Return-Path: Received: (qmail 26646 invoked from network); 31 Oct 2002 12:06:31 -0000 Received: from warrior.services.quay.plus.net (212.159.14.227) by mailstore with SMTP; 31 Oct 2002 12:06:31 -0000 Content-Transfer-Encoding: 8bit Received: (qmail 17797 invoked from network); 31 Oct 2002 12:07:49 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: from post.thorcom.com (193.82.116.70) by warrior.services.quay.plus.net with SMTP; 31 Oct 2002 12:07:49 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from majordom by post.thorcom.com with local (Exim 4.10) id 187E4R-000757-00 for rsgb_lf_group-outgoing@blacksheep.org; Thu, 31 Oct 2002 12:05:07 +0000 Received: from [147.197.200.9] (helo=hestia.herts.ac.uk) by post.thorcom.com with esmtp (Exim 4.10) id 187E4Q-00074y-00 for rsgb_lf_group@blacksheep.org; Thu, 31 Oct 2002 12:05:06 +0000 Received: from gemini ([147.197.200.44] helo=gemini.herts.ac.uk) by hestia.herts.ac.uk with esmtp (Exim 3.22 #1) id 187DrB-0003fb-00 for rsgb_lf_group@blacksheep.org; Thu, 31 Oct 2002 11:51:25 +0000 Received: from [147.197.232.252] (helo=rsch-15.herts.ac.uk) by gemini.herts.ac.uk with esmtp (Exim 3.33 #1) id 187Dr8-0003Fr-00 for rsgb_lf_group@blacksheep.org; Thu, 31 Oct 2002 11:51:23 +0000 Message-ID: <5.1.0.14.0.20021031110602.00a96c88@gemini.herts.ac.uk> X-Sender: mj9ar@gemini.herts.ac.uk X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Thu, 31 Oct 2002 11:51:18 +0000 To: rsgb_lf_group@blacksheep.org From: "James Moritz" In-reply-to: <002b01c28043$53e25060$045bfea9@RSGB613192> References: <78.2f74dacc.2af16f83@aol.com> MIME-Version: 1.0 X-MailScanner: No Virus detected Subject: Re: LF: sound cards...well amybe if..... Content-Type: text/plain; charset=us-ascii; format=flowed X-Spam-Status: No, hits=-0.5 required=5.0tests=IN_REP_TO,REFERENCES,SPAM_PHRASE_00_01version=2.42 Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Dear LF Group, Aliasing certainly is significant in (fairly recent) sound cards - I designed my Jason TX implementation to take audio from the sound card at 5kHz, and mix/filter it up to 137kHz. When I tried this, I found a spurious output on 6.025kHz only about 20dB down on the desired 5kHz output. Changing the output frequency to 4.5kHz effectively eliminated the problem. Evidently anti-aliasing filtering is there, but not very sharp cut-off. I have observed significant aliasing effects also when using SpecLab in "software receiver" mode. As far as dynamic range goes, with the current receiver/sound card combinations used, it seems dynamic range is almost always limited by the RX IF/audio stages rather than the sound card. In conventional use of the RX, all unwanted signals are removed by the IF filters, so designing the IF/Audio section to have high dynamic range would be redundant. However, in our narrow band LF applications, unwanted signal rejection is performed in the PC, and IF/AF non-linearity will have a major effect. Effectively we are putting a high-gain, not particularly linear amplifier in front of the main selectivity of the receiver. There would be a lot of benefit in taking IF output from the receiver directly from the filter outputs, and mixing it down to audio with minimal additional gain sufficient to get the band noise above the noise floor of the sound card. I expect this would give a much more radical improvement than adding more resolution bits to the sound card ADC. Cheers, Jim Moritz 73 de M0BMU