Videeomakers at all levels of expertise can benefit from a working knowledge of the tape transport's secrets. In this article, we describe the inner workings of this crucial part of your camcorder or VCR.
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?
How it Works
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.
The Mysterious Innards
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.
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.
8mm and DVC
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.
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.
Moving Parts: Here to Stay?
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.
SIDEBAR: Are Four Heads Better than Two?
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.