Digital change over problems

In article ,
says...


The teletext services were experimental until 1976,

Yes, but anybody buying a Teletext decoder at the time would have known
that ...

... anyone recall exactly when Texas Instruments brought out the first
decoder using ICs specifically designed for the purpose? (I ought to
know, as I had one of the earliest samples!)

In fact, it was rubbish, as proved when the Mullard/Philips chip set was
released ..

[snip]

Going back to the early teletext era, the (possible) non-broadcast
system I was developing had to be able to use mass-produced teletext
decoders. That meant that it was vital that I understood the broadcast
specifications (there were three successive versions over a period of
time, finishing up with the 'Unified UK Teletext Specification' - IIRC.)
down to the last comma and full stop.

If I suggested anything that violated that specification in any way, I
would have been in very serious trouble!

I look forward to the start of the official stable service.

Ignoring HD transmissions, which are a different matter, any decoder in
service which FULLY conforms to the published DVB specification should
work in a very stable manner, shouldn't it?

I take it the ellipsis is indicative of sarcasm?
The TI-FAX decoder is precisely what I had in mind in
my reply to Alan. We, (Granada Rentals) had an investment
in a Salora model using one of these decoders and I don't
recall anyone telling me it was experimental.

Thank you for reminding me of the name. The TI-FAX decoder was also in a
set produced under the Rank Arena name (probably done so as not to need
two receivers, one badged as Bush and one as Murphy).

However, before this decoder was released, I think you'll find that
teletext had got past the experimental stage. In fact the specification
had already been updated to include the new facilities, such as double
height and background colour that the TI-FAX couldn't handle, and in
that form became the permanent specification as all future enhancements
were caterered for although, of course, not defined.

When the Philips chip set was released, its performance was so vastly
superior to the Texas design that TI retired from the scene.

We also had a GEC model that had a Teletext/Viewdata decoder,
that performed even worse. There was a couple of pots on
the decoder that set slice level which we learned to adjust empirically
to try and minimise the errors.

Ah! Do you remember the isolation from the BT line? In the prototype GEC
receivers we had, it was in a massive red ABS box - about 6" x 4" x 2".
By contrast, the first Philips viewdata decoder we saw had the isolation
on one end of the pcb, with small grey plastic covers on the top and
bottom of the board.

As a viewdata receiver, we noticed that the GEC decoder interpreted
random line noise as errors and, if the page was not changed or
refreshed, a line of white blocks would start to march across the screen
from the top left hand corner, sometimes reaching as far as the third
row in a very short period of time, whereas the Philips one standing
beside it for comparison might clock up one error, if any, in the same
time.

Of course, this had to be documented and I got fed up counting the
number of errors of the screen. However, one of my jobs was designing
all the test pages for our system, so I tacked on a new one on that
looked like this:

TOPIC Error Test Page
0123456789012345678901234567890123456789
0 1 2 3
0123456789012345678901234567890123456789
0 1 2 3
0123456789012345678901234567890123456789
0 1 2 3


so that all I had to do was glance across at the screen and read off the
results.

Of course, this test page was used when the GEC Hirst Research Centre,
who produced the prototypes, were summoned to account for the lousy
error performance ...

.... so you can imagine my surprise when I walked past the GEC stand at
an exhibition a few months later where a sales guy was waxing lyrical
about this special page they'd got for error counting! To my amazement,
they'd copied MY page onto their demo system and where singing its
praises! Of course, I wasn't the only one who saw it that knew it had
been designed to prove that GEC were crap, so there were quite a few
people who had a laugh at GEC's expense that week!

GEC released their viewdata decoder as a set of boards which I built
into a rack for experimentation. Viewdata ran at the pitifully slow
speed of 1200bps on dial-up lines but we wanted to use private wires at
9600bps - eight times as fast. (This produces a full page in one
second.)

I disconnected the clock from the UART and replaced it with one 8 times
faster - and it worked! The only problem was that when data was being
received and written into the page store, it clashed with the read
operation and produced rubbish all over the screen. GEC were consulted
but said sorry, no can do, the software won't work any faster!

One of our programmers walked into my lab, asked for the EPROM from the
decoder, and marched off with it. He returned with a smile on his face
and two EPROMs, the original and a new one, which he asked me to try.

Perfect!

Needless to say, GEC did not provide any terminals or components for our
network ...!

--

Terry

"Terry Casey" wrote in message
om...
In article ,
says...


The teletext services were experimental until 1976,

Yes, but anybody buying a Teletext decoder at the time would have known
that ...

... anyone recall exactly when Texas Instruments brought out the first
decoder using ICs specifically designed for the purpose? (I ought to
know, as I had one of the earliest samples!)

In fact, it was rubbish, as proved when the Mullard/Philips chip set
was
released ..

[snip]

Going back to the early teletext era, the (possible) non-broadcast
system I was developing had to be able to use mass-produced teletext
decoders. That meant that it was vital that I understood the broadcast
specifications (there were three successive versions over a period of
time, finishing up with the 'Unified UK Teletext Specification' -
IIRC.)
down to the last comma and full stop.

If I suggested anything that violated that specification in any way, I
would have been in very serious trouble!

I look forward to the start of the official stable service.

Ignoring HD transmissions, which are a different matter, any decoder in
service which FULLY conforms to the published DVB specification should
work in a very stable manner, shouldn't it?

I take it the ellipsis is indicative of sarcasm?
The TI-FAX decoder is precisely what I had in mind in
my reply to Alan. We, (Granada Rentals) had an investment
in a Salora model using one of these decoders and I don't
recall anyone telling me it was experimental.

Thank you for reminding me of the name. The TI-FAX decoder was also in a
set produced under the Rank Arena name (probably done so as not to need
two receivers, one badged as Bush and one as Murphy).

However, before this decoder was released, I think you'll find that
teletext had got past the experimental stage. In fact the specification
had already been updated to include the new facilities, such as double
height and background colour that the TI-FAX couldn't handle, and in
that form became the permanent specification as all future enhancements
were caterered for although, of course, not defined.

When the Philips chip set was released, its performance was so vastly
superior to the Texas design that TI retired from the scene.

We also had a GEC model that had a Teletext/Viewdata decoder,
that performed even worse. There was a couple of pots on
the decoder that set slice level which we learned to adjust empirically
to try and minimise the errors.

Ah! Do you remember the isolation from the BT line? In the prototype GEC
receivers we had, it was in a massive red ABS box - about 6" x 4" x 2".
By contrast, the first Philips viewdata decoder we saw had the isolation
on one end of the pcb, with small grey plastic covers on the top and
bottom of the board.

As a viewdata receiver, we noticed that the GEC decoder interpreted
random line noise as errors and, if the page was not changed or
refreshed, a line of white blocks would start to march across the screen
from the top left hand corner, sometimes reaching as far as the third
row in a very short period of time, whereas the Philips one standing
beside it for comparison might clock up one error, if any, in the same
time.

Of course, this had to be documented and I got fed up counting the
number of errors of the screen. However, one of my jobs was designing
all the test pages for our system, so I tacked on a new one on that
looked like this:

TOPIC Error Test Page
0123456789012345678901234567890123456789
0 1 2 3
0123456789012345678901234567890123456789
0 1 2 3
0123456789012345678901234567890123456789
0 1 2 3


so that all I had to do was glance across at the screen and read off the
results.

Of course, this test page was used when the GEC Hirst Research Centre,
who produced the prototypes, were summoned to account for the lousy
error performance ...

... so you can imagine my surprise when I walked past the GEC stand at
an exhibition a few months later where a sales guy was waxing lyrical
about this special page they'd got for error counting! To my amazement,
they'd copied MY page onto their demo system and where singing its
praises! Of course, I wasn't the only one who saw it that knew it had
been designed to prove that GEC were crap, so there were quite a few
people who had a laugh at GEC's expense that week!

GEC released their viewdata decoder as a set of boards which I built
into a rack for experimentation. Viewdata ran at the pitifully slow
speed of 1200bps on dial-up lines but we wanted to use private wires at
9600bps - eight times as fast. (This produces a full page in one
second.)

I disconnected the clock from the UART and replaced it with one 8 times
faster - and it worked! The only problem was that when data was being
received and written into the page store, it clashed with the read
operation and produced rubbish all over the screen. GEC were consulted
but said sorry, no can do, the software won't work any faster!

One of our programmers walked into my lab, asked for the EPROM from the
decoder, and marched off with it. He returned with a smile on his face
and two EPROMs, the original and a new one, which he asked me to try.

Perfect!

Needless to say, GEC did not provide any terminals or components for our
network ...!

Yes, I remember the big line isolation transformer and to be honest
it was perfectly normal practice at the time. The GPO were still
the national PTT, and the line entered a wooden box that contained 25KV.

Yes the modest 1200/75 baud system was a challenge for the clockwork
PSTN of the time.

I provided two Labgear set-top box style Viewdata terminals for
Granada Studios for a project.
They had a weekly segment called The Granada One-Hundred which was
an early interactive audience participation experiment.
100 viewers had simple terminals with which they could vote on some
questions posed in the show, and at the end the results would be
shown from the output of one of my terminals.

The first problem was as supplied, these only had a UHF RF output,
so the maintenance dept. made a couple of emitter followers on verobord,
a I recall, the baseband was connected to the same Belling-Lee socket that
carried the RF simultaneously!

Each was connected to a channel of the studio mixer.

The programme went out at about 1800, but they discovered they got a
cleaner line to London if they dialled up at midday.

So both these units were connected to the Audience Research (BARB)
computer for 6 hours at the not inconsiderable daytime rate, hey, that's
showbuiness.


--
Graham.

%Profound_observation%

Contact Sony and ask them which make of electronics they would prefer you
to buy in future.

I was told a few years ago they were 8k compatible



wrote in message
...
Earlier this week BBC2 analogue was switched off on the Caradon Hill
transmitter and we were advised to retune TVs. My newer Toshiba retuned
OK. However, my older Sony KV-28DX30U digital TV will not now receive
BBC1, BBC2 or channel 5. I have tried both automatic programme search and
manual tuning and can't find these channels. Both TVs are on the same
aerial system and reception is 'good' according to the Toshiba. Can anyone
help/advise please? Perhaps my Sony - I believe one of the first digital
TVs- cannot receive these new channels. Its programme sort function on it
has always been dodgy.

Peter