Possible to draw in the VBI with an Amiga?

[chris]

Hi all

Is it possible to position the screen such that it is possible to draw in the VBI lines?

I read elsewhere that max vertical overscan is something like 566 which obviously doesn't expand far enough for PAL VBI (625 lines).

Can the monitor tweaking tools on Aminet create a screenmode which extends into this area?  Or is the hardware just not physically capable of such a thing?  Or are there nasty hacks which can achieve it?

I'm talking about classic Amiga hardware here btw.

Thanks

[freqmax]

Purpose?

[chris]

Purpose?

It came about from an email elsewhere about messing with the parameters of video card output under Linux, in order to write to the VBI lines and produce a teletext signal to the TV.

With the Amiga's native PAL output, you don't need a crazy adapter - if it is possible to write into the VBI.

The point?  The only potential real world use I can think of, is distributing live updating text information internally to TVs, maybe in a hotel.  But there doesn't need to be any real world use, right?  It's just a fun thing to try and do.

[alexh]

Phillips made programmable chips to do this back in the day costing only a few cents. Both extraction and insertion.

They used to use them in most integrated digital television chassis until analog teletext was all but eradicated about 2-3 years ago and they stopped using them.

[freqmax]

DVB killed it

Anyway consider that a teletext line is 360 bits per video line. And that a PAL video line takes 51,95 µs. You must get very near the bitrate of 6,9375 Mbit/s in order to get the receiver PLL to sync to your bitstream.

You might try with 640 pixel wide in black and white. But I dault it will work.

[Zac67]

It could be a bit hard to persuade Denise to output bitmap data - but it sure isn't to manipulate the background color with Copper (=Copper graphics). Dunno if the resolution is sufficient though.

[chris]

DVB killed it

Yeah, I know.  But that only makes it a more interesting project IMHO

Anyway consider that a teletext line is 360 bits per video line. And that a PAL video line takes 51,95 µs. You must get very near the bitrate of 6,9375 Mbit/s in order to get the receiver PLL to sync to your bitstream.

You might try with 640 pixel wide in black and white. But I dault it will work.

Black and white is all that is needed.  Hmm... so there's no physical limitation on the chipset writing video into that area of the "screen" (beyond possible bandwidth constraints)?

[freqmax]

You could put together a 360-bit shiftregister that is clocked to 6,9375 MHz. That you switch into and out of the videosignal upon horizontal sync pulse at the VBI lines.