What's Under The Hood: Inside Your VCR or Camcorder

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.