
middle of the buffer, not the most current sample.
Well, for a live meter delay is obviously very undesirable. Taking into
account
the "laziness" of human senses, I guess up to ~5ms might be tolerable,
that's
about 220 samples@44.1kHz. If the DC filter is placed after the upsampler
(as
they seem to imply in the standard) and we upsample by a factor of 8 that
becomes ~1760 samples...would that be enough for real-world DC offset
tracking?
But, more importantly, this might not be needed at all because, as I pointed
out
in my first mail, "they" (the ITU-R and EBU standard "developers") obviously
think/imply that using a plain IIR DC blocking filter is "just fine" (and
one
would certainly expect the standard not to err in such fundamentals,
especially
considering the amount of people that worked on it). The question, again, is
how
(can it be "just fine")? Unless the answer is in the JOS link (the near
zero-phase)..?
--
Domagoj Saric
Software Architect
www.LittleEndian.com
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