Buffer Size Recommendations

Transmit and Receive DSP Filtering depend on your sample rate and buffer size settings and you will directly see changes in tx/rx filter slope and latency as these settings vary. PowerSDR's internal handling of sample rate and buffer size changed significantly with version 3.2.24 (19 Feb 2015). Here is the info from the developers:

We have now also made a change in the sample rates at which the DSP internals operate. In particular, the DSP internals now always operate at the following rates:

  • Receivers: 48K for all modes except FM, 192K for FM.
  • Transmitter: 96K for all modes.

Do not confuse these "internal DSP rates" with the selectable sample rate on the Setup/Audio/Primary tab. You can still select 48K/96K/192K/384K as you previously have on the Audio/Primary tab. That selection will determine the sample rate at which the hardware returns data to the software and will determine the panadapter width as it has in the past. So, why do the "internal DSP rates" matter to you? They matter because the filters within the DSP obviously operate at the internal DSP rates and their "sharpness" is
determined by the combination of sample rate and DSP Buffer Size (see Setup/DSP/Options for buffer sizes). Recommended buffer sizes are now:

  • Receivers: >= 1024 except for, FM >= 4096.
  • Transmitter: >= 2048.

Note that before this release, if you changed the sample rate on the Setup/Audio/Primary tab, it was also advisable to change the DSP buffer size to compensate to provide the same filter "sharpness". That is no longer necessary.

Receive Buffer Size effect on filter slope


These are plots of the ANAN-100D receive passband in cw and ssb for rx buffers = 1024 (red), 2048 (yellow) and 4096 (green). These plots confirm the 1024 buffers recommendation. Increasing rx buffers to 2048 in cw will help sharpen the filtering at the cost of slightly increased latency.

CW Receive Buffer Size comparison


For comparison purposes, the Ten-Tec Orion is included. It compares with 1024 buffers. The Elecraft K3 is similar.

Transmit Buffer Size effect on filter slope


These are plots of the ANAN-100D ssb transmit filter slope for ssb tx buffers =256 (top) to 4096 (bottom). For comparison, here's the plot for my 8-pole xtal filtered Omni 6+:


Again, the recommended value of 2048 tx buffers seems good. FWIW, the Omni 6+ filtering seems to be equivalent to 2048 buffers.

Here's a close-up of the degradation in tx signal width when decreasing buffers from 2048 (top) to 1024 (bottom):


(Thanks to NR0V and W5WC)

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