Path: utzoo!attcan!uunet!husc6!mailrus!ames!umd5!umbc3!cbw1!brian From: brian@cbw1.UUCP (Brian Cuthie) Newsgroups: comp.dcom.modems Subject: Re: 9600 baud over high-quality audio channel Keywords: modem Message-ID: <190@cbw1.UUCP> Date: 2 Jul 88 21:27:44 GMT References: <17507@glacier.STANFORD.EDU> <5063@videovax.Tek.COM> <189@cbw1.UUCP> <403@trwind.UUCP> Reply-To: brian@cbw1.UMD.EDU (Brian Cuthie) Organization: CBW, Columbia, MD 21046 Lines: 53 In article <403@trwind.UUCP> johng@trwind.UUCP (John Greene) writes: >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: >>> >>>> I'd like to transmit 9600 baud, simplex, (one-way) over a good-quality >>>>audio channel that can handle at least 10KHz. >>> >>>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 could >> >>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 >>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 ?). >> > >This type of thing has been done before by Standard Microsystems Corporation >(SMC) with their ARCNET chipset. Data Rates of up to 2.5 MegaBits/sec have >been acheived on standard phone line. The Network driver IC (COM9032 I think) Actually, the reason that works is because you have a whole lot more bandwidth over that line that you would think. If you want to pass data through the PSTN (public switched telephone network) or any other bandwidth limited channel, then you MUST modulate the data on a carrier that will be passed by the channel. The "di-bit" scheme will not work through audio channels (phone lines, other than 3002 DC continuity circuits, are considered to be audio paths) because the channel will act as a low pass filter and change the shape of the pulses to the point that they will be completely meaningless by the time the get to the other end; if they even pass. What a modem does, is modulate the data onto a carrier that is well within the bandwidth limits of the channel presented by a phone line. You can get away without a modem so long as the channel (read: wire) doesn't have any low pass filters (like loading coils). This is exactly how the ISDN system is supposed to work. Since most audio paths are bandwidth limited to about 20 to 20k hz (at best), you are going to have big troubles passing raw di-bits through them. Good luck though. If you can make it work you will put the whole modem industry out of business (of course so will ISDN -- eventually). cheers, Brian Cuthie CASE communications Columbia, MD 21046 -- Brian D. Cuthie uunet!umbc3!cbw1!brian Columbia, MD brian@umbc3.umd.edu "Captain, Captain! All the stars have gone out!" "No, you fool, you've leaned on the button. Turn the viewer back on!"