As some of you might know, most digital European television broadcasts use the Digital Video Broadcast standard (DVB), this includes cable TV (DVB-C), satellite TV (DVB-S) and terrestrial TV (DVB-T).
In the good old days of analog broadcasts, subtitles (in those countries where they are used) where transmitted as part of the video image (and hence there was of course no way to turn them off, or allow multiple languages). Such subtitles are called ‘embedded’ or ‘burned-in’ subtitles.
DVB offers 3 additional ways to provide subtitles to the viewer:
The first is bitmap subtitles. In this case a separate image is transmitted for each subtitle language, allowing the viewer to select which (if any) subtitles to overlay on the video image.
The second is teletext subtitles (these are also sometimes available on analogue broadcasts). In this case some teletext pages are marked as containing subtitles, containing the subtitle text to be rendered.
The third is code-based subtitles, which are text subtitles similar to the teletext subtitles, but without the dependence on the old teletext standard.
Bitmap subtitles are not supported by all old set-top boxes, and therefore it is mostly used on broadcasts where people are buying new boxes anyway (High definition channels and DVB-T).
For most other channels either teletext subtitles or embedded subtitles are used. We are not aware of code-based subtitles being in use.
MediaPortal currently has some limited support for teletext subtitles by way of using the Teletext module found in MyTV and no support for bitmap subtitles (embedded subtitles are of course supported as they don’t actually need any support :)
However, thanks to tourettes we are now on our way to having a DVB subtitle DirectShow filter, which when it is finished will support both bitmap subtitles and teletext subtitles. We aim to create it in such a way that the user won’t have to worry about the difference between the two types of subtitles, and it will additionally automatically select the correct subtitle stream based on language preference. Subtitles will be available for both live and recorded TV. The recording format must be Transport Stream in order to preserve subtitles in recordings and timeshift buffer. As you might have guessed the filter is therefore only available with TVE3.
You can track the current status of the work-in-progress as well as find information on how to help test the filter in this thread :
The developers involved on this are tourettes who have been doing most of the work, and me (Ziphnor) working on the teletext part (and a bit on the rendering part).