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, and in 1972 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 of 1985 and now has over 4,000 stores.
The VCR itself has two jobs:
Both of 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 two or three inches 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 HSW article on Television, 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, since it:
The relationship between the tape and the rotating head drum is shown in this figure:
- tells the VCR whether the tape was recorded in SP, LP or EP mode
- tells the VCR how fast to pull the tape past the drum (since the tape may stretch or shrink over time)
- gets the heads lined up with the bands during playback. 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.