Video tapes and helical scan
For whatever reason, I really enjoy reading up on all the nitty
gritty technical details of old media formats. It usually proves
extremely interesting and education. Sometimes that's because
I'm reading about something that I've never seen before because
it wasn't commercially successful or launched at all in Australia
where I grew up. LaserDisc, for example, was a video format from
the late 70s which stored analogue video (in the familiar NTSC or PAL
format) on something which looks an awful lot like a CD or DVD, but
which was closer in size to an LP record, being 30cm in diameter!
The same "pits and lands" structure used to store digital data
on a CD or DVD is used on LaserDisc, but the discretely varying
reflectivity is interpreted as a pulse-width modulated signal whose
continuously varying duty cycle is the actual information content.
Other times, I end up learning a bunch of stuff about a format
that I used extensively but just never understood in any technical
detail because, even as a curious and technically-inclined kid,
in the pre-Wikipedia days it just wasn't so easy to learn stuff
you couldn't easily figure out with your own hands and eyes.
This entry is about one of those cases.
I grew up with casette tapes as the dominant audio format of my
youth, and they were pretty easy to understand. When the door of a
casette deck opens to accept the tape, you can peek inside it and
see what's there , and even when it's closed with a tape inside,
the doors were always transparent so you could keep looking and
be sure there were no surprises. There's nothing too involved,
mechanically. When you press play, the tape head moves into position
to contact the tape, and the tape is drawn across it, off one spool,
onto the next. You can visualise the audio signal as a straight
line along the tape, read off in a simple, linear fashion. Easy!
I had always just assumed that VHS casettes were more or less a
larger version of the same thing, but this is actually very wrong,
in interesting ways. Unlike a tape deck, the insides of a VCR
are rather more hidden from view - okay, you can lift up the flap
to peek inside, but it's dark in there, and the slot is narrow!
And it turns out there's neat stuff going where you can't see it.
The difference between these two tape formats ultimately comes
down to information density. There's a lot more information in
one second of video than in one second of audio. With the simple
linear reading solution used by audio casette tapes, the only way
to read more information off the tape in an given unit of time is
to increase the speed at which the tape moves across the head.
This imposes various practical limitations: ultra-fast tape is
liable to wear out quickly, requires larger, heavier, noisier motors,
and probably most important of all, means that a 2 hour film would
require such a great total length of tape that the spools to hold
it all would be enormous and unwieldy.
The solution to this problem used by both VHS and Betamax is called
helical scanning. The tape head itself actually rotates against the
tape at the same time as the tape is drawn past it, and furthermore
the tape head is inclined at an angle to the tape. As a result,
the signal isn't visualsed as a straight line drawn across the
tape, but rather as a series of closely spaced, roughly diagonal
(but actually slightly curved, hence the name "helical") lines.
That's probably quite unclear in words - check out the Wikipedia
article on helical scan for a diagram which hopefully makes it
clearer. With this approach, the relative speed of the head against
the tape is much greater than the actual speed at which tape is drawn
across it, resulting in a high information rate without any of the
drawbacks mentioned above, making the system practical for home use.
It's seriously clever and I'm really impressed that somebody thought
of this! It would never have occured to me.
There's a cost to this, of course, which is increased mechanical
complexity compared to the relatively simple situation in a tape
deck. The VCR actually extracts quite a length of tape from outside
of the casette, and uses a series of rollers on moving arms to press
it up against, and almost wrap it around, the rotating tape head.
There's a really nice video of this at Wikipedia, showing the
process from multiple angles:
I had no idea this was happening inside my VCR! To my memory,
audio casette tapes were a lot more liable to get chewed up by
the tape deck than VHS tapes were liable to get chewed up by the
VCR, but now knowing what happens under the hood this seems like
a perfectly backward state of affairs. Is my memory bad, or was
I lucky enough to grow up with an unusually high quality VCR?