Why 20-minute videotapes?
On the picture below you can see, right to left:
- a standard VHS-C cassette, which runs 20 minutes;
- an MII cassette, which runs 20 minutes;
- a 500-ft Beta cassette, which runs 20 minutes when used in a Betacam camcorder;
- a T-120 VHS cassette, which runs 20 minutes when used in an M-format camcorder.
If we look farther back, the small U-Matic cassette, developed for portable use in the early 1970s, runs 20 minutes.
And the first portable broadcast-quality VTR Ampex VR-3000 — only 35 pounds, excluding camera — used 8-inch reels, running up to 20 minutes.
Clearly, there is something about the 20-minute run time that made it a standard for portable video. But what is it?
The standard length of a 35 mm film reel is 1,000 feet (305 m), which runs approximately 11 minutes at 24 fps. It was established as a measurement for the size of the camera film magazine and for shipping.
For projection in theaters, movies were usually packaged as 2,000-foot “two-reelers”. A standard Hollywood movie comprised five or six 2,000-foot reels.
A short film, usually limited to 20 or 40 minutes, translates into one or two “two-reelers”. Film reels are transported in canisters that can accommodate one, two, three or even four “two-reelers”.
A short feature like “Heat Lightning”, which lasts a scant 63 minutes, translates into substantially reduced shipping costs — one clunky 35mm Goldberg shipping canister rather than two. — Kyle Westphal, Chicago Film Society, 2013
The 2,000-foot reel size is so ingrained in the movie production, that it has transcended cinefilm and became a part of digital workflow.
When we work on large movies, we break the film up into roughly what used to be 2,000-foot reels of 35-mm film, which is about 20 minutes of screen time. We still do that in order to keep the chunks of action manageable, it also allows my team to work on other bits of the movie while I am working on one separate piece of the movie. Also, we can turn the bits of the film to sound and music in chunks. We may have finished the middle section of the movie before we finish the beginning and the end. We can turn over, say, reels 2, 3, 4 and 5, and I am still working on reels 1, 6 and 7. It allows us to keep everything going at the same time. — Eddie Hamilton, ACE, explaining the editing of “Top Gun: Maverick” in Avid Media Composer.
So, the tapes shown above are the television equivalent of film reels. Film may be dead, but the concepts established during film heyday are still with us. They are not necessarily limitations, but useful conventions helping working on a feature movie, a documentary, or even an amateur home video.
