"Crimson_M" <crimson_m@hotmail.com> wrote in message
news:9ba2ff64.0411151405.75091349@posting.google.com...
> How can I go about compensating for FIR filter integer delay, for the
> case of M even (odd length filter)? How about for M odd? I'm trying to
> implement a FIR differentiator in Matlab, first using a Kaiser window,
> and then other windows with varying lengths for comparison.
>
> Thanks,
> Brandon

As Rune pointed out, you need to delay the unfiltered paths commensurate
with the filtered path.
If your FIR filter is linear-phase, then the imparted delay is indeed
(N-1)/2. This is nice for odd N, but, as you noticed, not so nice for even
N.
Try filtering the un-filtered path with what's called a "fractional delay
filter" that has the same group delay as your FIR filter. A fair
approximation of a fractional delay filter is a sampled SINC function:
  D=(N-1)/2;
  h=sinc((0:N-1)-D);
Where h is the FIR coefficients of the fractional delay filter.
I'm sure there are better ways to compute the coefficients that have better
edge effects (better over-all flat frequency response).
HTH,
Aj

allnor@tele.ntnu.no (Rune Allnor) wrote in message news:<f56893ae.0411160009.26702ef0@posting.google.com>...
> crimson_m@hotmail.com (Crimson_M) wrote in message news:<9ba2ff64.0411151405.75091349@posting.google.com>...
> > How can I go about compensating for FIR filter integer delay, for the
> > case of M even (odd length filter)? How about for M odd? I'm trying to
> > implement a FIR differentiator in Matlab, first using a Kaiser window,
> > and then other windows with varying lengths for comparison.
> 
> In general, you can't do much about the delay. In off-line applications, 
> you can avoid the delay by using zero phase filters. 
> 
> In on-line applications you just have to accept the delay. If you need 
> to syncronize two data streams, one filtered and one unfiltered, you will 
> have to delay the filtered stream to match up with the filtered stream.

Ouch, that didn't make much sense. Try "you will have to delay the 
UNfiltered stream to match up with the filtered stream" instead...

Rune

Dan Hensley <dan@somewhere.net> wrote in message news:<pan.2004.11.15.22.27.48.971492@somewhere.net>...
> On Mon, 15 Nov 2004 14:05:50 -0800, Crimson_M wrote:
> 
> > How can I go about compensating for FIR filter integer delay, for the
> > case of M even (odd length filter)? How about for M odd? I'm trying to
> > implement a FIR differentiator in Matlab, first using a Kaiser window,
> > and then other windows with varying lengths for comparison.
> > 
> > Thanks,
> > Brandon
> 
> Are you using filtfilt to do the filtering?
> 
> Dan

No. I was unaware of this function. However, on checking the help file
for filtfilt() it says:

    Note that FILTFILT should not be used with differentiator and
Hilbert FIR
    filters, since the operation of these filters depends heavily on
their
    phase response.

crimson_m@hotmail.com (Crimson_M) wrote in message news:<9ba2ff64.0411151405.75091349@posting.google.com>...
> How can I go about compensating for FIR filter integer delay, for the
> case of M even (odd length filter)? How about for M odd? I'm trying to
> implement a FIR differentiator in Matlab, first using a Kaiser window,
> and then other windows with varying lengths for comparison.

In general, you can't do much about the delay. In off-line applications, 
you can avoid the delay by using zero phase filters. 

In on-line applications you just have to accept the delay. If you need 
to syncronize two data streams, one filtered and one unfiltered, you will 
have to delay the filtered stream to match up with the filtered stream.

Rune

On Mon, 15 Nov 2004 14:05:50 -0800, Crimson_M wrote:

> How can I go about compensating for FIR filter integer delay, for the
> case of M even (odd length filter)? How about for M odd? I'm trying to
> implement a FIR differentiator in Matlab, first using a Kaiser window,
> and then other windows with varying lengths for comparison.
> 
> Thanks,
> Brandon

Are you using filtfilt to do the filtering?

Dan

How can I go about compensating for FIR filter integer delay, for the
case of M even (odd length filter)? How about for M odd? I'm trying to
implement a FIR differentiator in Matlab, first using a Kaiser window,
and then other windows with varying lengths for comparison.

Thanks,
Brandon