Heads Up!
Take a look around your video-rental joint. For all the
popularity of the DVD, it's still pretty much a videocassette world. And even if you have
embraced the new technology -- and startling numbers have -- chances are you still have a
shelf full of tapes that you're occasionally going to want to play. It may even be time to
replace that old VCR with a new model. These days, that's definitely cheaper than having
the old one refitted.
VCRs tend to be pretty much the same, technically, until
you reach the higher-priced Super VHS range. People who market video equipment therefore
have only a few hooks on which to sell their products. One of the most common is the
number of heads included in a particular model. Generally, entry-level recorders are
two-head machines, while more ambitious models have four heads, or more. But does it
matter?
What they are referring to are video heads, the
electromagnetic devices that turn TV images into magnetic patterns on tape and vice-versa.
In fact, VCRs are bristling with other heads as well -- to record and play the sound, to
erase old recordings, to synchronize the VCR with your TV -- but these never figure in the
two- or four-head numbers game.
The video heads are the most critical part of a VCR, and in
some ways it's astonishing that they work at all. The challenge facing designers when
consumer video systems were under development was that, in a given period of time, even a
fairly minimal video signal contains something like a hundred times the information needed
for stereo sound. The technological limits of the day meant that a videotape would have to
run 100 times faster than audiotape, and thus the tapes would have to be that much longer
to hold the same length of program.
Obviously that was less than practical. Instead, a system
was devised where the heads were mounted on a spinning metal drum, angled slightly to the
direction of tape motion. The result was a large number of very narrow parallel tracks
recorded diagonally across the tape. This allowed the tape to move relatively slowly as it
passed by the head assembly, but still gave the high head-to-tape speed (often called
"writing speed") needed to carry the video information.
One of the first improvements made to early VCRs was to
provide a slower speed so that longer programs could be recorded. The first of these,
long-play (LP) mode, halved the speed and doubled the recording time; all modern VHS
machines can play tapes made in this mode, although hardly any can record in it. Far more
popular is super-long-play (SLP) or extended-play (EP), which allows three times as much
material on a tape as normal speed.
In theory, slowing the tape down has no effect on the
writing speed, as the head spins at the same rate whatever the linear speed of the tape.
What does happen is that the diagonal tracks are packed closer together, and that can
increase the video noise in the signal, which we see as snow.
Ideally, for lowest noise, the video head should be exactly
as wide as the recorded track. If it's too narrow, the signal level drops and noise
increases. If it's the right width, it can pick up interference from adjacent tracks at
slower speeds where the tracks are closer.
In early machines, none of this made much difference, as
most of the TV sets of the day couldn't reproduce the noise anyway. But as video monitors
got better, some means had to be found to tame the snow, which was typically worse at the
slower speeds (performance could be optimized for either slow or fast mode, but it seemed
natural to choose the latter as that was what was most expensive for the user, at least in
terms of tape). To clean up the slow-speed noise, many manufacturers simply restricted the
picture sharpness to make the noise harder to see.
Such machines used just two heads (each frame of a video
image is created by a pair of successive diagonal tracks, recorded and played by two heads
on opposite sides of the head drum). Mostly, they weren't ideal for either speed, but a
cost-driven compromise. Better machines had two separate pairs of heads for the commonly
used speeds, each optimized for its function. This was theoretically a better solution,
and had a welcome byproduct for some users in that it improved such special effects as
stop-frame and fast search at all speeds.
Whether or not the extra facility is worth the cost to you
depends on how you plan to use your VCR.
...Ian G. Masters
ian@mastersonaudio.com
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