Re: [Discuss-gnuradio] raised cosine filter taps implementation

[Ben Reynwar]

PSK31 for ham radio uses a raised cosine filter rather than the RRC.

On Mon, Nov 14, 2011 at 9:26 AM, Tom Rondeau <address@hidden> wrote:
> On Mon, Nov 14, 2011 at 10:40 AM, Nowlan, Sean <address@hidden>
> wrote:
>>
>> Because I need a raised cosine filter. If I convolve root raised cosine
>> filter coefficients with themselves (generated with gr_firdes), do I need to
>> apply a scaling factor?
>
> Ok, but my question was Why do you need a raised cosine filter? I'm just
> curious about what applications use this instead of RRC filters?
> Tom
>
>>
>>
>>
>> From: Tom Rondeau [mailto:address@hidden
>> Sent: Monday, November 14, 2011 9:56 AM
>> To: Nowlan, Sean
>> Cc: address@hidden
>> Subject: Re: [Discuss-gnuradio] raised cosine filter taps implementation
>>
>>
>>
>> On Sun, Nov 13, 2011 at 6:56 PM, Nowlan, Sean
>> <address@hidden> wrote:
>>
>> I want to add a raised cosine filter to gr_firdes.* in
>> gnuradio/gnuradio-core/src/lib/general/. I see there's already a root-raised
>> cosine there. After looking through a few sources, I have the following
>> time-domain response of an RC filter:
>>
>> h(t) = [sin(pi * t / T_s) / (pi * t / T_s)] / [cos(pi * alpha * t / T_s) /
>> (1 - (2 * alpha * t / T_s )^2 )]
>>
>> As far as I can tell, I should be able to compute the taps using:
>>
>> for n = -FLOOR(ntaps/2) to FLOOR(ntaps/2), do
>>     h(n) = [sin(pi * n) / (pi * n)] / [cos(pi * alpha * n) / (1 - (2 *
>> alpha * n)^2 )]
>> end_for
>>
>> The things I'll have to worry about:
>> 1) if n is 0: h(n) = 1
>> 2) if n is +/- 1/(2 * alpha): h(n) = (1 / (8 * alpha) ) * sin(pi / (2 *
>> alpha) )    #I think I did this right...
>> 3) rounding errors: not sure what to do here given we're operating with
>> floats.
>>
>> Any tips/suggestions?
>>
>> Thanks,
>> Sean
>>
>>
>>
>> Sean,
>>
>> We've gotten this question before, and I'm again curious why you want a
>> raised cosine filter? If you really are using it for something, the easiest
>> thing to do is just create a root raised cosine filter and convolve it with
>> itself.
>>
>>
>>
>> Tom
>>
>>
>
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