Image Processing Reference
threshold coupled with a framing analyzer should be able to extract the information with-
out any difficulty if the lines are accessible in the digital domain.
Subtitles and Closed Captioning
The Teletext service in the United Kingdom (and elsewhere) carries an open-captioning
service on Page 888. If you can detect the Teletext data, you may be able to pull off the cap-
tion texts and insert them into a text stream within your video-storage file as the video is
ingested. Preserving the lines so they have to be cropped later allows this data extraction
to be done offline as a separate process.
The wide-screen signaling (WSS) data in a PAL transmission also serves to indicate
the kind of subtitling that is provided. For example, it indicates whether the subtitles exist
within the Teletext data.
It is certainly useful to know whether the content was intended for viewing in normal-aspect
ratio or wide screen. You may be able to detect these signals when ingesting some off-air
content. Extracting this from an analog service will be the same as Teletext-data extraction.
On the NTSC service, the signaling is placed on lines 20 and 283 but in the PAL sys-
tem the signaling is only present on line 20. The format of the signaling is different, too.
There are quite a few additional service-oriented data values in the WSS data on the PAL
If you use the S-VHS connection format, the wide-screen signaling is provided by a
DC offset voltage that does not affect the transmission of the picture information and can
be sensed easily at the receiving end.
This signaling is also implemented in the digital TV services but of course it is done
in a completely different way. It is possible to signal the aspect ratio in the MPEG header
but if the signal is delivered within the stream, it must appear on a GOP (Group of
Pictures) boundary. The header supports a 4-bit aspect-ratio field that can be set to indi-
cate square samples, 4:3 or 16:9. There are other values but they are not meaningful.
Another 4-bit field in the header describes the video frame rate. Again, not all the values
are used. These values are coded into the header of the video sequence. It is beyond the
scope of this topic to get into “bits and bytes” but some coverage of the bit-stream organ-
ization and structure was presented in Chapter 14.
When processing recordings off air, you might be able to use the wide-screen signal-
ing to identify where the advertising breaks are. In the United Kingdom at least, ads are
broadcast in wide screen even on the breaks during 4:3 aspect-ratio broadcasts.
D-Cinema and Subtitles
Now that films can be delivered digitally, the subtitles can also be added without
requiring an additional print to be created. The textual data is stored in a track that runs