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D.I.Y. TELETEXT DECODER PROJECT

  • 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. 
This project is designed to allow the home constructor to produce himself a full spec Teletext unit at around half the cost of comparable commercial units. The design requires no hard wiring into the set, as it contains its own modulator and works into the aerial socket. Definition usually suffers utilising this method, but here great attention has been paid to overcoming this problem.

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.

FACILITIES
Keyboard commands

RESET The screen is cleared and converter is ready for channel change (timed page is cancelled).
STATUS Television station identification appears top left of screen.
HOLD Displayed page is held.
TOP Large (2x) top half of display.
BOTTOM Large (2x) bottom half of display.
MIX PAGE Cancels both above displays channel video and teletext together.
TIMED PAGE On: - The selected time for page selected can be inserted and is displayed in the top right of screen (4 Digits).
Off: - Above cancelled.
REVEAL Displays hidden characters.
TEXT Calls up teletext. Page 100 selected automatically (currently for BBC 2 Ceefax key in 200).
CANCEL Cancels text.
DATA Used for external data (not used in current design).
TV ON Not used in current design. Last two facilities available for further expansions.

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

HOW IT WORKS

Click on the circuit board diagrams for higher resolution images


Fig. 1 Circuitry from Board 3, the video drive and mixing

Ultrasonic Receiver and Transmitter

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.

The Decoder

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
SAA 5040 TAC Teletext Data Acquisition And Control

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

SAA 5030 VIP Video Input Processor

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.

SAA 5020 TIC Timing Chain
The divider stages in the TIC integrated circuit sub-divide the 6MHz clock signal from the VIP down to 25Hz, the television frame rate, and generate all the timing signals for the teletext display. During the display period, a 1MHz clock signal RACK (Read Address Clock) takes over from WACK to step the character addresses. The address counters 74LS161 are cleared at the end of every line and reset to the first position. After every ten lines during the display, the TIC steps the row address on by one to access the next row of characters in the memory.

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.)

Memory Block
The memory block consists of seven 1k x 1 static RAMs.
SAA 5050 TROM Teletext Read-Only Memory
The read-only memory of the TROM converts the 7-bit character data from the memory into a dot matrix pattern. The matrix is in a 7-by-5 dot form for each character. It also contains a ‘character rounding’ facility which effectively increases this matrix to 14-by-10 dots, giving improved definition to the displayed characters.

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.

Set up

  1. Disconnect encoder video from the modulator board
  2. Disconnect blanking and picture on (PO) outputs from main board
  3. Connect UHF O/P to set, and UHF aerial to converter
  4. Select spare channel on T/V set
  5. Tune T/V for blank screen (ie. no noise)
  6. Switch off
  7. Link P.O. input of UHF and mixer board to 12V
  8. Switch on
  9. Tune RV 201 (front panel to obtain best picture on BBC1
  10. Re-adjust set for best colour picture, modulator RV 401 may need adjustment
  11. Repeat 7 and 8 as required
  12. Switch off
  13. Reconnect steps 1 and 2 remove link step 6
  14. Switch on
  15. Set RV 100 to midpoint
  16. Connect pin 1 1C103(VIP) to 12V
  17. Connect pin 7 via 5M6 to 12V
  1. With transmitter switch to mix mode
  2. Adjust CV101 until characters lock with picture
  3. Switch off
  4. Remove steps 14 and 15
  5. Switch on
  6. Adjust L101 to obtain page header and time clock stepping (note this setting is sharply defined). L101 should not need adjustment (ignore any colour flicker)
  7. Switch off
  8. Link pin 10 1C103 to 12V rail
  9. Switch on. Note CV102 and L101 interactive repeat 20 and 24 as necessary
  10. Adjust CV102 for best display (approx 1/4 closed)
  11. Switch off
  12. Remove step 22
  13. Switch on
  14. Switch to text mode
  15. Adjust CV301 for best colour
  16. Other channels can now be tuned (hit reset followed by channel No 1=BBC1; 2=ITV; 3=BBC2).


Fig. 4 - Circuit diagram for Board 1, the teletext decoder and RAM circuits


Above: a unit complete except for the mounting of the ultrasonic receiver


Fig. 5 - Tuning circuit

BUYLINES
The designers of this project - GMT - have a complete kit of parts available. This includes all metalwork, PCBs and hardware. A manual is also included. Cost is £155 plus VAT (total £178 inc p&p).

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.

Part 2 >

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