Why can't DV be imported faster than x1 speed?

[Gibson's Squares]

Please help me understand why DV has to be imported in real time from tapes?

Why can't a PC just read the stream as 0s and 1s, but quickly?

Is it something to do with there being no file headers on the DV footage?

[aedipuss]

it's on tape and the tape can only be played at normal speed. that's as fast as the spinning heads can go, it's like a vcr.

[smrpix]

Sony and Panasonic both made faster than realtime decks (DVCPro only for the Panasonic) but they were very expensive and HD made them obsolete in the pro market very quickly anyway.

The most practical -- least expensive way to speed up your imports (and I'm not necessarily saying it's cheap,) is to use multiple cameras and computers simultaneously. Assuming you have more than one tape, two systems will cut your total upload time in half!

[solarfox]

More specifically: DV can't be imported in faster-than-real-time for the same reason that you can't do it with analog videotape (VHS, 8mm, etc.); namely, because it uses a helical-scan recording method where the information is recorded as a series of diagonal "stripes" across the width of the tape, and the placement and angle of the stripes is a function of the tape's horizontal motion vs. the video heads' position and spin rate.

You can do high-speed reading and dubbing with optical discs because the data is recorded as a single long, effectively linear track, with each bit lined up behind the next, so all you have to do to read them faster is to crank up the spin rate of the disc so that the bits pass by the reading head at a faster rate. (A similar principle applies to hard drives, or analog audio tapes.) Effectively, there is only one "axis" of motion which the reading mechanism has to cope with.

Unfortunately, with helical-scan tape, you now have two axes of motion to deal with. Since the video heads' position and spin rate is fixed, trying to run the tape past them at a faster horizontal speed would throw off the relationship between horizontal tape motion vs. the spinning heads, which would cause the heads to "miss" portions of each stripe or to catch partial amounts of multiple adjacent stripes. (If you've ever used the "fast cue/review" buttons to skip past the commercials on a VHS deck, you've seen the results of disrupting this relationship.)

The only way such "faster-than-1X" reading could be made to work is if you had a deck which can adjust the video heads' spin rates and angles to compensate for the change in horizontal tape speeds. The problem is that this vastly increases the mechanical complexity of the tape deck, which leads to such decks being extremely expensive specialty items, as smrpix says.

[Gibson's Squares]

Hey solarfox, thank you for that extremely thorough insight!

It seems so obvious now you've explained it.