The Incredible Journey: Inside the Tape Transport

Richard Bennette
December 1995

Just what exactly happens when you pop your tape into a VCR or camcorder? If you've never done so with the protective cover removed, you've probably only watched the cassette disappear into the body of the mechanism and listened as hidden motors clicked and whirred in some mysterious unseen process.

What's happening is that the VCR or camcorder is pulling out a small amount of tape and lining it up along the complex mechanism that holds it in place, moves it, records it and plays it back--the tape transport mechanism, as it's appropriately called.

Understanding how the tape transport mechanism operates is not only useful for repair technicians; videomakers at all levels of expertise can benefit from a working knowledge of its secrets. Not only will this help when it's time to clean your equipment; it will aid you in identifying problems you might have when you try to play your tapes back on someone else's equipment, and increase your overall knowledge of how your equipment works.

And given the choice between knowledge and ignorance, you'd always choose the former, right?

An explanation of how the tape transport system works requires a visual analogy to better understand the process. The best analogy I can think of is the home 8mm sound movie films that were popular in the late seventies. Whether we use film or videotape to preserve our memories, we need to store three basic components: the image, the sound and the timing signal. If you examine a movie film, you can easily see these three components. The images are small squares that look like a series of miniature photographs. The sound is a brown magnetic strip that runs along one edge of the film. The timing is the punched holes on the other edge of the film. The physical distance relationship between these three components is essential for the proper recording and playback of the film.

On videotape, these three components are not visible to the eye. The tape looks the same whether or not it has video information on it. The image, the sound and the timing signals are invisible magnetic pulses on the tape. The tape itself is simply a precision coating of magnetic oxide bonded to a long plastic strip. If you were able to see the magnetic impulses on a recorded VHS videotape, they would look like a series of diagonal stripes down the middle, sandwiched between two linear stripes along the edges of the tape. Other formats vary slightly, but the magnetic recording principle is the same.

The videotape playback system is similar in principle to that used by 8mm movie film. In VHS systems, the linear audio track runs down one edge of the videotape, the control track (or timing signal) runs down the other edge of the tape and the images are the diagonal stripes down the center. In 8mm and DVC videotape sytems, all three types of data--audio, video and timing--are contained within the diagonal stripes.

As in the case of the film, the relationship of the distance between these three components is essential for proper recording and playback. As we shall see later, there are several factors that can distort the relationship between these three components. When this occurs, the picture and sound can suffer considerably.

When you insert a videotape into a VCR, the loading mechanism pulls the tape shell into its proper position. It opens the protective panel on the front of the shell to expose the tape. A series of electric motors and gears move the guide pins to pull the tape out of the shell and wrap it around the video head drum. This process takes a few seconds to complete, and is the source of the whirring and clicking noises heard when you insert the tape.

When you press the record button, the video drum begins to spin. In VHS models, the tape then moves forward as the control head records a pulse every sixtieth of a second to trigger the start of each new field of video. This pulse establishes the time base--an electronic reference that the VCR uses to synchronize the picture during recording and playback. The video heads on the spinning drum operate in synch with the timing pulses of the control head.

In 8mm and VHS machines, the video heads record one diagonal stripe for each field of video. Two of these fields combine to form one frame of video. There are 60 of these fields recorded per second, creating the 30 frames per second which is the NTSC video standard. With DVC hardware, 10 discrete track of digital information make up the complete video frame.

When you play a tape, the process is reversed. In the VHS realm, the control head reads the control track on the tape. It then sends the time base information to the electronics of the VCR, and uses it to synchronize the forward speed of the tape and the rotational speed of the video drum.

When you use the tracking control on a VHS VCR, it makes a minor adjustment to the forward speed of the tape until the video heads track directly over the center of the diagonally recorded video tracks. The tracking control only functions during the playback process; it has no effect during recording.

In an 8mm or DVC VCR or camcorder, there are no separate stationary heads to read and record the timing information. Instead, this information is recorded in the diagonal tracks that also holds the video and audio information.

If all goes well, the net result is a synchronized picture and sound coming out of your television set that faithfully resembles the original recorded program. But as we all know, this isn't a perfect world, and the process can go awry at any stage, leaving us with a distorted image, muffled audio or worse yet, a totally unusable tape.

Now that we know the basic process of how a VCR or camcorder records a video signal, we can now examine the particulars of the VHS and 8mm families of videotape.

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