Date : Fri, 31 Jul 2009 18:54:17 +0200
From : rick@... (Rick Murray)
Subject: Fw: Fwd: ITV Teletext to shut down in January
Philip Pemberton wrote:
> Rotating pages *should* be tagged as subpages.
"should" in quotes. :-)
> Stupid thing output at 0.3V, my monitor wanted 0V/5V TTL. A bit of
> wire over some transistors sorted *that* :)
Hehe, where there's a will....
> The Magic Box (tm) has its own rolling clock, and timestamps everything
> on the way out. Timing between the PC and the box is irrelevant, as long
> as you can fill up RAM2 before RAM1 is empty.
Don't have experience with the magic box. Was thinking more a really
simple inserter that reads directly from a RAM as the VBI is being
'drawn', and for the rest of the time (the picture time), the attached
computer is refreshing the contents of said RAM. If we get the computer
to translate it into broadcast form (including hamming bytes, run-in,
etc), d'you think a 2K/KB/KiB SRAM would do it? How much actual data
(all in, every bit) can fit on one VBI? There are about 20 lines.
Something like 46 bytes of data per line. That's under 1K. If we assume
50 bytes so's we can fake up run-in and such... still under 1K!
This is working on the idea of keeping the hardware REALLY SIMPLE and
making the software harder... well, that too is debatable, as waiting
for a VBI-done IRQ and stuffing data is also simpler than some really
delicate timing...
> You're right. Far simpler...
:-)
> Then you can rewrite pages at the beginning while pages later in memory
> are being played out. That is, if the RAM isn't big enough to play your
> entire Teletext block out as is.
Ah, you're still working on the downloading it all to a big RAM while
I'm looking at "tiny RAM, lots of refreshing". This isn't to dismiss
your idea, not at all, but I will ask what will arbitrate the RAM reads
to VBI - will it work in 1K chunks and just step through for 'x' chunks
before resetting?
Hey - I might finally find a use for a few ANCIENT 30-pin SIMMs I have
around here someplace! :-)
Or just use a 2K SRAM with no screwing around with refresh issues.
> PLL), but the point is not whether it happens, but if it *can* (or at
> least if it is *allowed to*) happen.
I was quite impressed that the IIC thingy inside the teletext chip still
worked when I was using "Teletext for DOS" in TurboC's debuggwer,
single-stepping it. Probably the slowest IIC transfer ever, but it still
worked!
> Even on a small FPGA,
Problem is, I have no way of programming an FPGA. How many people here can?
> Though that said, I don't think I'd even try and build something like
> this out of 74LS.
:-)
> I'd worry about wearing the floppy drive out with all the repeated
> reading and re-reading...
Or cache a bunch in RAM? With some clever software, if our own designed
frames, a lot of savings can be made - like "standard" page layout with
content built on the fly just prior to upload. Of course this is less
viable for serving presnarfed content, like a copy of Ceefax.
I am worried about the addressing logic, but actually warming to the
idea of a "big wodge" of memory. Upload it it, then let it run itself.
Then you can use the computer for other stuff, like perhaps a teletext
decoder. :-)
If this is the case, may I suggest some sort of buffering so it can be
disconnected from the host and left to run itself if so desired.
> OK, so worst case you need to update the clock for each frame --
> calculate it beforehand. Or just add an RTC to the Magic Box (tm) and
> let it insert the latest date/time whenever it sees a header row being
> transmitted.
Or just be like some crappy European channels and don't have a clock...
> It's actually easier to talk to USB devices. You use libusb-win32, which
> is a Windows device driver that allows you to talk to USB devices at a
> very low level. Basically, you initialise the device, then start reading
> and writing USB packets from/to the device.
Mmmm, is it regular DLL calls? Possible from VB? I might look into that.
> Don't you have a VGA card? :)
Erm, yes, but that's not composite PAL! I'm in 1024x600 60Hz. Looking at
the options, I don't think this display panel can do anything BUT 60Hz.
Probably originally designed for portable DVD players... :-)
> Get an FTDI FT245 development kit. It's basically a USB controllable
> 8-bit port with two modes -- read/write, and bit-bang. The API is really
> easy to use (it's called "FT2XX") and IIRC there's some support in the
> chipset for a few common serial buses (like I2C and SPI).
Sounds very interesting. Before I get all excited, this is where you
tell me the price tag...
Best wishes,
Rick.
--
Rick Murray, irregular internet access at local library.
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...