[hpsdr] FIR filtering and HPSDR

[Warren C. Pratt]

Hi Eric,

I thought this approach was creative; so, I read part of the paper.  As 
he notes, it's somewhat difficult to implement; however, I'm sure we 
could overcome that.  The real problem with using this algorithm with 
what we're doing on the Jetson board appears to be the amount of 
computation it requires.  If you look at Figure 6, and then project 
those lines out to, for example, 1,000,000, you can see that there's a 
huge difference in the amount of computation required versus the "large 
block" approach.

By the way, I also disagree with his position the 
direct-time-domain-convolution FIR and his approach are "zero delay."  
The delay of a linear-phase direct-convolution filter is 
Sample_rate*(N-1)/2 where N is the number of taps.  His filter approach 
will be the same.

Window functions are sometimes used in the generation of the filter 
coefficients.  I think he just didn't really discuss that process.

73,
Warren  NR0V

On 9/9/2014 10:01 AM, Erik Anderson wrote:
>
>     A random article I ran into yesterday seems like it would have
>     value here, it is focused on "zero-latency convolution" and uses a
>     hybrid of convolution and FFT calculations.  It talks about
>     overlap-keep, but if you ignore all references to ho it also is
>     creating an arbitrary-latency FFT computation by disassembling a
>     "decimate-by-frequency" FFT formula and precomputing/reusing as
>     many of the calculations as possible.
>
>     I haven't fully parsed everything in here yet, but I do have two
>     questions:
>
>     (*) Would this approach have any value in say the Jetson board
>     people have been talking about?  It could be used to create a heck
>     of a lot of overlapped FFT results.
>
>     (*) How come there is no mention of window functions in here?
>      They seem rather important for getting good outputs, but as soon
>     as we start tinkering with the inputs they no longer exist as even
>     something to struggle with.
>
>     Erik KM2G
>
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