Re: [Discuss-gnuradio] Coherent de-dispersion

[Lamar Owen]

On Thursday, July 01, 2010 11:35:13 am Marcus D. Leech wrote:
> Oh, I agree that it's in the tradition of Gnu Radio filters.  It's just
> that after you've finished reading
>   paper after paper about FFT filters for de-dispersion, with everything
> being expressed in the
>   frequency domain, it's a bit jolting to find that the FFT filter takes
> "taps" in the time-domain.

The old way with hardware was a massive filter bank; we have an old hardware 
de-dispersion machine here that isn't working at the moment.  Given that 
dispersion produces a 'chirp' from a pulse, de-dispersion is essentially 
de-chirping, and providing programmable delay per band.

But a low DM pulsar shows up very well on a waterfall display.

Or to put it exceptionally bluntly, the frequency domain doesn't really exist 
anyway, as all signals are in the time-domain to begin with.  Even hardware 
filters act on the time domain signal to produce a frequency domain result, 
whether they are lumped constant LC or RC filters or distributed constant 
cavities.  

But yes I sympathize with your mind-bending there, Marcus.  Much like the 
mind-bending required when dealing with audio feedback; it doesn't seem to make 
sense that the amount of feedback doesn't change the amplitude of the end 
oscillation, but rather the amount (and phase) of feedback determines the 
frequency of the oscillation.  And due to the acoustics of some venues, there 
is actually a 'sour' spot on the volume level where the feedback is worse, and 
goes away when the level is turned up (not down, but up).  Counterintuitive, 
but definitely a real effect.