The Incredible Journey: Inside the Tape Transport (page 3)

Another common problem is tape dropouts. On screen, a dropout appears as a series of small white specks or lines in the television picture. Some dropouts are the result of imperfections in the videotape manufacturing process, and cannot be corrected. Better tapes yield less dropouts. Scanning a tape repeatedly can wear the oxide coating off the tape, creating more dropouts. Worse yet, this material can become temporarily lodged in the video heads, creating larger dropouts. When the material becomes permanently lodged in the video heads, the VCR is said to have a head clog.

When head clogs occur, the dropouts can become so monumental as to nearly wipe out the top or bottom half of the picture. Usually, a professional cleaning is necessary to solve this problem.

As you can see, the mechanical tolerances of videotape transport mechanisms are extremely critical. They are measured in ten thousandths of an inch. For this reason, most VCR problems are due to mechanical problems in the transport mechanisms.

In the future, video signals might be recorded digitally onto electronic media much like videogame cartridges today. When that happens, there will be no moving parts to become mis-aligned or wear out, duplication will be almost instantaneous and perfect and editing will be simple and precise. But for now, we'll have to make the most of what videotape--and its highly complex mechanical transport--has to offer.

A four-head VHS VCR uses only two of its four heads when recording or playing a tape. The other two heads come into play to provide a clearer picture during scanning or slow-motion playback. Some manufacturers have made these extra heads narrower in width to function as the primary heads during the slower EP (extended play) speed.

Though many four-head machines are higher-quality machines in other respects, it is the quality of the heads that produce a better recording, not the number of heads.

Often, a VCR will employ two additional heads to record hi-fi stereo sound. These heads record the hi-fi signal directly over the video tracks at a different "depth" and angle in a process called depth multiplexing. For this reason, you cannot dub hi-fi audio onto a previously recorded tape without also destroying the video.

On some high-quality VCRs (and the vast majority of camcorders), the drum holds still another head to erase the tape track by track. This is the flying erase head, so called because it "flies" on the rotating head drum, as opposed to the stationary master erase head.

Flying erase heads are essential for editing, because they allow videomakers to do two things: make clean, glitch-free edits, and perform video insert edits without destroying the audio track.

--R.B.

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