Return-Path: Received: (qmail 2857 invoked from network); 11 Aug 2002 22:27:19 -0000 Received: from murphys.services.quay.plus.net (212.159.14.225) by mailstore with SMTP; 11 Aug 2002 22:27:19 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: (qmail 25592 invoked from network); 11 Aug 2002 22:27:31 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from post.thorcom.com (193.82.116.70) by murphys.services.quay.plus.net with SMTP; 11 Aug 2002 22:27:31 -0000 X-SQ: A Received: from majordom by post.thorcom.com with local (Exim 3.33 #2) id 17e2s3-0004Av-00 for rsgb_lf_group-outgoing@blacksheep.org; Mon, 12 Aug 2002 01:15:43 +0100 Received: from imo-d05.mx.aol.com ([205.188.157.37]) by post.thorcom.com with esmtp (Exim 3.33 #2) id 17e2s2-0004Aq-00 for rsgb_lf_group@blacksheep.org; Mon, 12 Aug 2002 01:15:42 +0100 Received: from MarkusVester@aol.com by imo-d05.mx.aol.com (mail_out_v33.5.) id l.e6.2c69267d (4362) for ; Sun, 11 Aug 2002 18:23:00 -0400 (EDT) From: MarkusVester@aol.com Message-ID: Date: Sun, 11 Aug 2002 18:23:00 EDT Subject: LF: Noise blanking for narrowband modes? To: rsgb_lf_group@blacksheep.org MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 8bit X-Mailer: AOL 4.0.i for Windows 95 sub 72 Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: 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