S E T U P F O R M
C H A P T E R 4
Therefore, you should set your Audio Buffer as low as your computer system will tolerate at the
set Sample Rate. Next you should set your DSP Buffer as high as you can, without experiencing
noticeable latency. Finally, for the narrowest (CW) filters you may need to lower your Sample
Rate further, especially in extreme conditions such as contests.
Noise Blanker
This controls the detection threshold for impulse noise. If a signal sample exceeds this detection
threshold, the sample will be set to zero and the filtering in the radio serves to interpolate through this
zero sample. This noise blanker is identical in theory to those in traditional radios. The detection
threshold in our noise blanker has the unique feature that it is signal strength dependent. This enables
it to function properly at all signal levels.
This control is preferable when the spikes are very large in comparison to the average signal. However,
when the spike is smaller,
Noise Blanker 2
provides a much cleaner reconstruction of the signal since
the signal is more likely to look like the mean. For this reason, the Noise Blanker 2 threshold should
always be about four or five less than the Noise Blanker threshold.
Noise Blanker 2
This controls the detection threshold for a pulse. If a signal, pulse or not, exceeds this detection
threshold, the sample will be replaced by a computed estimation of what the signal sample should have
been given an interpolation of the signal samples around it in time. By replacing the noise pulse with an
interpolation of the signal, distortion is greatly reduced over that of traditional noise bankers.
When seeing a significant amount of impulsive noise, being too aggressive with
Noise Blanker
(
NB
)
can damage the signal. However, completely removing the large pulses is desirable prior to operating
the smoother acting
Noise Blanker 2
(
NB2
). Therefore, when seeing many repetitive noise pulses, it
is probably best to use both NB and NB2. The NB Threshold is adjusted to just begin to lower the noise
from the pulses, after which NB2 is turned on, with a threshold of four or five less than that of NB. Both
together can spectacularly reduce impulse noise, resulting in increased intelligibility of the signal under
severely adverse conditions.
Window
This control selects the DSP windowing function that will be applied to the power spectrum in the main
display when using Spectrum, Panadapter, Histogram, and Waterfall displays. The default is Blackman-
Harris, which is the best setting for many high-level signal measurement needs. The purpose of the
windowing is to diminish bleed-through to adjacent FFT “bins” which results from a tone that is not
exactly on the center frequency of one of the “bins” (or parallel filters) in the power spectrum
calculation. The bleed-through is caused by using the Fast Fourier Transform (FFT) to calculate the
power spectrum, which we need to use for the sake of efficiency. That said, it is important to
understand that the FFT writes the data (mathematically speaking) on a circle and not on a line. When
the last sample meets the first sample on a circle, it is very probable that it will not meet up or join in a
continuous fashion. This discontinuity acts in exactly the same manner a key click causes a wide
spectrum. The window is used to mitigate this key click-like phenomenon. The Rectangular Window
bleeds through the worst. The best in our selection is the Blackman-Harris Window, which bleeds
through the least, but at a penalty of a slightly reduced spectral resolution (=wider filter). Appendix D
describes in more detail several of the window functions available in PowerSDR.
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2003-2008 FlexRadio Systems
