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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='color:#1F497D'>The airlines use tgone signaling on HF for the over water runs. In order to make that work they use AM for the signaling but switch to USB for the voice communications. Primarily because the accuracy of the frequency in order for the tones to decode was much more critical than what they could maintain carrier frequency. The tones depend on tuning on sideband but on AM frequency changes do not change the frequency bveing transmitted. I occasionally listen to Aircraft HF and always wondered. One of our ham club members designed the decoders used by the airlines for this so I asked him and that was his answer. I then would switch to AM when they annouceed a selcal test and sure enough the tones are crystal clear in that mode.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>For what it is worth.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Erv<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>K0OTZ<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'> <o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> aprssig-bounces@tapr.org [mailto:aprssig-bounces@tapr.org] <b>On Behalf Of </b>Robert Bruninga<br><b>Sent:</b> Friday, January 10, 2014 10:10<br><b>To:</b> aprssig@tapr.org<br><b>Cc:</b> amsat-bb@amsat.org<br><b>Subject:</b> [aprssig] DTMF on HF?<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Has anyone had any experience with any success at DTMF working on HF for some rudimentary commands?<o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal>I know tuning is critical as well as inter-tone noise must be way down.<o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal>Just thought maybe someone has experimented with it.<o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal>Bob, WB4aPR<o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p></div></body></html>