• A complete ultrasonic controlled Teletext design employing the newly released Mullard chip set.
• Design by GMT Electronics for ETI
• Facilities include double size characters and video superimpose. 

As with all decent designs remote control is ultrasonic, and gives both full and half page displays. The keyboard arrives already fitted to the PCB, and only needs the decoder chip and transducer soldering in to produce a complete unit.

A complete kit is available from GMT electronics, which includes plated-through hole PCBs, full metalwork and the hand controller. See Buylines for final details.

Construct-a-Text
Despite the complexity of this project construction is amazingly straightforward, all that is required is to assemble the four hoards carefully following the overlays, and fit these into the chassis. Inter-wiring between the PCBs is dealt with by following the list given here, and referring to the wire nos. shown on the overlays. Don't be tempted to change this, best results - indeed any results - will only be obtained by strict adherence! Once you're satisfied that all is as it should be, fit the ICs into their sockets and move on to the setting up.

Above and below:
Two typical screen displays from the ITV, Oracle service.
 Now do you see what you're missing out on?

In the transmitter the keyboard, com mands are encoded by the SAA 5000 which switches the HEF 4069 transmit ter IC in the correct code sequence.

This pulse coded 40Hz transmission is received by the TDB 1033 which provides 90dB of gain in AGC system and a carrier filter. The output is fed to the decoder section.

This design is based on the Mullard L.S.I. design and uses four main ICs and a memory section of seven 2102’s.

The signal from the TDB 1033 is fed to the SAA 5010 receiver decoder and checked for error content and then produces various outputs.

1. Analogue Controls - Not used in this design
2. Station Selector Drive Output - Used via an HEF 4011 inverter to step an HEF 4017 station selector.
3. Message Received Output - Used to drive an LED and audible indicator
4. Control Signals for the SAA 5040 TAC

The principal function of the data acquisition section of the TAC integrated circuit is to process the teletext data so that it can be written into the memory. The control section processes the information from the remote control

The data retrieval section of IC, slices the incoming data signal by means of an automatic adaptive data slicer circuit. This circuit sets the threshold level for slicing at half the data amplitude, regardess of the amplitude of the incoming signal, and provides some compensation for distortion such as co-channel interference; the performance of the system under noisy conditions is thus improved. A clock signal is generated from the sliced data by using an external 6M9375Hz tuned circuit, and this signal is used to clock the data into the TAC integrated circuit.

A 6MHz display system clock is also included in the VIP, the output of which is divided in the TIC to produce a clock pulse every 64us. This signal is passed back to the VIP where it is compared with the incoming line sync signals. By this means, the timing system of the teletext display is phase-locked with the incoming television picture signal.

A ‘signal quality’ detector circuit is also included. When a signal with a high noise content is being received, or in the absence of an incoming signal, the signal quality detector cuts off the teletext data to the TAC and allows the display system to free-run. Thus the detector prevents the data stored in the memory from being corrupted by noise.

This facility, combined with the local display clock, allows a stable display even in the absence of an incoming television signal. Both are essential for after-hours display.

The IC also contains an adaptive sync separator which extracts the sync signals from the incoming video signal and also provides a sync output signal for the timebases of the television receiver. When a full page of text is displayed, the sync output signal is derived from the SAA 5020 TIC system, and uses this information to operate the various display functions of the teletext decoder system such as selection of television, teletext, or viewdata modes; page hold, time display, or timed page select.

The data acquisition section, divides the data from the VIP into its cornponent parts. The Hamming-coded address words are checked, and words having a single wrong bit are corrected. Address words having two wrongs bits are rejected. The row address of the incoming data line (one of twenty-four) is fed by this section to the 5-bit row address bus, and the character date is fed through the data to the memory as a sequence of forty 7-bit parallel words.

A signal denoted as WOK (Write O.K.) indicates to the memory when valid data is to be written in, and a WACK (Write Address Clock) signal causes the address counters 74LS161 to step on after each character.

The IC also contains circuits for the implementation of the control bits for the page header.

In addition to providing all the timing signals for the display, the IC also generates a complete composite sync signal. This signal can be used to drive the timebases of the television receiver without the need for the transmitted sync signal. (This form of operation is also termed 'after-hours' operation.)

Additional circuits enable various control functions to be performed. These functions are determined by control characters received from the memory. Examples of these control functions are the selection of graphics or alphanumerics, 'flashing' words, or newsflashes and subtitles displayed in boxes within television pictures.

A 'concealed display' function is also provided which can be operated by the user.

Fig. 5 - Tuning circuit

As an alternative the teletext decoder board and control system is available separately at £125 for those who wish to wire into their own television.

PCBs and chip sets are available separately also - but are PoA.

Next month we conclude the project with component overlays, parts lists and some erudite hints on getting the best results from this superlative design.