RF modulator
When the machine is playing back, a TV signal at baseband is produced, and this cannot be applied to a TV receiver whose only signal input facil- ity is via its aerial socket. To cater for this, an RF modulator is provided within the VCR, working as a tiny TV transmitter. Baseband audio and video signals are applied to its input for modulation onto a UHF carrier, whose frequency is chosen to fall into a convenient gap in the broadcast spectrum. A rear-access preset control swings this frequency to avoid beat effects with other RF signals in the vicinity. VCR modulators are normally programmable for any channel in the broadcast band. Output level is set to be 1–3 mV (the optimum for a TV receiver), and this modulated RF car- rier signal is added to the booster output for application to the VCR’s RF output socket.
During record, regardless of the signal source which may be off-air, TV camera, STB, cable, another VCR or whatever, the signal being recorded on tape is applied to the RF modulator so that it may be monitored on the TV set if desired. This is called the E–E (electronics to electronics) mode to distinguish it from off-tape playback mode. The E–E signal is taken off from the record electronics as late as possible, and several of the record and playback circuits (such as the luminance/chrominance adding stage) are usually included in the loop.
Clock back-up
Although most types of EEPROM can retain stored data (for 10 years or more) without the need for a sustaining voltage, it is necessary to keep the clock oscillator—but not its display—ticking over during power cuts and while the VCR is temporarily disconnected from the mains. It saves hav- ing to reset the time, and retains stored timer-recording data. Clock back- up supply comes from a small battery or electrically large capacitor which becomes isolated from the main +5 or +3.3 V supply by a diode when mains power is lost, and can sustain the oscillator and timer-programme memory for a period varying from a few minutes to several hours, depending on the design of the machine.
Video tape formats
There are four major analogue home video tape formats in the consumer field: VHS, S-VHS, Video-8 and Hi-8. VHS owes its origin to JVC, while the Video-8 formats are later developments, using more advanced fea- tures. Over the years, the competing formats have shaken down into well- defined categories. Video-8 is favoured for camcorder use, while VHS is the system for general-purpose homedeck use, and virtually the only one in which pre-recorded cassettes are produced for sale and rental. The high- band variants Hi-8 and S-VHS find a relatively small market, even though they are capable of producing better pictures than the others.
VHS stands for video home system: this format was designed by the JVC company in Japan. It was released in Europe in 1978. A clever adap- tation of the standard tape and cassette package is VHS-C (compact) which permits a small and light VHS camcorder. The cassette is a small
(92 mm X 59 mm X 23 mm) housing containing up to one hour’s worth of standard 12.7 mm VHS tape. It fits a small camcorder weighing less than 1 kg, and incorporating a small head drum, thin direct-drive motors and a solid-state image sensor. Back at home the small cassette is loaded, piggy- back style, into a normal-size adaptor shell for replay or editing in the standard VHS machine.
Super-VHS is an advanced ‘high-band’ variant of the established format. It uses a high FM carrier, with deviation from 5.4 to 7 MHz. To permit this, an advanced video tape formula is used in a cassette of conventional size, shape and running time. A very small video head gap, and ‘fine grain’ magnetic tape permits a baseband video frequency response approaching 5 MHz, and on-screen resolution better than 400 lines. Noise ratio is also better than that of the low-band formats.
The Video-8 format uses 8 mm-wide tape, from which it gets its name. Unlike the others, this system was developed by a consortium of compa- nies and accepted for use by over 127 of the world’s major audio/video manufacturers. It was the first domestic video system to use digital audio recording. Its other advantages are a small cassette (92 mm X 58 mm X 20 mm), facilitating miniaturisation of the equipment, be it portable or home-base type; a flying erase head for good edits, also featured in some VHS equipment; and the exploitation of new tape and head materials and techniques for better performance. Although offered in home-base form for tabletop use, Video-8 is seen mainly as a camcorder format, whose pri- mary advantages are excellent sound, light weight and high performance.
As with VHS, development of the basic Video-8 format led to the pro- duction of a high-band variant, with picture resolution exceeding 400 lines under favourable circumstances. The Hi-8 variant, has higher FM record frequencies and a greater sideband spread than Video-8, as shown in Figure 21.42. It is used with metal evaporated (ME) and high-grade metal powder (MP) tapes. In camcorders, which is the form in which nearly all Hi-8 hardware is produced, the lens and CCD image sensor are upgraded to accommodate the higher resolution capability, and the bandwidth of the luminance path widened to suit. As with S-VHS, provision is made for separate handling of Y and C video components throughout the chain, and for their passage through S-links to suitably equipped TVs, monitors and copy-VCRs. In other respects, Hi-8 is similar to Video-8 format, with the same cassette dimensions, sound systems, tracking and transport arrangements.