#Matlab rcosfir function

Started by bogfrog 4 years ago4 replieslatest reply 4 years ago521 views
figure; plot(rcosfir(.2, 3, 12, 1/9600, 'sqrt') - rcosfir(.2, 3, 12, 1, 'sqrt'))

The difference is negligible. Why does the 'T' parameter (symbol period) seem to have no effect at all? For these two, it's literally equal:

isequal(rcosfir(.2, 3, 12, 1/2400, 'sqrt'), rcosfir(.2, 3, 12, 1/9600, 'sqrt'))



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Reply by mblokJuly 28, 2017

Notice that rcosfir generaly computes impulse response:

h(t) = SINC(t/T) COS(pi R t/T)/(1 - 4 R^2 t^2 /T^2)

at time instants t = n*T (where n is integer), which gives us

h(n) = SINC(n) COS(pi R n)/(1 - 4 R^2 n^2)

This, us you noticed, is independent from input signal sampling period. 

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Reply by bogfrogJuly 28, 2017

Thank you! Can you explain why they made it an input argument? It seems useless.

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Reply by mblokJuly 28, 2017

Not sure. Probably this resulted from continous time domain based approach to the design. Notice that currenlty, rcosdesign is recomended instead of rcosfir. The new script does not relate to continous time and sampling rate.

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Reply by napiermJuly 28, 2017

Just so you are aware, the raised cosine filter is not always the best choice for Nyquist filtering.