[Lf] [Fwd: LF: Noise blanking for narrowband modes?]

[Andre Kesteloot]

MarkusVester at aol.com wrote:

> Hi Alberto, Jim and LF group,
>
> oh I just love those rainy sundays ;-). - It's been interesting to read about
> the improvements of Jason decoding with hard-limited audio. In times of heavy
> QRN, I have often observed a significant improvement of weak QRSS signals
> when connecting a pair of 1N4148 diodes parallel to the PC's line input. My
> RX has full SSB bandwidth and no AGC. The best performance occured when the
> frontend attenuator was set such that normal background noise was just below
> the threshold of the limiter. Driving the system into hard limiting did not
> seem to help any more, but tended to show blocking and IM, caused by strong
> in-band carriers and DBF39.
>
> Narrowband reception goes along with long FFT integration times, and in a
> linear system, the effective noise level is given by the total energy of
> background and burst noise. If for example the QRN covers 10% of the
> aquisition time with a level of 30 dB above the background, it would look
> like 100-fold increased Gaussian noise, a desastrous 20 dB SNR loss. If we
> however clip the QRN at say 6 dB above background, the average noise energy
> will be increased by only 40% (1.5 dB). And the signal will virtually
> disappear during 10% of the time (-0.8dB), adding up to only 2.3 dB SNR
> degradation.
>
> But I am wondering, if not a noise-blanking scheme would be more effective
> than simple limiting. In a software implementation with some buffering, one
> could even avoid switching clicks by smoothly reducing the gain around each
> burst (eg. a 10 ms cosine shape), keeping the spectral widening of strong
> carriers moderate. The user could preset a fraction of blanked time (e.g 10%,
> dependent on the severeness of the QRN), and the SW could then automatically
> control the threshold to approach that.
>
> Going one step further, we could try to give an optimal estimate of a signal
> in the presence of a time-variing noise level. In analogy to a Wiener filter,
> the noise contribution of each sample (ie. 10 ms block) to the FFT input
> would then have to be inversly proportional to its SNR. If the noise level
> went up temporarily by 3 dB, we should reduce the gain by 6 dB. For high
> noise peaks, this will of course result in almost complete blanking. One nice
> feature of the scheme is that we don't have to worry about a threshold, all
> we have to do is keep track of the wideband power from the RX (assuming that
> this is noise and our weak signal is below that), and dynamically control the
> gain.
>
> 73 es gn
> Markus, DF6NM