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 of cable 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 $75! 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 article, 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!
The VCR Tape
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 contains:
- 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
- Several screws to hold the whole thing together
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.
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:
- 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 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.
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:
- 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 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 relationship between the tape and the rotating head drum is shown in this figure:
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 with VCRs:
- 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 segments.
- 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.
For more information on VCRs and related topics, check out the links on the next page.