Re: [Discuss-gnuradio] Combining filters

[Marcus D. Leech]

On 08/14/2010 08:08 PM, Brian Padalino wrote:
> On Sat, Aug 14, 2010 at 7:59 PM, Marcus D. Leech <address@hidden> wrote:
>   
>> I have an application that's running on some fairly "spartan" hardware,
>> so I'm trying to find ways to make the
>>  flow-graph more computationally efficient, so that I have more
>> headroom for inevitable feature creep.
>>
>> Part of my flow-graph has a FIR bandpass filter, which is followed
>> immediately by an FFT filter.  The FIR bandpass is simply
>>  to define my passband of interest (I bring in 1MHz, but only need/want
>> 700KHz in the middle), and the FFT filter is designed
>>  to allow me to dynamically notch-out narrowband interferers.   Is
>> there an efficient way to combine these two into a single
>>  filter, that will be more computationally efficient than two filters
>> in series?
>>     
> You should be able to convolve the time domain taps of both filters to
> achieve the cascaded performance of the two filters in series.
>   
Oh, I knew somebody would say "just convolve 'em", but that yields a
filter that's
  computationally the some order as the two in series, give or take.  Sigh.


> I am not sure this would really yield any better computational
> results, but it's the easiest thing that comes to mind.  Switching to
> fixed point for your filtering may be the best bang-for-your-buck as
> long as you don't need a massive amount of dynamic range.
>
>   
My impression is that floating-point performance these days, on *86
hardware,
  is generally better than integer, particular with the SIMD floating-point
  instructions.  The particular platform is Atom based, a D510, which is
  a dual-core CPU running at about 1.7GHz.  The current app is taking up
  about 65% of the combined CPU, and I just want to get a little more
headroom.

> I'd be interested to hear what solution you come up with.
>
>   
Me too :-)

Actually, something I'd thought of would be to treat the "edges" as
multiple contiguous
  notches, and run that in the FFT filter only, and eliminate the FIR
bandpass filter.  But I'm not sure
  I'll get really good stop-band attenuation that way.


-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org