it1@ra.MsState.Edu (Tim Tsai) (08/01/90)
Hello.. I'm using a 19.2K bauds VDM (Voice Data Multiplexer) remote unit made by Universal Data Systems that allows me to connect to the campus terminal server over my regular phone line. The unit works beautifully, except that whenever someone picks up the phone, I get so much noise FROM the unit I generally have to wait until the person hangs up the phone. Very little noise is sent out to the remote end, however. (Note that this unit is designed to be able to handle simultaneous voice and data communications. I do know that noise is expected with a regular modem when you pick up the phone.. :-) The phone company is unable to help me (this unit is a rental from them), because they think that I live too far away from the central switching unit. Someone I know, however, has absolutely no problem with it, and he lives only a few blocks away, so I'd hope distance is not the problem here. I have slowed the unit down to 1200 bauds, and the problem persists. The unit has all the normal indicators lights (RS CS CD, etc) of an external modem, and the carrier detect LED always comes on when I pick up the phone. The ringing of the phone only introduces a little bit of noise, and is tolerable, but as soon as I pick up the handset, the screen starts to fill up with garbage. The only thing that the phone compnay has suggested is something called an "isolation filter", but I have been unable to locate one. Although I don't believe this is going to solve my problem, I'm willing to give it a try. The folks at the phone company here are new with these units themselves, so my doubts aren't without reasons (They couldn't get the unit to work the first few days and also told me that I lived too far away, until I mentioned that my terminal program was set to 19.2K.. Aha! They had the unit set at 9600 bauds at the main office..) Anyway, does anybody have any ideas on either the "isolation filter" or the unit? An excerpt from the manual follows: "The Universal Data Systems Voice Data Multiplexer Remote Unit (VDM-R) provides full-duplex synchronous or asynchronous operation at data rates up to 19.2 kbps. The VDM-R operates on unloaded lines and acts as a real time multiplexer. Voice energy is transceived in the frequency band from 0 to 5000 Hz and digital data is transmitted in the band from 30 kHz to 70 kHz. Digital reception from the central office is in the 80 kHz to 120 kHz range. Bandpass filtering and full voltage isolation insure that normal voice communication and data transfer can be simultaneous with no interference. Digital commands sent to the telephone company central office equipment allow users of the VDM-R to communicate with any other VDM-R user that the telephone company has programmed into the VDM network." Thanks a bunch in advance for any help... Tim, -- it1@ra.msstate.edu Mississippi State University