Return-Path: Received: (qmail 10495 invoked from network); 23 Jul 2002 16:17:12 -0000 Content-Transfer-Encoding: 8bit Received: from murphys.services.quay.plus.net (212.159.14.225) by mailstore with SMTP; 23 Jul 2002 16:17:12 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: (qmail 16527 invoked from network); 23 Jul 2002 16:16:05 -0000 Received: from post.thorcom.com (193.82.116.70) by murphys.services.quay.plus.net with SMTP; 23 Jul 2002 16:16:05 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 X-SQ: A Received: from majordom by post.thorcom.com with local (Exim 3.33 #2) id 17X2DU-0005Fu-00 for rsgb_lf_group-outgoing@blacksheep.org; Tue, 23 Jul 2002 17:08:52 +0100 Received: from hestia.herts.ac.uk ([147.197.200.9]) by post.thorcom.com with esmtp (Exim 3.33 #2) id 17X2DT-0005Fp-00 for rsgb_lf_group@blacksheep.org; Tue, 23 Jul 2002 17:08:51 +0100 Received: from gemini ([147.197.200.44] helo=gemini.herts.ac.uk) by hestia.herts.ac.uk with esmtp (Exim 3.22 #1) id 17X2DS-0005fn-00 for rsgb_lf_group@blacksheep.org; Tue, 23 Jul 2002 17:08:50 +0100 Received: from [147.197.232.252] (helo=rsch-15.herts.ac.uk) by gemini.herts.ac.uk with esmtp (Exim 3.33 #1) id 17X2DR-0005A2-00 for rsgb_lf_group@blacksheep.org; Tue, 23 Jul 2002 17:08:49 +0100 Message-ID: <5.1.0.14.0.20020723153951.00af7b98@gemini.herts.ac.uk> X-Sender: mj9ar@gemini.herts.ac.uk X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Tue, 23 Jul 2002 17:08:48 +0100 To: rsgb_lf_group@blacksheep.org From: "James Moritz" Subject: Re: LF: Sound Cards In-reply-to: <5.1.1.6.0.20020722224253.009efa70@pop.erols.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Dear Sandy, LF group, As I recall, the average sound card ADC claims about 70 or 80dB dynamic range - while I have no doubt some have much better performance, the RX IF/detector/AF sections are almost never going to be good enough to see any benefit from an improved ADC. This is understandable - as far as the designers are concerned, in a normal HF/LF communications receiver design, all the channel filtering is performed by the IF filter, and whatever signal is present at the filter output is the "wanted" signal. So all the subsequent stages of the receiver are expected to do is maintain an adequate signal to noise/distortion ratio - for communications purposes, 40dB would be more than enough. With LF narrow-band modes the situation is different - the selectivity and unwanted signal rejection is defined entirely by the software, so all circuits up to and including the sound card ADC are effectively the front-end of the receiver, and have to handle large amounts of unwanted signals and noise without distorting a weak wanted signal - not something they were ever designed to do. This is the same as the argument against add-on CW audio filters. However, in practice it usually seems to be possible to set the gain so that neither receiver nor sound card are overloaded, and the receiver noise level is only a tiny fraction of the band noise, so it probably does not matter much most of the time. Where it is a problem is when there are large unwanted signals in the IF passband - this sometimes happens here when you are trying to copy a weak QRSS signal, and there is a local station operating at the same time. During the transatlantic tests I found that with the right adjustment of receiver and sound card gain controls, along with the display parameters, it was possible to copy a weak signal 50 - 60 dB below a strong local signal, when 30s dots were being used. It would certainly be possible to design an RX with high dynamic range, low noise IF/detector/Audio stages with optimised gain distribution to complement a high performance sound card. A more integrated approach is a receiver with IF DSP, with the ADC digitising the IF signal instead of the audio. But with current LF usage, especially in the US, it would probably be a benefit only 1% of the time. Cheers, Jim Moritz 73 de M0BMU PS - even for the top-notch sound cards on the web site you mention, it still says nothing about the sample rate accuracy! At 23:11 22/07/2002 -0400, you wrote: > I have a question? How important is the sound card to the operation of > DSP software (Argo etc). I think we have proved that almost any card will > work but would we gain any thing by buying a card with a better signal to > noise ratio or better dynamic range? > This web page has test results showing many DB difference between cards > (click on the card name for details on each card). > > http://www.pcavtech.com/soundcards/summary/index.htm > > Sandy > WB5MMB > >