Abstract :
An efficient and low-cost teletext decoder based on l.s.i. i.c.s is essential for the success of broadcast teletext and viewdata. However, other vital peripheral areas, such as teletext remote control, interface with the video circuits, low-cost modem functions, surface acoustic wave (s.a.w.) i.f. filters and so on, are not ignored in a comprehensive component solution from Mullard. Remote control for teletext must have up to 32 commands available together with a very fast action to allow rapid keying of page numbers and must also control all the normal t.v. functions. A system employing infra-red serial data transmission, and which interfaces directly with the teletext decoder, is realised in two low-cost l.s.i. i.c.s. Four l.s.i. i.c.s together with appropriate page memory i.c.s comprise the teletext decoder; three, in m.o.s. technology, carry out the digital functions of data acquisition, display timing and character video generation; the other, a linear bipolar i.c., provides a very efficient data slicer and data clock system which is vital to good off-air teletext performance. The decoder is designed for easy extension for viewdata use. A viewdata extension system based upon the MP8048 microcomputer includes a data-recording facility, optional full-keyboard operation, automatic dialling of several alternative numbers and a comprehensive set of on-screen legends to aid the viewer. A modem unit, designed to meet the British Post Office performance and safety requirements for installation in a t.v. reciever or v.d.u. forms part of the system. The performance of the i.f. amplifier in a t.v. receiver is more critical for the reception of data than t.v. signals and the use of a s.a.w. i.f. filter with excellent amplitude and group-delay response is beneficial to teletext. Feeding decoded data video into the RGB channels of a receiver is greatly simplified by colour decoder i.c.s designed for data insertion, particularly if text inlay into the t.v. picture is required- as an option.
