Sinclair ZX80 / ZX81 / Z88 Forums

Tele text is data sent between picture frames to a TV.

Could a custom display routine on the ZX81 create Tele text data, and thereby let the Tele text decoder create the screen?

Then the rest of the screen could be used in "FAST mode", because it is not shown on a Tele text decoder. And then the ZX81 would have color text mode without additional hardware.

Requirement: TV capable of Tele text.

Just a wierd thought.

I would say, in principle yes.
But you have to create a valid TV picture with correct syncs and probably coded frames (odd/even) as the TV would not accept data on a miss synced signal, I guess. Second a bit has a duration of 144ns vs. 154ns with the 6.5 MHz resonator in the ZX81. So I think you have to replace the resonator with a 7MHz type (gives 143ns pulses).

http://www.mouser.de/ProductDetail/Mura ... 6kmLVNY%3d

Then you should prepare an own display routine with HRG and a filled buffer, make the screen in the top margin for teletext data, can use NMI's in the middle to produce a blank picture (even a bit text display would be possible, too) and do the correct timing.

Could be a funny project to work on.

http://www.bighole.nl/pub/mirror/homepa ... A%20TIMING

I think this document is more readable and goes deeper into the details:

http://www.etsi.org/deliver/etsi_i_ets/ ... 06e01p.pdf

When thinking further, you have to do the horizontal sync as well, at least has to be stretched from 207/414 clock cycles to 222/444. This could be maybe done at the end of a line by reseting the sync counter inside the ULA. Here I am not sure if this is possible. This is a very sophisticated project I think. Not 100% sure if possible but worth a try.

There are different coding formats possible, so it may work even with 6.5 MHz and 154ns bit width when choosing data very carefully to compensate timing issues which will manipulate data. So including some space char or similar there and then could help. Even not an easy goal. But still worth a try with the original resonator to have just a software to be loaded on any ZX81 later.

PokeMon wrote:Second a bit has a duration of 144ns vs. 154ns with the 6.5 MHz resonator in the ZX81. So I think you have to replace the resonator with a 7MHz type (gives 143ns pulses).

Err... the need of a change to the ZX81 almost beats the purpose.

Could be possible by right choosen data with original 6.5 MHz as well, I think.

A really interesting idea!
This is far from my programming skills but I would really love to see if it's possible.

Great idea, worth some further investigation I'm sure, however before trying to make the zeddy do such a feat i think it would be prudent to use an FPGA, or maybe a microcontroller ( avr , pic, propeller, ect ) with much faster clock rates that shod allow one to create any arbitrary waveform with the appropriate bitstream to create a teletext page.

How often could you 'refresh' the content of the page.

If frequently you can indeed use the settings of teletext as a screen and resolution.

(when it works the emulators must get add a TELETEXT screen)

As others have said, a very interesting idea and well worth investigating

I do know one thing, some teletext TVs need a properly synchronised picture, as some teletext circuitry uses this to lock the internally generated text display to the broadcast picture (essential in mixed picture / overlay mode). Even if the full teletext mode is used, some TVs still need the properly synchronised picture, otherwise the generated text screen becomes unstable.

Mark