Date   : Wed, 10 Jun 2009 22:10:58 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: Morley Teletext Adapter

 
"Tim Matthews" wrote:
> I wouldn't have a clue. But ISTR to remember from the page number
> indicator in the top left of the Ceefax screen that they were broadcast
> at perhaps a couple per second?
 
PAL TV consists of 625 lines of "data", transmitted 25 times per
second. Odd numbered lines are tranmitted every 50th of a second
and even numbered lines every other 50th of a second.
 
Of the 625 lines, 575 lines are used for the display of 275 visible
interlaced lines. 50 lines in each scan are in the blanking
interval when the electron beam returns to the top of the display.
 
The blanking interval occupies lines 623-625,1-23 in the odd scan
and lines 311-335 in the even scan. Lines 623-6 and 311-319 are
unusable to carry data as they carry the vertical syncronisation
signal.
 
The teletext specification allocted lines 7-18 and 320-331 for
teletext data, but originally only transmitted on lines
17,18,330,331. This means it would need 6 1/25th second scans to
transmit a single 24-line page, so giving two pages per second.
Later teletext data expanded to occupy all the allocated lines, 24
data lines per 25/th second scan, so allowing a whole 24-line page
in every scan, 25 pages per second. Broadcasters would also
transmit frequently accessed pages (such as index pages) more
frequently to speed up user response.
 
Theoretically, you can use all 610 non-sync lines to carry teletext
data, carrying 610*40 bytes per 25th second, 610,000 bytes per
second. Some systems use this "whole-scan" teletext transmission
system. The Hong Kong Stock Exchange transmits data over coax links
as whole-scan PAL teletext. I believe some betting chains in the UK
used it to transmit betting data from events to shops.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
Our chief weapons are 'who', 'ps -aux', 'kill -9', and a fanatical
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