Date   : Fri, 24 Jul 2009 13:37:10 +0100
From   : philpem@... (Philip Pemberton)
Subject: Fw: Fwd: ITV Teletext to shut down in January

Rick Murray wrote:
> So it seems that now may be the time to start thinking about a little 
> teletext encoder for old time's sake. If somebody can make a WiFi 
> antenna out of an old Pringles tube [*], then a teletext inserter based 
> around a microcontroller and a wodge of memory shouldn't be impossible!

Could be a fun afternoon project. You can go two ways with it:
   (1)- Adding VBI data to an existing video signal. For this you need a 
composite input and a sync separator. The LM1881 costs about ?3 and 
needs three or four external parts. Simple!
   (2)- Creating your own video signal as well. Bit more complex, now 
you need a framebuffer and a sync generator.

If you go with (1), you need a *stable* video signal. So that'll be live 
stream from a camcorder (a *good* camcorder), PC composite output, or 
something similar. A VHS recorder probably won't cut it, but Betamax 
might (just about) work.

Next you need a line-counter. Reset this every time the sync 
separator/generator's Vertical Sync output goes active. Increment it 
every time you get a Composite Sync (horiz sync) pulse. Run a separate 
frame counter off the reset pulse for the line counter.

Run a PLL off the composite sync to generate the pixel clock. Give it a 
decently long loop time, say a second or so. That way the vertical sync 
interval won't screw it up as much.

Have two external RAMs. While one is in use, you can change the data in 
the other. Swap between the RAMs for each frame or field (whichever you 
prefer). Or use a dual-port RAM, but those tend to be expensive. Feed 
the upper address lines off the frame counter, and the lower ones from 
the LSbits of the line counter.

Add a bit of analogue support circuitry, a few level translators and 
you're done.

That's how I'd do it anyway :)

-- 
Phil.
philpem@...          
http://www.philpem.me.uk/