Path: utzoo!attcan!uunet!husc6!cmcl2!nrl-cmf!ames!killer!netsys!cucstud!c3pe!c3engr!charles From: charles@c3engr.UUCP (Charles Green) Newsgroups: comp.dcom.modems Subject: Re: 9600 baud over high-quality audio channel Summary: Schmitt trigger recovers edges from ac coupling Keywords: Schmitt trigger Message-ID: <2856@c3engr.UUCP> Date: 27 Jun 88 04:49:37 GMT References: <17507@glacier.STANFORD.EDU> <5063@videovax.Tek.COM> <189@cbw1.UUCP> Reply-To: charles@c3engr.UUCP (Charles Green) Organization: C3, Inc. Engineering Division, Herndon, VA 22070-5201 Lines: 37 In article <189@cbw1.UUCP> brian@cbw1.UMD.EDU (Brian Cuthie) writes: >In article <5063@videovax.Tek.COM> bill@videovax.Tek.COM (William K. McFadden) writes: >>This might seem too obvious, but why not connect the output of your RS-232 >>port directly to the input of your audio channel (using suitable attenuation >>and DC blocking)? At the receive end you would probably want some sort of >>active filtering and schmitt triggering to recover the data. The problems I >The first problem I see with this is that the audio path is not DC coupled >and thus you would have big problems with data that was all marking. Now if I would expect his Schmitt trigger to take care of that; data that was all marking would be seen as approaching 0 volts at the receiving end. The trigger will fire only on the edges of the input waveform, as the output voltage exceeds the hysteresis. For example: +V --- --- --- --- --- Input 0V (data all marking) -V --- -------------------------- --- --- --- _ ++V -- _ +V -__ -__ (ac coupling goes to 0) -- -__ __ Output 0V ____------------------- __ Threshold +V _-- _-- - _-- _-- -- Note that after the marking condition, you have a higher-than-normal positive voltage, but provided that your receive threshold is correctly set, you should still be okay, although noise certainly needs to be dealt with. >you MFM encoded the data you might get by, but I doubt it. This is why >people invented modems for phone lines (audio channels -- right ?). I would expect that the technique of locking onto a frequency-shift signal (a la Bell modems) to be relatively immune to noise, as is FM ("No Static at All!"). -- #ident "@(#).signature 1.5 :~charles/s..signature 04/27/88 21:40:43" TwoBitNet: 460 Herndon Pkwy, Herndon, VA 22091-1928 UUCP:decuac!c3*!charles