When you think about the history of television,
there are a handful of events that stand out as extremely
important. The invention of the black-and-white TV set and the
first broadcasts of television signals in 1939 and 1940 were
obviously important. Then there is the advent of color TV and
its huge popularity starting in the 1950s. There is the rise
television and cable channels like HBO and CNN competing
with the three big networks in the 1970s. In this same list
must certainly go the development and popularization of the
VCR starting in the 1970s and '80s.
The VCR marks one of the most important events in the
history of TV because, for the first time, it gave people
control of what they could watch on their TV sets. Prior to
the VCR, there was no such thing as a video store, and when
you consider that there is now a video store on nearly every
street corner in the United States, you can see what a big
impact the VCR has had!
The other interesting thing about VCRs is how incredibly
intricate and interesting they are inside. They are certainly
the most complex mechanical systems most people own outside of
their automobiles, yet VCRs can cost as little as $100! There
are motorized tape loading and ejection systems, complex
motorized tape paths, drum-mounted rotating read/record
heads... VCRs really are neat inside!
In this edition of HowStuffWorks,
we will look inside a VCR to understand the basics of its
operation, both from a tape transport standpoint and a signal
standpoint. Once you have seen what is going on, you will be
both amazed and impressed!
A VCR tape is pretty intricate
device in its own right, and it's a classic miracle of mass
production. It is amazing what you can buy for $2!
If you take a video tape apart, you will find that it
- The top and bottom pieces of the outer shell
- A moving, spring-loaded door to protect the tape
- Two spools to hold the tape
- An 800-foot (244-m) long, 1/2-inch (1.3-cm) wide piece
of oxide-coated Mylar tape that acts as the recording medium
- Several low-friction rollers to guide the tape across
the front of the shell
- Two spring-loaded locks to prevent the tape from
unrolling inside the cassette
When the tape is inserted into a VCR, the VCR uses a
lever to release and open the door, exposing the tape.
It also inserts a pin into the hole to disengage the
two locks on the spools. At that point, the drive can extract
the tape and play it.
- Several screws to hold the whole thing together
In 1956, the first commercial
reel-to-reel videotape recorder was created by Charles
Ginsburg and Ray Dolby while working for the Ampex
Corporation. This new device was a major development for
television broadcasters because it marked the first time that
shows could be recorded and broadcast later. Prior to 1956,
all shows on television were live.
Sony created the first inexpensive VCR in 1969. In 1976,
the VHS tape format appeared and began its domination of the
market. Video stores were soon to follow. Blockbuster
opened its first video store in Dallas, TX, in October 1985,
and now has over 8,000 stores.
The VCR itself has two jobs:
these are formidable tasks, and the second one was a big
technological challenge. In sound recording, the sound
information is stored linearly on the tape. That is, the tape
moves past the recording head and the sound information is
laid down as a long line following the length of the tape. The
tape might move past the head at a speed of 2 or 3 inches (5 -
8 cm) per second. A video signal contains perhaps 500 times
more information than a sound signal, so the same approach
cannot work. The tape would have to be moving past the head at
a rate of many feet per second.
- It must deal with the tape -- an extremely thin, fairly
fragile and incredibly long piece of plastic. As we will see
in a moment, it is amazing what a VCR does with the tape.
- It must read the signals off of the tape and convert
them to signals that a TV can understand.
To solve this problem, two recording heads are mounted on a
rotating drum that is tilted with respect to the tape. If you
have read the HowStuffWorks article How Television
Works, then you know that a television image is divided
into a series of 525 horizontal scan lines, half of which are
displayed every 60th of a second. Each pass of the VCR's
rotating head reads or writes the data for one field (262.5
scan lines) of the television image. Therefore, the data
recorded on the tape looks like this:
In this figure, the light blue bands are individual fields
laid down by the recording head of the rotating head drum.
Since the drum contains two heads on opposite sides of the
drum (180 degrees apart), the two heads alternate, each one
reading or writing every other band. The yellow tracks
represent the audio and control tracks. The control
track is especially important:
The relationship between the tape and the
rotating head drum is shown in this figure:
- It tells the VCR whether the tape was recorded in SP, LP
or EP mode.
- It tells the VCR how quickly to pull the tape past the
drum (since the tape may stretch or shrink over time).
- It gets the heads lined up with the bands during
When you play with the "tracking" control on your VCR,
what you are doing is adjusting the skew between the control
track and the actual head position on the tape. Usually,
this is not necessary, but if a tape is badly worn or
stretched you may have to adjust the tracking.
The head is rotating at 1,800 revolutions per minute (rpm),
or 30 revolutions per second. In SP mode, the tape is moving
past the head at 1.31 linear inches per second (33.35 mmps).
[In LP mode, it's 0.66 ips (16.7 mmps), and in EP mode it's
0.44 ips (11.12 mmps).] Because of the head rotation, however,
the head is moving over the tape at 228.5 inches (5804 mm) per
second, or about 25 miles per hour (41 kph)! That means that
if the video information were being stored linearly, you would
need a 50-mile-long (80-km-long) tape to store a two-hour
movie. Obviously, the rotating head approach, also known as
helical scanning, saves a lot of tape!
The only problem this creates is that a VCR designer has to
get the video tape to wrap around the rotating head in order
to record or play back the tape. In addition, the device has
to read the audio and control tracks from the tape, keep the
tape moving at exactly the right speed and detect the end of
the tape. To do all of this, the tape has to follow a tortuous
path, like this:
Different VCRs use different approaches, but you get the
idea. The drive mechanism in the VCR has to extract a good
long piece of tape from the cassette and wrap it around a
variety of rollers, drums and heads in order to play the tape.
It is absolutely amazing that a VCR ever works!
Inside a VCR
If you ever take apart a VCR
and look inside (something you will probably want to do on a
dead VCR), you can now understand a good bit about what you
are seeing. Here is a typical view inside an RCA VCR:
Taking a closer look at the drum, you can see that it is
tilted with respect to the tape:
When the VCR loads the tape, the two rollers identified by
the green arrows will actually pull the tape out of the
cassette. They will move along the tracks identified by the
red arrows and wrap the tape around the drum:
Here's a better look at the two rollers:
When the tape is first seated in the VCR, these two rollers
are actually inside the cassette, behind the tape. The
cassette has cutouts to allow these rollers to fit into the
cassette. In this picture, the tape is clear, and you can see
the rollers behind the tape:
Once the rollers fully extend on their tracks, the tape
fits very nicely around the drum:
The pinch roller and inertia roller engage, and the tape is
pressed onto the erase and audio heads. It's like watching a
ballet when you see all of this take place -- there are some
pretty amazing mechanical engineers working on this stuff!
Now that you know something about a
VCR, you can understand several of the controls and terms used
- Tracking control - The tape contains a linear
control track that helps the VCR synchronize the rotating
heads with the actual bands recorded on the tape. When you
adjust the tracking control, you are skewing the
relationship between the control track and the heads to try
to get a closer match to the bands on the tape.
- Flying erase head - VCRs have two types of erase
heads. The low-cost kind simply erases the entire half-inch
wide tape. This can cause a lot of snow between different
segments recorded on the tape. A "flying erase head" is
actually mounted on the rotating drum. It is able to erase
bands individually, allowing very clean splices between
- SP, LP and EP settings - The three speed settings
on a normal VCR simply control the speed of the tape with
relation to the rotating drum. In SP mode, the tape is
moving past the head at 1.31 linear inches (33.35 mm) per
second. In LP mode, it's 0.66 ips (16.7 mm/s), and in EP
mode it is 0.44 ips (11.12 mmps). As the tape speed
decreases, the bands on the tape get closer together,
reducing the quality of the image but increasing the amount
of material that fits on the tape.
- Four-head vs. two-head - A VCR needs only two
heads to record or play back a tape at SP speeds. A problem
arises, however, at LP and EP speeds because the tape is
moving much more slowly. Many VCRs therefore include two
wider heads for SP speed and two narrow heads for use at
slower tape speeds. These four-head systems offer better
performance at slower tape speeds.
- End-of-tape sensing - The leader on video tapes
is clear. A VCR shines a light through the tape and can
detect the end of the tape when it "sees" the clear leader.
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