The Incredible Journey: Inside the Tape Transport (page 2)

The VHS tape transport mechanism contains several recording and playback heads. There is a stationary erase head which clears the entire videotape of any previously recorded signal (only during the record mode). Next comes the rotating drum, which contains at least two video record and playback heads. Most VHS-family camcorders also have a flying erase head (more on this in a minute). The last head contains a stationary linear audio head and the control track head.

While manufacturing techniques vary from one VCR to the next, the distances between the video, audio and control heads must remain constant. In more expensive VCRs, especially industrial models, the transports use thicker and heavier mounting plates, posts and gears. This helps maintain the ever-critical tape signal distances over many more hours of usage. An inexpensive transport can warp or bend, causing time base errors in the video signals. That's why a $189 VCR typically has a useful life of two years while a $4000 industrial deck will run fine for 15 or more years.

VHS format camcorders use a tape transport very similar to the VCR, but in an effort to make VHS camcorders as small as possible, manufacturers have modified the video head. The head drum of most VHS camcorders is 41 millimeters in diameter--smaller than the 62 millimeter drum found in the VCR. Also, the tape wraps around it 270 degrees, as opposed to 180 in the VCR. It spins faster; 2700 RPM instead of 1800 in the VCR, and uses 4 heads rather than two to record the video.

Although the mathematics works out close enough for the camcorder to maintain compatibility with the VCR, the tighter curve of the smaller head drum introduces a minute distortion of the image which you cannot correct without electronic time base correction during playback. The visual result shows up as curves in images with vertical lines, such as a doorway. Some camcorders use a full-size head drum specifically to eliminate this problem.

The VHS format is suitable for VCRs which remain mounted in one place at home, but for camcorder use, the large size of the tape limits the ability to make small camcorders. Hence, the VHS-C cassette. VHS-C cassettes use the same tape as VHS, only with a smaller shell and hence less tape.

The adapter used to play VHS-C tapes in a VCR uses part of the tape as a leader. This means the first and last ten seconds of video might not appear on screen when you use the adapter, since this portion of the tape never makes contact with the heads. For this reason, it's a good idea to leave 20 seconds of unused tape at the beginning and end of a VHS-C cassette.

Another drawback of VHS-C is the lack of locking mechanisms on the cassette hubs. This means you can't prevent them from turning when they're outside the VCR or camcorder, and the tape tends to go slack and cause jams when you re-insert it into the VCR or camcorder. You can turn the hubs by hand when the tape is out of the VCR to reduce this problem.

In 8mm video, the tape transports are much smaller and simpler. Audio and timing signals exist within the diagonal tracks. There are no linear tracks along the edges of the tape, which allows manufacturers to make the transport smaller. A third head on the drum is the only erase head. There are no linear stationary heads as with VHS. Consequently, all 8mm camcorders and VCRs come equipped with flying erase heads as an integral part of the technology.

The tape is about half as wide as VHS (eight millimeters, to be precise). To make up for the narrow size, 8mm tapes use metal particle (MP) or metal evaporated (ME) tape, which is capable of packing more information into the smaller surface area. This allows two hours of playing time per tape at the normal speed, and four hours at the slower speed.

The DVC format converts all audio and video signals to digital, storing on the tape as "0s" and "1s." Because it records 10 tracks per frame, DVC's head drum spins at an extremely fast 9000 revolutions per minute. DVC formats have no stationary heads to slip out of alignment. For a complete overview of this exciting format, watch for an upcoming feature on DVC technology.

Now that we've covered the basics of how the transport works, let's discuss some of the most common tape transport problems that videomakers encounter.

One of the joys of videomaking is the ability to swap videotapes with friends. But even with improvements in video technology, sharing tapes can still lead to compatibility problems among various machines. This is due to the tape transport mechanism more than any other factor. Understanding the shortcomings of the tape transport mechanism can help you understand how to work around these problems, and in some cases prevent such problems from affecting your viewing.

In all video formats, a complex series of specially machined and angled guide pins maintain the proper position and tension of the tape against the heads for proper recording and playback. A problem can occur as soon as you insert the tape into the VCR. If one of the guide posts is slightly out of alignment, the signal will not be made in the proper location of the tape. If the post is mis-aligned the same way all the time, the tape might play fine on that machine because the machine's playback process tracks the same error it recorded. But the tape will probably not play on any other machine. If the problem is a one-time occurrence due to forcing the tape into the VCR too fast, the tape may not play properly on that VCR either. There are always very small mechanical tolerances allowed in each machine. It is a build up of these tolerances from one machine to the next which prevents a tape made on your VCR from playing on your friend's VCR. If the tolerances are within specification, the tape will play on either machine with no adjustments. If the tolerances are slightly out of spec, you may be able to adjust this on some formats by using the tracking control. If the tolerances are beyond the range of the tracking control, the tape may play with distorted results; usually seen as lines through part of the picture. Another VCR with a wider tracking range may be able to play the tape with better results.

Sometimes, VHS tapes load without making a firm contact with the control head. If this happens, the video and audio signals may record properly, but the control track signal is too weak. This mostly occurs as a result of trying to push the tape in faster than the loading mechanism can handle it. The visual result of this on the TV set is a rolling picture that refuses to stabilize.

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