I understand the theory but in practice I've never seen such a failure happen, and it's never happened to anyone I know.
I've seen a number of decks and cameras fail and need service at the post houses I've worked at. I recently had my deck serviced and it came back working better than new. Even though my heads and transports hadn't failed, they had worn down over time (so slowly that I never noticed) and getting them replaced made a night and day difference for how my deck performed.
It all depends on how much you use your equipment though. If you only run it a few hours a month that's nothing. But if you run it for 10-12 hours a day then it will wear out a need service much sooner. I don't know if consumer gear has ratings like this, but pro gear typically has a service schedule for how many hours particular parts are designed to be run before they need to be serviced/replaced.
That's true in the analog world. However, as it's digital I contend that the camera can send the data without being able to make sense of it.
Otherwise,
DV Backup could not
function.
According to the DV Backup FAQ tapes made on NTSC equipment are not compatible w/PAL equipment, and vice versa, which, IMO, is another indication that the cameras/decks have to be "understand" the signal on the tape and aren't just "dumb" devices that can send/receive any type of data via the FW port. Also, the DV Back FAQ states, in regards to a Q about normal vs. strict mode, that "Strict mode generates the DV video stream in a different way to suit certain cameras which don't accept the normal format." Again another indication, IMO, that the camera needs a DV signal, not just any old data stream, in order to function properly.
The theory behind this I don't think is much much different than the theory behind a modem (data->dv steam->data as opposed to data->audio frequencies->data). Also, there is already non-image data embedded into video signals such as closed captioning, date/time stamp, timecode, and sync data. NTSC is made up of 525 vertical lines, but only 480 of them are used for the actual picture.
Lethal