sets and adapters use a special character-generator chip and a few, mostly British-manufactured, micros use that chip also-- the Acorn Atom was one example. The BBC has a teletext mode which adopts the same display. But for most micros, viewdata emulation is a matter of using hi-res graphics to mimic the qualities of the real thing, or to strip out most of the graphics. Viewdata works on a screen 40 characters by 24 rows, and as some popular home micros have 'native' displays smaller than that, some considerable fiddling is necessary to get them to handle viewdata at all. In some emulators, the option is referred to as Prestel or Micronet--they are all the same thing. Micronet-type software usually has additional facilities for fetching down telesoftware programs (see Chapter 10). Viewdata emulators must attend not only to the graphics presentation, but also to split-speed operation: the usual speeds are 1200 receive from host, 75 transmit to host. USA users of such services may get them via a packet-switched network, in which case they will receive it either at 1200/1200 full duplex or at 300/300. Integrated terminal emulators offering both 'ordinary' asynchronous emulation and viewdata emulation are rare: I have to use completely different and non-compatible bits of software on my own home set-up. Modems Every account of what a modem is and does begins with the classic explanation of the derivation of the term: let this be no exception. Modem is a contraction of modulator-demodulator. A modem taking instructions from a computer (pin 2 on RS232C) converts the binary 0's and 1's into specific single tones, according to which 'standard' is being used. In RS232C/V24, binary 0 (ON) appears as positive volts and binary 1 (OFF) appears as negative volts. ** Page
25
The tones are then fed, either acoustically via the telephone mouth-piece into the telephone line, or electrically, by generating the electrical equivalent direct onto the line. This is the modulating process. In the demodulating stage, the equipment sits on the phone line listening for occurrences of pre-selected tones (again according to whichever 'standard' is in operation) and, when it hears one, delivers a binary 0 or binary 1 in the form of positive or negative voltage pulses into pin 3 of the computer's serial port. This explanation holds true for modems operating at up to 1200 baud; above this speed, the modem must be able to originate tones, and detect them according to phase as well, but since higher-speed