[sdiy] Digital interpolation filtering

Eric Brombaugh ebrombaugh1 at cox.net
Sun May 31 18:23:14 CEST 2009

Interesting site.

The 'raised cosine' FIR filter design they're doing here is of the  
type I described first in my previous email. Generate a raised cosine  
frequency response & transform into time domain. As noted on the site,  
this approach is often used for creating filters used in data  
communications. The 'beta' they refer to is also known as 'rolloff  
factor' and relates to the steepness of the passband shape. This type  
of filter has characteristics that really aren't necessary for basic  
interpolation and you probably shouldn't use it unless you're doing  
data pulse shaping.


On May 31, 2009, at 9:08 AM, Tom Wiltshire wrote:

> Thanks Neil.
> I had a look at this page, and tried out the raised-cosine FIR page,  
> but a couple of things confused me:
> Beta. What the hell is beta? I'm told to enter a value between 0 and  
> 1, but no clue is given as to what this is. I expect people that  
> know don't need to be told, but it just made me feel ignorant.
> Why does the FIR impulse have to have an odd number of taps?
> Other than that, it seems to be a simple way to get a look at the  
> frequency response, which is the part I understand best when I'm  
> thinking about filters. The "truncate" to x bits option is dead  
> handy too. As I understand more what I'm doing, I expect I might  
> agree that this is a very useful page. Currently I think it's a bit  
> over my head.
> Thanks,
> Tom
> On 31 May 2009, at 16:28, Neil Johnson wrote:
>> Tom,
>> Please try out the page at York.  Here's the URL again:
>>      http://www-users.cs.york.ac.uk/~fisher/mkfilter/
>> You could start with a raised-cosine FIR filter, and you can also  
>> specify truncating the filter to 16 bit data.
>> It will give you back C code, plus graphs of responses.
>> A very useful page.
>> Cheers,
>> Neil
>> --
>> http://www.njohnson.co.uk
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