A
Buffers and Sample Rate
Rather than a theoretical discourse on the consequences of Buffer Size and Sample Rate settings, this
appendix will attempt to illustrate how these settings effect both the minimum possible filter
bandwidth, filter slope or steepness and latency for each of the settings available in PowerSDR. We will
start with the filter effects and then move on to latency. Finally a little underlying theory is offered to
help gain some more insight without the need to delve into a DSP tome.
Filter Effects
The minimum filter width possible at a given DSP Buffer Size N and a Sample Rate f
s
equals 1.5*f
s
/N,
where the factor 1.5 is due to the additional roll-off due to the Blackman-Harris window function.
Therefore, for the steepest and narrowest filters we want a low Sample Rate and a high DSP Buffer
Size. Exactly how this effects the filter shapes is shown on the following two pages in Figure 170 and
171 for a 2.7kHz LSB filter and in Figure 172 and 173 for a 25Hz CW filter.
It is clear that as the Sample Rate increases and/or the DSP Buffer size decreases, the filter skirts
become less “brick-wall” and more “roll-off”, thus reducing a filter's effectiveness.
[The rest of this page has been left blank intentionally]
173 2003-2009 FlexRadio Systems
Appendix
A
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