Hidden Signals: Not Always Transferred
TV contains many hidden signals. One signal type that is not transferred, except on some very expensive converters, is the closed captioning signal.
Teletext signals do not need to be transferred, but the captioning data stream should be if it is technologically possible to do so.
With HDTV broadcasting, this is less of an issue, for the most part meaning only passing the captioning datastream on to the new source material. However, DVB and ATSC have significantly different captioning datastream types.
Read more about this topic: Television Standards Conversion