Hey Laura, that technique sounds just fine. You're right that the
fft_wideband_real block wouldn't do it for you in this case, you'd have to
do a complex FFT. This would be pretty easy to stitch together from
fft_biplex and fft_direct modules (consider how they are stitched together
in the fft_wi
Hey Laura,
Have you tried the vanilla complex 'fft' block? If you generate the full
spectra including negative frequencies before inputting (I think there's a
mirror_spectrum block that might do this(?)), I would have thought you
could add two input streams together, as streamA + j*streamB. Since
Hi Ryan,
I have used a method of similar simplicity that involves swapping the real
and imaginary parts of samples before and after the fft, so a mathematical
equivalent of multiplying by j after taking the conjugate of the samples.
For that design I used the fft_direct block and operated only on
IIRC, an inverse FFT can be implemented as
1. Complex conjugate
2. Fft
3. Complex conjugate
Which is mathematically identical iirc to an ifft, if slightly less
efficient computationally.
In general, the output will not be real valued of course
On Tue, Dec 9, 2014, 2:45 PM Jonathan Weintroub
wro
Hi Jonathan
I wrote down the problem and immediately see the problem with inversion.
So I retract my earlier comment about straightforward inversion being
possible.
Since all methods are approximate to a greater or lesser degree, I'd be
interested to try a simple PFB would handle the inversi
Thanks to Richard and everyone who responded earlier for the comments, which in
some cases are very detailed. It is good to know we are not the only ones
worrying about this. Our DSP group is digesting the material and looking at
options, and other followup will likely follow. I did not want
Hi,
I thought I'd comment as this is a problem we've been having to deal
with recently for some VLBI observations. Fortunately we've had some
success with an offline least-squares inversion of the PFB. This is
probably not the scheme that you want, as it essentially operates on
the whole PFB'd tim
Hi Jonathan,
I thought that I sent some replies to the Casper mailing list, but it
turns out I accidentally only sent them only to Gerry Hart. So, I resend
them here.
-
(originally sent Dec 5)
Hi Jonathan,
Even discounting small numerical errors in any implantation, one cannot
get perfec
hi Aaron,
I think this depends on what you use for the filter coefficients. In
theory there are pairs of "analysis" and "synthesis" filters that let
you exactly reconstruct the original input signal. In practice I think
errors caused by quantization, overflow, etc are probably the main
sour
I think the PFB FIR is not a reversible process. It sums samples and
decimates, so that you have fundamentally lost the information that would
be required to recover the input time series. However, as Gerry points
out, it is possible to invert just the FFT component, leaving what is
essentially a
Hi Gerry,
I am glad someone is interested in this. To be clear we have a need for an
inverse PFB, but have not developed one ourselves—no where near to ready to
publish. ;) Our take was the “step by step” was needed, inverse FFT followed
by FIR, and that probably it would not be entirely tri
Hi Jonathan
This is interesting. Is an inverse PFB is just a PFB using an inverse
FFT? That should be very close by possibly not bit-perfect inversion.
Or are you considering a step-by-step inverse of the PFB algorithm? I'm
interested because there are traps. Small numerical errors are magni
Hello CASPERites,
Has anyone implemented an _inverse_ PFB? That is a block taking channelized
PFB data and reproducing the original time series.
If so, is the code/mdl/yellow block available?
Thanks,
Jonathan
13 matches
Mail list logo
