VideoPreservation Website

Chapter 5

The VTR

Quadruplex and Helical Scan Recording

As we've mentioned, the largest VTRs may be the size of two refrigerators and could cost $200,000 or more. These are the big QUAD format machines that use 2-inch wide videotape and produce the best quality color pictures.

Quad or QUADRUPLEX VTRs reside mainly in TV broadcasters' studios and require expensive human and technical support systems. Basically, the quad is a 4-head video recording system that produces pictures on the tape by scanning the videotape at a 90° angle as it travels by at high speed-15 IPS (inches per second). One roll of 1-hour quad tape costs $250-S300. The quad is an expensive animal to feed.

The other type of video recording process is known as HELICAL SCAN RECORDING. Invented by Toshiba Corporation of Japan in 1953, helical scan utilizes a screw-like scanning process that places the video information DIAGONALLY along the tape. The helical scan mechanism uses only 1 or 2 heads, is much simpler than the quad process, and moves the tape along much slower—from 9 to 11/2 ips. This slower rate of tape travel and the use of smaller tape greatly reduces tape consumption and operational and maintenance costs of helical scan recording relative to quad recording.

Tape size or width determines the TAPE FO R-MAT. A particular VTR is designed for one particular tape format only. At present, there are 5 different tape formats-2-inch, 1-inch, 3/4-inch, 1/2-inch and 1/4-inch. A 3/4-inch wide videotape can only be used with a VTR specifically designed to use 3/4-inch tape. Thus the VTR becomes known as a 3/4-inch VTR or a 3/4-inch format VTR.

None of the different tape formats are compatible with any other format and in some cases, there is no compatibility even within the same format. For example, Sony has manufactured three different yet incompatible 1-inch VTR formats. It sounds crazy, but don't despair.

Although you can't take a 1/2-inch tape which was recorded on a '/2-inch VTR and play it back on a 1-inch VTR, it's quite possible electronically to TRANSFER or COPY the tape to any other size VTR format. In this case, you are making a SECOND GENERATION copy of the tape which will probably suffer a slight quality loss but still be quite acceptable. So, even though VTRs may not be physically compatible, they are all electronically compatible, especially with the help of a time base corrector.

A Comparison of Film and Audio Recording

There are 2 common methods of recording picture and sound—chemically, as in film recording, and electronically, as in video recording. In the first process, the film image undergoes a chemical change in reaction to light. In the second instance, the video image and audio elements undergo an electronic conversion from light and sound to voltage and magnetism.

Since video and audio are both electronic media, they bear a closer relationship to each other than video does to film. It is important to realize that the video recording process evolved from the audio recording process rather than from the film recording process.

THE VTR RECORDING PROCESS Recording Heads

Like an audio recorder, a recording VTR uses a series of magnetic RECORDING, ERASE and PLAYBACK HEADS. These heads contain electromagnetic coils which when energized, create a magnetic field in the coil. This in turn causes the magnetic particles on the videotape to respond and store the electronic information.

The VTR is unique because it employs special rotating VIDEO HEADS which record and play back the picture component of the electronic information. The video heads are made up of tiny magnetic coils with a small gap between them.

Unlike stationary audio heads, video heads will not work if they are standing still. Instead, video heads must rotate at a high speed, in the opposite direction to the tape travel. This rotation is apparent when the VTR is placed in PLAY or RECORD mode and a pronounced whirring sound is heard emanating from the video head drum.

Inside the Video Head Drum

On all 1/2-inch EIAJ VTRs, two video heads are used, one located on each end of a bar which is mounted on a circular rotating plate.

The head bar rotates at 1800 rpm (revolutions per minute) if it has 2 record/play video heads, and 3600 rpm if it has only one record/play video head—as is the case with many 1-inch VTRs.

Sophisticated editing VTR s such as the Sony AV-8650 and VO-2850 and the Panasonic NV-3160 employ 4 rotating video heads-2 heads to record and to play back the picture, and 2 additional erase heads mounted just ahead of the record/play heads to insure that erasing of the old picture information is complete right up to the point of the new recording. The stationary erase head is still used, but the distance between the rotating video heads and the stationary erase head can prevent precise editing of the new picture. Therefore, ROTARY ERASE HEADS are used to insure perfect erasure of old picture information right up to the point of the new edit. In 3/4- inch videocassette recorders, the video heads are integrated into the rotating head drum itself. This system is simpler and has fewer problems. Some new generation 1-inch VTRs (Ampex and Sony) also use 1 or 2 additional heads, as well as the flying erase and record/playback heads for improved tracking, broadcast quality slow-motion and for monitoring playback during recording.

The Rotating Video Head

Video heads must rotate at high rates of speed because of the necessity to record and reproduce the very wide range of frequencies required by the video picture. Even the most sophisticated of stationary recording heads used in high quality audiotape recorders can record and play back only 20 to 20,000 Hz. HERTZ (Hz) is the official designation for cycles per second. Video, however, requires a range of 2.5 MHz (million, or MEGA-HERTZ) for black-and-white, and 3.5 to 4.5 MHz for color—or 18 octaves! Frequency response is traditionally extended in several ways:

1. improve the electronics and the tape

2. increase the size of the tape

3. increase the speed of the tape past the head