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Teletext display

Teletext is dead, but there are many teletext chips that can still be useful as displays for embedded projects

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This project started as a teletext receiver for the PC, in the days when you could bit-bash the printer port. Circa 2000! I got some DOS code bit-bashing I2C, but got distracted by work! Analogue TV switched off, and it became junk.

Then some hobbyists said they wanted text output on their projects. An easy and cheap way to get coloured 40-column text and block graphics on any project is to make use of the large stock of obsolete and cheap teletext chips.

At the moment I have an SAA5246 in DIP48, a 27 MHz crystal and an 8K RAM, which is enough to start the project. It is the L font option but I can live with that for now.

There is a chip with an amazing feature list - the SDA5273 - but I think it will be hard to program and there is no programming guide for it on the internet.

My first thought was to use the MV1815, a chip I had used before.
It needs a 27.75 MHz crystal which is not common.
I got a quote to get them made, for about £140 set-up charge and about 20p per unit. Not impossible but not cheap either.

Googling teletext chips, I found the SDA5273 which seems to be one of the very last - and advanced - chips made before digital TV effectively halted teletext technology.

https://www.ebay.co.uk/itm/4pcs-SDA5273-2PGEG-SDA5273-Teletext-MEGATEX-Ics-PLCC84-MICRONAS/263383835491

Suitable DRAM chip:

https://www.ebay.co.uk/itm/HM514400CS6-Hitachi-DRAM-1-048-576-Word-x-4-bit-20pin-SOJ-DRAM-4-PER-SALE/222248086779?epid=1548393060&hash=item33bf04acfb:g:cwcAAOSwFe5X1sr6

And 20.48 MHz or 8 MHz crystal, depending on which data sheet you read.

Features:

● Single chip teletext IC
● Stores up to 14 teletext pages on chip
● Stores up to 2048 teletext pages with external 16 M memory
● Analog RGB-output
● 41 latin script languages
● 12 × 10 character size
● Parallel display attributes
● 64 from 4096 colors selectable
● Enhanced flash modes
● Dynamically redefinable character set (DRCS, PCS)
● Pixel graphics
● Fullscreen display (64 × 32 or 80 × 24 character positions)
● Horizontal and vertical scrolling
● Graphic cursors
● 4:3 and 16:9 display
● Multinorm display (50/60/100/120 Hz)
● RISC-processor
● Firmware downloadable
● I2C / 3 wire UART-interface (1 Mbit/s)
● Independent clocks for acquisition and display
● Tools for greatly simplified software development
● P-LCC-68-1
● P-SDIP-52-1
● 24-Kbyte on-chip reconfigurable DRAM
● 44160-bit character ROM
● One external crystal for all standards

Teletext Features:

● Analog CVBS-input with on-chip clamping circuitry
● Data Slicer
● Supports level 1, 2.5 and 3.5 ETSI teletext standard
● full level 2.5 processing

See 

https://en.wikipedia.org/wiki/Teletext

https://www.etsi.org/deliver/etsi_i_ets/300700_300799/300706/01_60/ets_300706e01p.pdf

At the moment, I'm inclined to use the SAA5246, because it is simple and there is code for it on the web.

I have removed the tuner module (which was dead anyway) to make room for the new circuitry.

MV1817-Datasheet.pdf

Like an MV1815, but in DIP48, uses 4-bit data bus DRAM, and 9.9375 MHz crystal (half of the more available 13.875 MHz crystal).

Adobe Portable Document Format - 1.05 MB - 03/01/2021 at 03:11

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SAA5246.pdf

SAA5246 datasheet

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MV1815-Datasheet.pdf

MV1815 datasheet

Adobe Portable Document Format - 1.00 MB - 11/15/2020 at 21:53

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SDA5273-5275_data_sheet(1997-09-01).pdf

SDA5273-5275 data sheet (1997-09-01)

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SDA5273-3CP_Delta_Specification-Application_Notes(1991-01-27).pdf

SDA5273-3CP Delta Specification Application Notes (1991-01-27)

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  • Hardware for MV1815 Telepod Module Block Diagram

    Keith05/30/2022 at 13:34 0 comments

    Telepod Module Block Diagram

    Decoupling: I have put 100 nF ceramics close to chips. I assume the tuner has adequate internal decouplers. I have also put a 220 μF 16 V electrolytic on the 12 V rail.

    My prototype is built on a 100 x 160 mm square-pad Eurocard. The power connector is the same as used on a 5.25" disk drive. This allows it to fit in a standard 19" Eurorack, and take power from a spare disk power lead, but bear in mind this is often an electrically noisy environment and the sensitive decoder section may need shielding.

    If you wish to power it from 12V only (e.g. for use in a car with a laptop) then there is plenty of room left for a 7805 regulator circuit. And a small audio amplifier if you wish.


    12V to 33V charge pump module

    This circuit is used in Maplin's NICAM tuner project, but here the connections to the hex inverter (HCF40106BE) have been rearranged to make the layout easier. The circuit has been drawn this way to show this. It produces 75 V when the 33 V zener diode and the tuner are not connected. I have not measured the current consumption of this module.


    Video buffer module

    This is simply a unity-gain emitter follower. The example circuit in the tuner module data sheet did not specify a particular transistor. Perhaps a 2N2222A may be suitable. This should be located near the tuner module.


    Audio module

    This is simply a unity-gain emitter follower. The example circuit in the tuner module data sheet did not specify a particular transistor. Perhaps a 2N2222A may be suitable. This should be located near the tuner module. The socket is a 3.5 mm jack, as used by common portable stereo earphones. LK1 allows mono audio to be heard in both ears.


    Teletext receiver module (MV1815 based)

    The 40-pin chip is the MV1815 from GEC Plessey. A more modern design than the Philips family, the use of dynamic memory allows it to address 1 M x 2 bits = 256 x 1 Kbyte pages. In comparison, the 48-pin SAA5246 addresses eight 1Kbyte pages. The DRAM shown here is a 64k x 4-bit 4464. Although half its memory is unused, its 32 x 1K pages is adequate. Especially if pages are cached in the PC. It also saves construction time, cost of sockets, and current consumption. The MV1815 takes 15 mA, the 4464 takes 55 mA, and the optional LED takes about 10 mA. Crystal is 27.75 MHz, rather unusual but I have some free ones spare.


    LPT to I2C Interface (1)

    This used the least hardware, taking advantage of the open-collector outputs of some LPT signals.

    LPT to I2C Interface (2)

    This circuit is an improvement on (1). The buffers drive the input lines of the cable to the PC, so they do not present capacitive load to the I2C bus. The buffers are powered by the 5V from the application circuit. The type of buffer is deliberately unspecified - you may use spare gates from your application circuit. Note that the buffers should not be at the PC end of the cable where they would provide no benefit!

    You could put this circuit in a small Centronics adaptor box (e.g. Maplin JW56L or JW540) with a Centronics connector socket for a printer cable, and I2C signal outputs. There is no standard I2C connector, but you could follow the Elektor mini-DIN connector convention used in their I2C projects, or the 4-pin FCC68 connector convention used by the AccessBus standard. Irritatingly, neither PCB-mounted connector has pins on a decimal-inch grid used on most prototyping boards, and FCC68 requires a special crimping tool.


    Disclaimers:

    This design has not been tested, so I cannot guarantee it will do anything. The only thing I can say is that I managed to get teletext data out of the circuit involving the I2C interface number 1, the MV1815 and the 4464 DRAM, using a small MSDOS & C program. The video signal was from the SCART socket of a TV.

    I have tested the tuner circuit. The charge pump works, driving the appropriate tuner pin to 33V. A hiss can be heard with Walkman earphones, exactly like...

    Read more »

  • Software

    Keith02/29/2020 at 19:32 0 comments

    2020-02-29

    I see that teletext characters have been proposed for addition to the Unicode character set.

    http://teletextart.co.uk/teletext-graphics-characters-among-those-added-to-unicode

    https://www.unicode.org/L2/L2019/19025-terminals-prop.pdf

    I don't know what the values are, or whether browsers support them yet.

  • Hardware

    Keith04/01/2019 at 23:30 0 comments

    2019-04-01

    Parcel of goodies arrived including MV1815 and SDA5273 chips. I removed the dead chips and fitted new a new MV1815 and 64Kx4 DRAM chip. The phono socket used to be the UHF output from the tuner. As the analogue tuner was dead and obsolete, it has been removed. The phono socket can now be the video input to the teletext chip. The MV1815 outputs are red, green and blue, and will be routed to the RGB input pins of a SCART socket. This project will require a SCART-socketed colour TV, which I have.

    I gave some chips to a workmate to play with. They might be useful as text overlay chips, but all the monitors at work are baseband monochrome. A few colour LCD monitors have SCART inputs. But there are obsolete analogue colour TVs available cheaply on ebay - collection only, they are too heavy to post.

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