Car enthusiasts spend hours under the hoods of their machines, but few video enthusiasts know what goes on inside a VCR. Join us for a nuts-and-bolts look at how home video equipment produces a moving, talking picture from a reel of magnetic tape.
Did you ever notice that sometimes the movies that you rent from your local video store don't play very well on your VCR? Perhaps that ball game that you taped last week has a white fuzzy streak running across the middle of it, or your camcorder munches your tape as you record that once-in-a-lifetime event. These are symptoms of your VCR or camcorder being out of whack. This is either because the tape you are watching is bad, or the natural wear and tear from hours of use. You can fix most of these problems. All you have to do is look under your VCR or camcorder's hood, recognize what you are looking at and take care of the problem. For serious problems, take the time to talk to a reputable VCR technician.
To help you understand the gadgets, wheels, buttons and pegs under your VCR's hood; we will look at the tape transport system. We'll also look at the various kinds of VCR head drums. We'll look at the differences between formats and the advantages and disadvantages each format has with tape transport and recording styles. Finally, we will discuss some troubleshooting techniques that could save you money on expensive repair bills, and explain why some tapes just don't want to play on your machine.
The Parts Department
Before you can fully understand a system, it is a good idea to know the parts. In a VCR, it's important to understand the functions of a number of parts. The largest part of the VCR's tape path mechanism is the head drum. You will find this large, precisely tilted cylinder at the center of the VCR. The tape travels across it diagonally in a method called a helical scan. As you look at the drum, you will notice small objects called "heads."
The heads of a VCR are small electromagnets, and either erase the signal from a tape, read the signal that is already on the tape or record a signal to the tape. They do this by reading or changing the polarity of the oxides found on the surface of the tape.
In a VCR, there are eight different types of heads. The full erase head, found on VHS machines, is a stationary head located just before the head drum in the tape path. It erases everything that is on the portion of the tape that passes over the head. This head erases the video track, all audio tracks and the control track. It does this so the new information that you record onto the tape will have no trace of the old information to interfere with it.
You will find a flying erase head, located on the main drum assembly, on some VHS and virtually all S-VHS, 8mm and Hi8 format VCRs. This head erases along the same path as the record heads, and gives the VCR the ability to handle precise, glitch-free editing.
The video playback heads, located on the drum assembly, convert the signal recorded on the tape back into an electrical signal and feeds it to the outputs of your VCR.
You will find the audio playback heads in different locations for different formats. The VHS hi-fi, S-VHS hi-fi and all 8mm audio playback heads are found on the drum assembly. See the positions of the flying erase, audio and video heads in Figure 2. If your camcorder or VCR is a VHS unit, you will find the standard analog audio playback head on a post located next to the drum assembly.
Each video record head records one field of a video signal. There are usually two video record heads on the drum. Each head records a video field, and the two fields combine to make up the video frame. Some VCRs or camcorders have four heads. Increasing the number of heads by two gives the recorder the ability to record video using a smaller drum. It also wraps the tape three-quarters of the way around the drum instead of the usual one-half. Since there are four heads, the recorder can switch between the working heads to ensure proper placement of each video track. On some VCRs the four heads split, so that two heads record video and the other two heads provide clearer slow motion and pause playback.
There are a couple of different types of audio record heads. The hi-fi stereo audio head, located on the drum assembly, records high-quality stereo in the same track as the video. The audio signal records deep within the oxides of the tape, and the video signal records in a layer closer to the surface of the tape. Although this provides for great audio, it makes it impossible to dub new audio to the tape without destroying the video on top of the audio. Hi-fi VHS, S-VHS and AFM 8mm all have this type of audio record head.
Another audio record head is located after the drum assembly for linear VHS and S-VHS audio. It records a linear analog audio track. This track is often mono, but some models split the audio into low-fidelity stereo. To accompany the linear audio record head, decks that are capable of audio dubbing will also have an audio erase head. This head, located immediately before the linear audio head, erases anything already on the audio tracks.
The final type of head is the control track head. This head records a sync signal onto the tape to enable the VCR to play back the tape at the same speed every time. This recorded signal also controls the counter on the front of your VCR. Located on the same post as the linear audio record head on VHS family models, this head lays down a continuous linear control track. On 8mm and Hi8 recorders, the control/tracking head, located on the drum assembly, lays down a signal on the same track as the video signal.
The Tale of the Tape
When you slide your favorite movie or a blank cassette into the VCR, a number of things happen. The protective flap on the video cassette opens, and when you hit play or record, two slim, smooth pins called loading pins slide up behind the tape and pull it into the VCR's mechanism. Rollers and guide pins gently press the tape against a series of heads located on the VCR's head drum. These various heads alternately erase, record or play back the audio and video information depending on what buttons you press on the front of the deck.
Let's look at what the tape does on its journey through the system. In a standard VHS VCR or camcorder, the tape first encounters the full erase head. When you press the record button, this head erases the tape as it passes across it. The tape is now ready to record a new signal.
After erasing the tape, it then wraps part way around the spinning drum found at the center of the VCR. This drum contains all or some of the following: video record and playback heads, stereo audio heads and a flying erase head. Remember helical scan? As mentioned before, the drum is spinning at a high rate of speed at a very precise angle. The tape passes across this slanted drum creating a pattern called a helix. This helical scan is the secret to how VCRs and camcorders can record so much information on a relatively short tape. Because the tape runs across the drum at an angle, the recorder can lay down more and longer parallel tracks. This gives the record and playback heads the chance to read or record onto the slim diagonal strips provided. As the drum spins and the tape feeds over it, it records or plays back the video, and in some formats, the audio information.
In the VHS family, the tape then passes across two fixed heads, stacked one on top of the other. These two heads are the linear audio record head and the control track head. If your VCR is capable of dubbing linear audio, the tape will pass a linear audio erase head before it gets to the audio record head.
The final major part of the tape transport system is the capstan. This slender metal post rotates at a very consistent and precise speed. A rubber roller presses the tape against the capstan so that when it spins, it pulls the tape along through the mechanism at a constant rate. From there, the tape then goes to its take-up reel.
Different Formats--Different Tracks
As we described earlier, the VHS tape has one or two linear analog audio tracks along the top edge of the tape. There are video tracks in the center of the tape, and if your equipment permits, hi-fi stereo audio tracks. Along the bottom of the tape is a control track. However, not all formats are created equal.
Unlike the VHF family, the 8mm family of tape formats has no linear tracks and there is no need for a stationary full-track erase head. The drum assembly contains all the heads; audio, video, control and flying erase heads. The tape is 8mm wide, two-thirds as wide as VHS tape.
The tracks on the 8mm tape are all diagonal, including the tracking section that is equivalent to the VHS linear control track. Sectors divide each diagonal track. This format combines video, AFM audio and the tracking signal all within the same sector. It also includes a digital stereo PCM audio sector and a time code sector. Like the hi-fi tracks in VHS tape, you cannot change the AFM audio without changing the video. You can edit the PCM audio sector however, without changing your original video.
DV formats have a similar transport system as the 8mm format. There are no stationary heads that could become misaligned. The drum contains all the necessary heads. Like the 8mm format, all the tracks are diagonal, and there are no linear tracks. The big difference in this format is the way it records all the different information.
The DV format divides each track into four sectors: sub code, video, audio, and Insert Track Information (ITI). Each sector contains digital information and has a specific layout and function.
- The sub code sector contains the time code information and index identification that you can use to locate specific shots on the tape or identify specific still pictures.
- The video sector is unique among all the formats. Each video sector contains one-tenth of a frame of digital video information. This is possible because the DV format head is revolving at 9000 Revolutions Per Minute (rpm) compared to 2700rpm for camcorders and 1800rpm for standard VCRs. The video sector also contains useful information such as the recording time and date, the input source and the video processing data needed for recording in the 16x9 aspect ratio.
- The audio sector contains stereo digital PCM audio. You may choose between one 16-bit, CD-quality stereo signal, or two 12-bit, near-CD-quality stereo signals. You can edit either one of these choices.
- The ITI sector contains the signal needed to maintain tracking, and allows video and audio insert editing. While the DV format is relatively new, its high-quality and versatility, as well as simple tape transport mechanism, should make it a star in the video production field.
Cleaning and Troubleshooting
Now that you know the parts and function of the tape transport system, lets look at some typical problems related to the system. If you insert a tape in your VCR and get nothing but snow, one or two simple things may be wrong, or a few serious problems may have developed. The first and most obvious answer is that the tape is blank. The second and more troublesome answer is that the playback head is dirty. You can use a good cleaning tape quite successfully to take care of this problem. Just follow directions. There are two types of cleaning tapes available, a wet type cleaning tape that uses a cleaning fluid on the tape and a dry abrasive type cleaning tape. The wet type is better for your deck. Read your owner's manual and follow the suggestions given. Snow may also be an indication of more serious problems. If cleaning the heads does not help, see your local repair shop.
If you'd like to get into your machine to clean it manually, take the time to buy "head cleaning sticks" with chamois tips and head cleaning fluid. Do not use cotton swabs on video heads! The cotton fibers can stick in the heads and cause more problems. Always clean across the heads in the direction of tape travel. Don't clean up and down or perpendicular to the tape travel. Remember to be careful in there, bumping tape guides or other parts can throw things out of alignment. Always remember to disconnect the VCR or camcorder from power before opening it. Unless you have problems with video snow, intermittent color or loss of hi-fi audio, leave the video head drum alone!
If the tape you are watching is suddenly "eaten", it is probably due to a dirty or worn idler tire (which controls the take-up reel), preventing the take-up reel from turning. If this seems to be the problem, consult your dealer or VCR repair service.
If the top of the video is constantly shifting, or it rolls inconsistently, the tape may need "packed," which is accomplished by fast-forwarding to the end of the tape and then rewinding back to the beginning. If the tape is in the cassette unevenly, it can pull the tape mechanisms out of sync and create problems. By fast-forwarding and rewinding the tape, you distribute the tape evenly in the cassette and alleviate this problem.
The most common problem you may come across is bad "tracking." If you recorded the tape you are playing in a different VCR or camcorder, its alignment might be slightly off. If this is the case, the picture will have a fuzzy, noisy area in one section of the screen. You can fix this problem by turning the tracking control knob on your VCR. This knob moves the guide pins in your VCR to duplicate the alignment of the machine that recorded the tape. The change in alignment may be very slight, but in the videotape transport system, being a little out of whack can cause a whole lot of problems.
Shutting the Hood
Taking the mystery out of the tape transport system should give you a better understanding of how the various format systems work. Keep this in mind when you go shopping for a new VCR or camcorder. If you have specific editing and audio needs, you want to make sure you buy a format and VCR that will enable you to perform the functions you need.