VCR Page

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 colour TV and its huge popularity starting in the 1960s. There is the rise of cable television and satellite competing with the terrestrial networks in the 1980s. In this same list must certainly go the development and popularisation 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 TVs. 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 corner of the U.K. 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 cars, yet VCRs can cost as little as a hundred pounds! There's motorised tape loading and ejection systems, complex motorised tape paths, drum-mounted rotating read/record heads...!

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.

The VCR itself has two jobs: 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. 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. To solve this problem, two recording heads are mounted on a rotating drum that is tilted with respect to the tape. A television image is divided into a series of 625 horizontal scan lines, half of which are displayed every 50th of a second. Each pass of the VCR's rotating head reads or writes the data for one field (312.5 scan lines) of the television image. The Video rotating head lays down the video signal onto the tape 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. On the top and bottom of the tape the audio and control signals are laid onto the tape by the audio/control head which is fixed.

The control track is especially important, since it: 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. The head is rotating at 1,800 RPM, or 30 revolutions per second. 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 mmps) and in EP mode it is 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 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 designer has to also 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. 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!

Now that you know something about a VCR, you can understand several of the controls and terms used with VCRs:

Tracking Control - as mentioned above, 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 head. 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 segments.

SP, LP and EP settings - As mentioned above, 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.

4-head vs. 2-head - A VCR needs only 2 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 then two additional narrow heads for use at slower tape speeds. These 4-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 at either end of the tape.