rbarth@TUMTUM.CS.UMD.EDU (Richard Rbarth) (09/25/89)
Timothy Stark (11startk@gallux.gallaudet.edu) asked in Issue 255 for a "blueprint" of a TDD modem. Information on the design of TDD modems is available on the Handicapped Educational Exchange (HEX) BBS in Silver Spring MD. Phone lines are (301) 593-7033 (TDD or 300 baud ASCII) (301) 593-7357 (300/1200 baud, ASCII only) For those interested, a TDD modem uses FSK with 1400 Hz mark and 1800 Hz space tones. The same tone pair is used in each direction, so the modem must be designed to shut off its tones a fraction of a second after the last outgoing character has been sent. TDDs in this country use the Baudot code at 45.5 baud. The software that drives the modem must do ASCII/Baudot code conversion. Some commercial modems do the conversion internally.