S E T U P F O R M
C H A P T E R 4
Buffer Size
The DSP buffers can be preset separately for receive (RX) and transmit (TX), for each of the Phone, CW
and Digital modes. Selecting an operating mode on the front console will then automatically select the
corresponding (RX or TX) preset DSP buffer: Phone for LSB, USB, DSB, FMN, AM and SAM; CW for
CWL and CWU; Digital for DIGL, DIGU and DRM.
The size of the the DSP buffer, which determines the size of the FFT filter and therefore the group delay
(latency) through the digital filter. Higher values will result in more latency and sharper (“brick wall”)
filters. Lower values will allow nearly real time monitoring, but with wider filter skirts that “roll off” (as
opposed to the typical “brick wall” filters). Therefore a trade-off needs to be made and this trade-off is
often different, depending on the mode of operation.
For example if the sample rate (see Figure 75 on page 90) equals 192kHz and the DSP buffer
equals 2048 then the minimum attainable 3dB filter width equals 1.5*192000/2048 ≈ 140Hz,
where the factor 1.5 is due to the Blackman-Harris windowing function (see below).
lowering the sample rate or increasing the DSP buffer size will enable narrower and sharper
filters.
To dramatically illustrate this effect Figure 92 below shows two traces of the same 25Hz CW
filter. The red trace is at a sample rate of 192kHz and a DSP Buffer size of 512, yielding a
minimum filter width of 1.5*192000/512 ≈ 563Hz!! The blue trace at 48kHz and 4096 yields a
minimum 3dB filter width of 1.5*48000/4096 ≈ 18Hz.
The minimum audio latency is determined by the maximum of the group delay due to the DSP
Buffer and the delay due to audio buffering (see page 90). In the above example the group
delay equals 2048/192000 = 11ms. If the audio buffer is set to say 1024 then its latency
equals 1024/192000 = 5ms. In this case the overall latency is determined by the DSP buffer
size and the group delay of 11ms.
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The same filter shape is achieved for 96kHz and 1024 or 48kHz and 512.
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2003-2008 FlexRadio Systems
Figure 92: 25Hz CWL Filter at 192kHz Sample Rate/512 Buffer Size (Red) and 48kHz/4096 (Blue)
