Sample Chapter: 7 Analogue video recording
Sample chapter from Mike Constant's book: The Principles & Practice of CCTV
Analogue Video Recording
The human eye is an incredibly adaptable device that can focus on distant objects and immediately refocus on something close by. It can look into the distance or at a wide angle nearby. It can see in bright light or at dusk, adjusting automatically as it does so. It also has a long 'depth of field'; therefore, scenes over a long distance can be in focus simultaneously. It sees colour when there is sufficient light, but switches to monochrome vision when there is not. It is also connected to a brain that has a faster updating and retentive memory than any computer. Therefore, the eyes can swivel from side to side and up and down, retaining a clear picture of what was scanned. The brain accepts all the data and makes an immediate decision to move to a particular image of interest, select the appropriate angle of view and refocus. The eye has another clever trick in that it can view a scene of great contrast and adjust only to the part of it that is of interest.
The predominant method of recording video pictures at the time of publication of this book is by analogue video recording. In analogue recording, the voltages that make the composite video signal are recorded on to magnetic tape; the changes in voltage magnetise and demagnetise the tape. To play back the recording the changes in magnetism on the tape are converted back in to voltages and the composite video signal is re-created for connection to a video monitor.
A video tape recorder is a complex integration of electronics and extremely high precision mechanics. There have been several types of recording systems in recent years, the main contenders being 'Betamax' from Sony, 'Video 2000' from Philips and 'VHS' from Matsushita. They are all based around a tape contained in a cassette with a supply spool and a take up spool. However, there were both electronic and mechanical differences that prevented one tape being used on another make. The one to emerge as the standard throughout the world is the VHS system. VHS means Video Home System and was developed by the JVC Company in Japan.
The VHS Video Recorder
All video tape recorders follow the same principles as an audiocassette recorder. That is, a tape containing thousands of tiny magnets, each with a north and a south pole is passed through a varying magnetic field. The magnetic field is generated in a revolving drum from the video signal. This reproduces the video signal onto the tape.
The tape is stored in a sealed cassette with a flap at the front protecting the tape. When the tape is loaded into the recorder, a mechanism draws the cassette into and down the machine.
The catch holding the front cover is released and the cover opened. The cassette drops over two threading posts as shown in the first diagram. When one of the functions such as play or record is operated the tape is drawn around the head drum as shown in the second diagram.
Principles of Video Recording
The descriptions give here are of necessity over simplified and are intended to illustrate the basic principles of recording. As stated before, the two essential elements of a video tape recorder are a rotating head assembly and the tape passing around a drum and head. The head consists of a ferrite ring with its continuity broken by a small gap. A coil is wound round the ring which, when energised, creates a magnetic field. The magnetic field in the ring concentrates in the gap. An essential aspect of design is that head gap is in the order of 0.3 microns. A micron is one-millionth of a metre. Therefore, 0.3 microns is about one-hundredth the thickness of a human hair.
The video signal is fed to the magnetic coil and creates an analogue version in the form of a magnetic field. As the tape passes the gap in the head the magnetic field causes the 'internal magnets' to align according to the signal passing through the head.
This makes a magnetic copy of the signal on the tape. The tape passes the drum at a fixed speed, therefore low frequencies will create long 'magnets' in the tape, and high frequencies will create short 'magnets'.
Tracks On Tape
The tape consists of an insulated base material with a fine oxide coating. For various reasons, the head is displaced at an angle to the tape. This is known as helical scanning and is standard for all recorders. The magnetic information is recorded at an angle across the tape.
The width of tape for standard VHS is 12.65mm (1/2"). The speed for standard real time recording is 23.39 mm/sec. Early video recorders and some domestic VHS recorders still available today had two coils, or heads, on each head cylinder. This worked well while the tape was moving, producing moving pictures on playback. However, when the pause function on the recorder was activated to view a single still picture horizontal noise bars would appear on the picture because the head was not moving fast enough to capture the single picture from the tape accurately.
The solution to this problem was introduced when the first four-head video recorders were made. These use four coils or heads, two each on opposite sides of the head cylinder. By using four heads instead of two twice the amount of information could be written to or read from the tape. Four head video recorders can replay still images without any noise bars and this has led to their general use in domestic and CCTV video recorders, replacing the older two-head design.
The heads are spaced 65 microns apart for a standard VHS time-lapse recorder and these lay down tracks on to the tape, which are 58 microns wide. Head cylinders of this design are known as type SP heads.