Electronics > Projects, Designs, and Technical Stuff
Digital Signal Processing, stop bands and transition width.
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Aethelstan:
Hi, I am currently working on a DSP assignment for my degree and I have been having quite a bit of trouble meeting the required specification. I have double and triple checked my calculations, all my normalised values seem correct, cut-off frequencies are ok, attenuation is within expected values. However, I cannot for the life of me to get my transition width to match specified or calculated values. I'm not asking for anyone to tell me how to do it, but I do have one question that I cannot find an answer to, despite searching 5 books and Google seems to be letting me down.

Is it reasonable for the transition width to be greater than half of the stop-band width? As in, would having the transition widths overlapping be a legal or sane digital filter? If so, I will delve back into it and try to figure it out. If not, then it is likely that my filter will never work to specification and I need to write about that.

Thanks :)
the_eagle:
Can you be more precise to describe your problem? What type of filter are you using? What is the sampling rate? What are the ideal parameters of your filter?

And, if you wish, can you show the calculations you have done so far?
Aethelstan:
Thanks for the reply, I am creating a band stop filter with lower cut off of 1.5kHz, upper of 3.225kHz, passband ripple of <0.003dB, a transition width of 870Hz and a sampling rate of 16kHz. I am using a Kaiser window function with beta of 8.96. Normalised values are lower cut off of 0.09375, upper of 0.2015625, transition width of 0.054375, and these are calculated by dividing the prenormalised figures by the sampling frequency. I have calculated to number of coefficients required to be 105 by rearranging ∆F= 5.71/N which is given for the Kaiser window. My concern is that the lower and upper transition areas overlap in the centre of the stop band and hence interfere with each other. I'm about to try and draw it on a graph, and will attach it shortly. Every example I have seen in the books I have and even the lecture notes have a gap in between the upper and lower transition areas.
Marco:
Yeah, if the transition bandwidth is that large you will get not get a "flat" bottom response.

Did a quick google for an example of how to design it with a flat bottom.
Aethelstan:
Cheers, that's very useful. I can use that to show that the upper stop band edge is lower that the lower stop band edge, which I think would be an invalid filter specification. :)
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