VideoPreservation Website

Chapter 6

All About Videotape

Introduction

No video recording is possible without videotape. The technical quality of a finished production depends greatly on the quality of the videotape used. Old, worn or poor quality videotape can produce a distorted picture or can even clog the video heads, resulting in no recording at all.

It is most important to understand how the videotape functions in the video recording process. As was mentioned in Chapter 5, a recording head produces a magnetic field which stimulates the magnetically-sensitive material on the videotape in such a way as to record and store picture and sound information which can be played back later.

Comparison with Film Recording

Film consists of various layers of chemicals on an acetate base. In contrast, videotape is a layered configuration whose surface has a magnetic coating of needle-shaped iron-oxide or chromium-dioxide particles which lie in a length-wise direction on the tape (helical scan only). A binder material holds these particles in place and makes them adhere to the polyester base. The magnetic material determines the tape's magnetic properties such as frequency response, sensitivity and SIGNAL-TO-NOISE RATIO.

Signal-to-noise ratio measures the amount of pure picture or sound signal as compared to the amount of grain or "noise" (interference) in the picture. The higher the ratio as rated in DECIBELS (db), the cleaner the picture. The surface of the tape (emulsion side),which is the side facing the VTR heads, is dark gray and is highly polished to minimize wear on the heads and to maximize tape-to-head contact. The back of the tape is usually flat black and coated with carbon which reduces static electricity caused by the friction of the tape against the tape guides. This type of tape is often called CARBON-BACKED.

There are several kinds of tape on the market, The most common isthe IRON OXIDE type tape, in use for many years and now improved considerably. Newer type tapes such as CHROMIUM DIOXIDE and COBALT-DOPED type produce greater frequency response and have a longer life than the older iron oxide type. Eventually, magnetic tape itself will be superseded by a solid-state random access memory system that will take the form of a small magnetic card, cube or cylinder.

High-Energy or High-Density Tape

All 3/4-inch, Betamax, Beta-2 and other new 1/2- inch formats utilize only HIGH-DENSITY tape. This tape requires a higher erase and recording current than standard iron-oxide tape and should not be used for recording on VTRs that are not designed for high-density tape. Certain 1/2-inch EIAJ color editors, specifically the Sony AV-8560 and the Panasonic NV-3160, have a dual-function switch that allows them to use both standard 1/2- inch tape and high-density tape (Sony V-72). High-density tape should always be used when possible, especially in editing as the picture quality and image sharpness is considerably superior (about 3 db) to standard tape.

The ability of the tape store information is expressed in density of particles or OERSTEDS OF COERCIVITY. Standard 1/2-inch EIAJ reel-to reel tape (Sony V-32 or V-30H) is rated at 300 oersteds, whereas the Sony V-72 1/2-inch cobalt-doped high-density tape is rated at 530 oersteads, and Betamax tape is rated at 750 oersteds.

Videotape Characteristics

Unlike film, any piece of videotape will record both black-and-white and color signals. However, color is NOT a function of the videotape, rather it depends on whether or not the camera, VTR or TV monitor is black-and-white or color.

Videotape determines picture quality. No matter how sophisticated the video hardware, the resulting picture quality is only as good as the videotape used. Manufacturing videotape properly requires expensive equipment and extremely strict quality control. Unfortunately, videotape tends to be expensive because the process of manufacturing is complex and critical and requires costly petroleum as a major raw ingredient. An hour of 3/4-inch videotape retails for $25-$35 with slightly lower costs for 30 and 20 minute tapes,

There are many brands of videotape on the market, some very inexpensive, which are usually used tape from TV studios-2-inches wide that is cut up into 1/2-inch tapes. Often this tape clogs the VTR and leads to expensive breakdowns and video head replacement. Unless you test it first, you can't tell how bad the tape really is until after you have recorded some very important programs on it. Then it begins to break down and loose particles will clog the machine. Use of computer or instrumentation tapes also involves a definite risk to the VTR and program material. Most people who use these brand "X" videotapes regret it sooner or later,

The best rule to follow is to use only tape recommended by the VTR manufacturer. If this is impractical, Sony tape is the helical scan standard for videotape by which all other brands are measured. Sony tape will work well on nearly any helical scan VTR. Other brands are MEMOREX and 3M. Even Sony manufactures bad batches of tape occasionally. If this happens, tape can be returned to the distributor and exchanged.

Extended-Play Tapes

Certain manufacturers offer EXTENDED PLAY TAPES. Dupont has a 90-minute 3/4-inch standard size videocassette and a 30-minute KCS type (smaller size 3/4-inch) videocassette. Panasonic hasa 90-minute''/2-inch EIAJ reel-to-reel tape and 3M manufacturers the 30-minute extended play tape for use with the Panasonic and Hitachi 1/2-inch EIAJ cartridge VTRs. The only way to gain the additional recording and playback time is to make the tape thinner—a factor which can increase the chances of the tape breaking, developing more dropouts, deteriorating faster and jamming the cartridge or cassette VTR.

CAUTION: Use of thinner tapes is not recommended for heavy use situations, such as mastering, still framing, editing or mass duplication of important programs intended for large scale distribution.

How Videotape Works

Like film, videotape is the image storage medium. But unlike most film, videotape contains the capacity to store both picture and sound. Both film and videotape record individual pictures or frames of information. 16 mm motion picture film records 24 individual frames every second while video records 30 frames a second.

These frames are visible on the film along with the sprocket holes which control the rate at which the images are run through the camera or projector. The sound track is placed next to the picture images and synchronized to match them. Soundto-film sync often requires additional equipment

A nice feature of movie film is that you actually can see the individual pictures. With videotape, the frames are not visible to the eye, but they are present in a series of diagonal lines formed by the magnetic particles on the tape.

Videotape Dropouts

Videotape DROPOUTS occur when a piece of the tape's magnetic oxide or coating flakes off or is rough. This causes a "hole" or missing line of information in the picture when viewed on the monitor during tape playback. Since dropouts are a function of the tape and not the VTR, they are visible only when the tape is played back.

These "holes" or missing lines of information are created when the video heads fail to contact the tape firmly and skip over it because of roughness, hills and valleys or actual missing flaked-off pieces of the oxide. When the tape particles flake off, the tape can never contain or store information at that point. Once a dropout occurs, it can't be corrected physically. Nearly all newer VTRs, especially the color models, feature electronic DROPOUT COMPENSATORS (DOC) or ONE-LINE DELAY circuits which electronically substitute or repeat the previous intact line of video information, thus electronically "covering up" the dropout. The circuitry senses the dropout while it occurs and replaces it with a good line of information, thus eliminating the appearance of the dropout on the monitor.

Dropouts are bad enough in black-and-white but can become intolerable in color because the dropout is so much more apparent in color. For this reason, all color VTRs (except some early model Sony AV-8600s) contain one-line delay systems. Occasionally, dropouts will exceed the ability of the one-line delay system to compensate, and some dropouts will sneak through.

One-Line Delay System Adjustment

The one-line-delay system may need adjustment when it fails to cover up dropouts adequately. On the Sony AV-8650 a marked control becomes accessible by removing the VTR from its case. While watching the tape playback on the monitor, you can play the tape with the dropouts and adjust this control for maximum compensation.

Dropouts often occur at the beginning and the end of a reel of tape, and because of this, you should never record important scenes on these portions of the reel. Tape will accumulate dropouts with use, age and abuse. Sony videotape specifications indicate that up to 25 dro pouts-perminute can be expected throughout a reel of 1/2- inch tape. If the dropouts really become constant and annoying the tape should be discarded or used for expendable programs. Loss of sync and picture instability can occur when you have 2 or 3 dropouts per second.

Videocassette Tape

The appeal of videotape recorders to home users and nontechnical persons always has been limited by the hassles of threading and handling reels of videotape. Far greater use of the audio medium immediately followed the introduction of audiocassettes and cartridges and the design of self-threading self-contained audio systems. The same has been true with video systems. The videocassette system has made it possible for many more people to deal confidently with videotape and videotape recorders.

Although videocassette tapes have many positive features, they also have certain drawbacks. These features and problems are dealt with specifically in CHAPTER 11—All About Basic Videocassette Systems. In the meantime, we will proceed to examine how everything gets plugged together, and how you can make the right connections.

CARE OF VIDEOTAPE

• Do not smoke in or near or within several rooms of VTR equipment. Check air flows

through building.

• Keep VTRs covered at all times when not in use to minimize dust accumulation.

• Keep VTRs as clean as possible.

-Use same size reels for supply and take up.

-Minimize hand contact with recording part of tape.

-Do not wrinkle or bend tape.

Do not expose tape to dust, moisture, heat or extreme cold.

•Avoid high humidity.

•Always keep tape sealed in package when not in use.

•Never stack tapes on top of each other—store tapes vertically at room temperature. •Cut off any wrinkles and severely damaged tape ends.