Re: Linear phase IIR filter

John Herman wrote:
In article <1147034634.336519.223150@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, "robert bristow-johnson" <rbj@xxxxxxxxxxxxxxxxxxxx> wrote:

John Herman wrote:

http://www.dspguru.com/info/faqs/fir/props.htm

for some reason, my domain name server says there is no dspguru.com .
geez, i hope that Grant didn't shut it down.

dspguru.com = 66.75.164.90

now it says that the connection was refused by 66.75.164.90. maybe
Grant doesn't like me and won't let my IP get in there.

The reason I pointed th OP tp DSP Guru was that they were talking about FIR
filters. For decimation and linear phase, there isn't an IIR filter that can
compete in terms of operation count or constant delay characteristics.

especially since an FIR can be perfectly linear phase and an IIR can't.

but, seriously John, you might want to Google "Avery Wang", "Julius
Smith", and "TIIR" and look up this thing called Truncated IIR filters.
they are really FIR but implemented with IIRs and a delay line. the
CIC would be a simple non-trivial example. at least from what i read
(and i thunk that i understood it pretty well) they can do a pretty
sharp phase linear LPF with cutoff at low frequencies for a lot less
MACs than a normal transversal FIR. might not beat "fast convolution"
using the FFT.


Greg Berchin wrote:
On 7 May 2006 13:43:54 -0700, "robert bristow-johnson"
<rbj@xxxxxxxxxxxxxxxxxxxx> wrote:

2. with all-pass filters, you can smooth out the humps, in the
group-delay (or phase-delay) response of a reasonably well-behaved
analog filter (such as a Butterworth or maybe Tchebyshev or inverse
Tchebyshev, but certainly not Elliptical),

I have in my possession a set of TTE passive analog 5th order elliptic
"delay compensated" lowpass filters that are within a few degrees of
linear phase from DC to just above the cutoff frequency (~20 kHz). I
purchased these filters for a project in 1993. I contacted TTE a few
years ago to find out how they were constructed, but they indicated that
they no longer sell the type and could not provide any design info.

Maybe one of these days I should break open the encapsulation and see
what kind of magic topology and components they used.

wow. to make such an aminal outa R's, L's, and C's seems like a
formidable task. one thing is that a few degrees of phase mean a lot
of time at low frequencies. i wonder what the group delay or phase
delay would look like.

my recollection of the group delay or Elliptical filters was the same
as that of Grover and Deller ( http://www.redcedar.com/revaes.htm )
which was something like "Drunk fly on cross-country skis in tornado".

it would be truly interesting to know the circuit, figger out the
transfer function, and see what they did.

r b-j

.

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