[hpsdr] FFT latency

[Scott Traurig]

Hi Warren,

Does that mean the high latency, linear phase FIR filters as currently
implemented in openHPSDR as the "DSP Phone Buffer" et al are going away in
the new implementation?

Thanks & 73,

Scott/w-u-2-o

On Sun, Apr 5, 2015 at 1:08 PM, Warren C. Pratt <warren at wpratt.com> wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
> Excellent comments.  There are examples of various types of filtering in
> the wdsp library we use.
>
> When a FIR filter exceeds somewhere around 50 to 100 taps, it is more
> computationally efficient to implement it using FFTs.  The reason being, if
> N is the number of taps, that using FFTs the computation grows as N log2 N
> where implementing in the time domain the computation grows as N^2.  Our
> main bandpass filter is implemented as a linear-phase FIR, implemented with
> an FFT/iFFT in an overlap-save configuration.  No "windowing" of the data
> is required in this case and we use an overlap factor of 2.
>
> The new noise reduction algorithm, NR2, is a frequency domain algorithm,
> also implemented with FFTs.  In this case, we perform an FFT, multiply each
> frequency bin by a calculated gain value, and perform an inverse FFT.
> Since the gains are time varying, we window the data both before the FFT
> and after the iFFT.  For various reasons, an overlap of four is being used.
>
> In contrast, for the APF (audio peaking filter), and in a few other
> places, IIR filters are used.
>
> 73,
> Warren  NR0V
>
>
>
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