From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Platt Subject: Re: Which tone comes first? Date: Sun, 11 Jun 2006 12:30:05 -0700 Message-ID: References: <448C6D95.2080007@sharon.esrac.ele.tue.nl> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <448C6D95.2080007@sharon.esrac.ele.tue.nl> Sender: linux-hams-owner@vger.kernel.org List-Id: Content-Type: text/plain; charset="us-ascii" To: pe1icq@sharon.esrac.ele.tue.nl, linux-hams@vger.kernel.org > With 1200bd AFSK NRZI a logical 1 is transmitted as a constant tone, and > a logical 0 as a change in tone. > I'm wondering which tone comes first when we start a sequence. Should it > start with 1200 Hz or start with 2200 Hz? Or doesn't it make any difference? It really doesn't matter. Use whichever you wish, or switch deterministically or randomly on each individual packet transmssion. At the receiving end, all "knowledge" of the actual tone being transmitted is wiped out by the first stage of the NRZI detector, which simply compares current-tone to previous-tone via an XOR arrangement. The only time it _might_ make a difference is if you're using a sideband transmission system, and want to set up a convention that the first tone burst is at one frequency or the other so as to allow for automatic per-packet fine tuning. I don't know of anyone who actually does things that way, though. On FM packet, this doesn't apply, since the modulation is symmetrical around the carrier frequency.