How can I edit a scene change onto a closed caption master without destroying the captions?
When inserting video onto a closed caption master, the closed captions will be replaced as well. This is because captions are encoded onto line 21, the first line of active video, part of the video frame. This is why caption encoding is the last thing done in the chain of post-production.
Often, it is necessary to make a last-minute shot change or video insertion. With a little care, these scene changes CAN be made without disrupting the caption data. There are two ways to do it, and it will depend on your switcher and whether you are editing with digital machines capable of pre-read.
The easiest way is to insert the video scene in pre-read mode while pre-reading a horizontal wipe of the master itself from the top down through line 21 and the new video from line 22 on down to 525. It's basically just a small manipulation of the wipe bar during the edit laydown. You should be able to preview this through a monitor with a decoder to verify that the caption will appear before committing to the edit.
The second way is to do the same as above but create a B-roll with matching time code of the CC master (or at least the portion that you are inserting video over) and perform a wipe as described above between the B-roll and the new material while putting in and out of the CC master.
Digital editing systems such as Avid can be set up to edit similarly.
One thing to be aware of, if you are planning to do any editing with CC masters, is that caption ontimes and offtimes do not match the audio but rather precede it by 1 or 2 seconds depending on the length of the caption. Therefore, if you think you are cutting out a sentence of dialog and the corresponding captions, you'd be wrong. The captions will be garbled at the cut in and cut out points because of this lag of transmission versus when the captions appear. There IS a way around this, though, using a combination of the method above and careful observation of when and where the caption transmission time starts, but call or email Max if you want to go that route. I'd be happy to tell you of my many late-night editing and reformatting of caption masters that had no time for re-encoding, which, as we all know is really the best way to go about the process of ensuring caption integrity to changed masters.