You've come to this page because you've asked about digital cable TV and teletext, posting questions similar to:
Why doesn't my TV display teletext when I tell it to ? Why doesn't my TV display subtitles when I tell it to ? How do I enable subtitles ?
These are the Frequently Given Answer to these questions.
First let's clear up some terminology: Some people make a false distinction between "analogue teletext" and "digital teletext". This is incorrect. Both forms of teletext are digital. Much more accurate terms are, ironically, the seemingly simplistic "old teletext" and "new teletext". Those are the terms that will be used here.
"Old teletext" is encoded (in digital form) in some of the (normally off-screen and thus invisible) scan lines at the top of an "analogue" television picture. It is decoded into text and block graphics by your television itself, which displays them instead of or superimposes them upon the picture.
Whether your television decodes and displays teletext (and subtitles) when you tell it to depends from whether the analogue picture signal that it receives from your cable decoder box contains the encoded "old teletext" data in the first place.
For "pass-through" analogue channels, available in a few areas, where you aren't actually watching digitally transmitted channels at all, the "old teletext" data will be there, of course, just as they are in those same channels as received via an aerial. This discussion will not touch such "pass-through" channels further.
In the case of digitally transmitted channels, whether the signal that your television receives from your cable decoder box contains the "old teletext" data is entirely at the whim of the channel provider. If the channel provider places the "old teletext" data into the picture at source, your cable box will decode them (since, to it, they are just another part of the picture), your television will receive them in the decoded video signal from your cable decoder box, and your television will thus display teletext when told to.
There is a popular misconception that NTL "strips off" the "old teletext" data from the picture signal when transmitting them via digital cable. This is untrue.
In the case of major channels, such as the BBC and ITV channels, Channel 4, Channel 5, and so forth, the channel providers do not add the "old teletext" data in the first place to the digital feed that they give to NTL. The data are simply not present at source at all. There is nothing there in the picture signal for your television to decode because nothing was encoded. Those parts of the picture are simply empty.
This is because, it seems, the major channel providers agree that devoting a significant amount of the limited bandwidth that is available to a digital TV channel signal to encoding "old teletext" data into the picture signal is a waste of bandwidth that would be far better devoted to encoding actual visible picture information; especially given that those channel providers (or, in some cases, the teletext providers to which they subcontract the provision of their teletext services) provide the much superior "new teletext" anyway, that anyone capable of receiving a digital television channel must also be capable of receiving.
( To quote the BBC's frequently given answer on this subject:
Ceefax is not [available on our digital channels] and is replaced by our interactive service.To quote Teletext U.K.'s frequently given answer on this subject:
As TV channels upgrade to digital transmission, the old analogue-style teletext services will die off to be replaced by the interactive TV format. It means the end of Teletext's direct link to ITV and Channel 4, but means we can offer a better-looking, more reliable service, with enormous potential.)
The major channel providers provide separate, different, feeds for digital and for analogue transmission. You'll notice that what is recived on a digital transmission is noticably different to what is received on an analogue transmission. (For example: The BBC channels have different stills, continuity announcements, and trailers for digital and for analogue.)
However, minor channel providers often have what is effectively one, single, feed, with both analogue and digital being taken from a single source. Most of them don't provide a "new teletext" service. Indeed, few of them provide even an "old teletext" service. Where the channel provider does provide "old teletext", the fact that their analogue and digital feeds are mostly taken from the same source means that the "old teletext" data are present in the picture signal that is transmitted digitally. Some examples of such channels are "The Box", "Kerrang!", and "Paramount Comedy". For these channels, your television can thus decode the "old teletext" data.
"New teletext" on digital cable is transmitted separately from "new subtitles". Subtitles are not part of the teletext service. Neither of them are transmitted in "unused scan lines", because there is no such thing in digital television transmission. They are interwoven with the MPEG data stream that your cable decoder converts into the analogue video/audio signals for your television. They are both decoded into text and graphics by your cable decoder box, and replace or are superimposed upon the picture signal that is then passed to your television.
So with the "new" system it is not your television that you have to instruct to display teletext and subtitles, it is your cable decoder box.
How one displays "new subtitles" depends from what particular cable decoder box one has. For some boxes, for example, one presses the "Settings" button on the remote control (or "Guide" then Yellow if one has the smaller remote control) to bring up the settings screen, and switches "Subtitles" from "Off" to "On".
Of course, you are still subject to the whim of the channel provider as to whether the "new subtitles" for its programmes are provided or not. Whilst the major channels provide "new subtitles", the minor channels generally, and disappointingly, do not.
The same is true of "new teletext". The capability is there for any channel provider to provide a direct link from its channel to a page on the "new teletext" service, which will be invoked by pressing the Red button when viewing the channel. Indeed, the BBC regularly takes advantage of this capability. It is up to the channel provider to provide the "new teletext" service and to arrange the Red button link with NTL.