wtm@neoucom.UUCP (Bill Mayhew) (11/27/87)
The modem noise seen at 2400 baud nearly always affects two adjacent characters. This is due to the fact that a scrambler circuit is used to encode the outgoing characters. The reason a scrambler is used is to ensure that the phase modulation used is more-or-less spread out evenly across all four possible phase angles, and that the line is busy continuously so that the receiver's demodulator can stay in sync. I have experineced increased incidence of noise when a call passes though a digital switching circuit. What seems to happen is that there are small time shifts due to the quantization of the modem signal as it passes though the digital switch. The time shifts introduced by the switch are tantamount phase modulation and thus produce extraneous caharacters at the receiver. There really isn't much that can be done to remedy the problem, unless you find a cooperative person at the telephone company. Priveately owned digital branch exchanges are even more of a problem, as the installers often do not have technical expertise to understand your problem and invariably blame the modem. Note that the telephone company is under no obligation to give you anything better than a line that is acceptible for carrying a voice call (unless you have a leased data line). Minor phase shifts don't seem to affect voice calls but do drive modems crazy. When the phone company in the area where I live switched over to T1 circuits between central offices, we had terrible modem noise for a while. This is due to the fact that the T1 lines were operated at a raw rate of 64K bits per second. Of that, 8K bits per second is used for telco supervisory stuff. That leaves 56K bits per second for the data. There was a problem where the time shift created by the removal of the supervisory data blocks was not being handled correctly, thus resulting in modem noise. The phone compnay eventaully did fix the problem. One thing to notice when sluething modem noise problems is which end, if any is more seriously affected. Under normal circumstances, when a host answers, it sends out a 2400 Hz carrier and listens for a 1200 Hz carrier from the person calling in. It seems that more often than not, there is high frequency attenuation on phone lines (that is to say, greater than normal high frequncy attenuation). This means that typically the person calling in will see more noise than the host does. Under such circumstances, equalization of the line will help. Essentially the same reasoning as putting a grphic equalizer on a stereo is employed, in that a "flat" frequecny response is better. I have problems with a particualy bad line between my computer, impulse, and the next nearest node, neoucom. I couldn't get the phone comany to equalize the line. Since impulse is on a residential line, the phone compnay isn't terribly sympathetic. I can understand their position. I think if you have a business line, you might get a little more attention from Ma Bell. My solution was to get a Telebit Trailblazer. Cleaning up 1200 bps calls wasn't really why I got the T. T., but it was a fringe benefit. The T. T. apparently has a built-in equalizer which it seemingly uses even on low speed calls. I had tried a real Hayes and several other compatibles on that line and had never been able to get even a login: on neoucom. I don't really want to provide a free plug for Telebit, so I should mention that any modem that has equalization would improve the quality of the connection. Also, equalization is not sufficient to cure time slippage problems introduced by digitial switching equipment. If possilbe, host modems should have lines that don't go though pbx systems. One other solution is to get modems that have NMP error correction. NMP does a 2:1 data compression as well as error correction. It is lik having a 4800 baud line. NMP is only effective if both ends of the conversation have modems with the capability. Supporting random Joe Any User is a problem, but at least you can issue a disclaimer that if Joe wants a clean call, he'll have to get an NMP modem. Hope this helps, Bill Mayhew (wtm@neoucom.UUCP) or ...![cbosgd|mandrill]!neoucom!wtm or ...![cobogd|mandrill]!neoucom!impulse!wtm