On 1.8.2012. 6:29, robert bristow-johnson wrote:
if DC is slowly varying, small displacements
of a windowed section of DC (which is what comes out of any weighted
moving-average filter) does not change it much. the difference between the IIR
vs FIR, minimum phase vs. linear phase, is just the shape of that window
function (it's the same as the impulse response)
I must admit you kind of lost me with that last sentence... :)
Unless the answer is in the JOS link (the near zero-phase)..?
you mean the zero-phase filters using truncated IIR? for DC blocking (or for
LPF metering) or whatever, using that Powell/Chau/Smith/Wang kind of
linear-phase TIIR filtering is using a cannon to kill a fly, no?
No, I mean
https://ccrma.stanford.edu/~jos/filters/DC_Blocker_Frequency_Response.html which
shows that when R=0 you get a linear phase FIR but when R approaches 1 (which is
the case in DC filters) the phase response approaches zero for frequencies not
near zero/DC.
for a moving average or for DC blocking, just use an IIR. simple and plenty
good enough. if it's done in fixed-point arithmetic, then using noise shaping
will help the filter behave itself.
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?
--
Domagoj Saric
Software Architect
www.LittleEndian.com
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