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[Discuss-gnuradio] Fwd: Transition width


From: Kyeong Su Shin
Subject: [Discuss-gnuradio] Fwd: Transition width
Date: Thu, 2 Nov 2017 02:33:33 -0700

Oops, sorry, I just realized that I did not post this to GNU Radio Discussion List, and only sent this to the poster's e-mail. :)

Here's a yet another response that I sent.

Regards,
Kyeong Su Shin


---------- Forwarded message ----------
From: Kyeong Su Shin <address@hidden>
Date: Thu, Nov 2, 2017 at 2:07 AM
Subject: Re: [Discuss-gnuradio] Transition width
To: Thom L <address@hidden>


Hello Thom Lavarenne:



This is well covered on signal processing books, so I suggest digging for them. (Wikipedia page, for a quick overview: https://en.wikipedia.org/wiki/Transition_band )

Basically, real-world filters (e​specially FIR filters) do not look rectangular; Instead, it looks like the figure attached (showing a simple low-pass filter). It will start rejecting signal from freq 'f1', where the 'pass band' ends (roughly; not a precise definition). The performance of the filter will keep improve until freq. 'f2', where the stop band starts (again, roughly). Transition width is the difference between these two frequencies. In case of the figure above, the cutoff frequency is 50kHz, and the transition width is (60kHz - 50kHz) = 10kHz.

If you use a FIR filter and use a small transition width, processing speed of the program will degrade, as more filter taps are needed. If you use a large transition width, the speed of the program improves (less taps are needed), but you have to deal with the lousy frequency response of the filter. There are filters which can give you a small transition width without increasing the computation complexity, but those filters often do not have desired phase responses (you usually want to have a linear phase response; non-linear phase response).

Low Pass Filter block on GNU Radio Companion will ask you to input these parameters in frequency (Hertz), and generate a FIR filter using 'firdes' function ( https://github.com/gnuradio/gnuradio/blob/master/gr-filter/lib/firdes.cc ) . Other blocks (especially OOT blocks) may have different implementations. 

If you want to customize your filter for the best results, you can use GNU Radio Filter Design Tool. (GNU Radio Companion - Tools - Filter Design Tool).

Regards,
Kyeong Su Shin

On Thu, Nov 2, 2017 at 12:54 AM, Thom L <address@hidden> wrote:
Hello,
I can't understand what the "transition width" parameter represents when filtering in gnuradio .. And how to adjust it effectively?
Thanks 
Thomas

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