ALTERNATIVE SURROUND SYSTEMS
The passage of time and the advent of digital transmission and stor- age media for TV has introduced new systems of conveying sur- round sound. THX is an enhanced variant of Dolby Pro-Logic processing requiring its own software, decoder and speaker system. AC-3, now known as Dolby Digital, has five separate channels of audio: left, right, centre, left rear and right rear, plus a dedicated sub- woofer channel. It gives tighter ‘targeting’ of the soundfield, and can offer completely separate signals in the two rear channels. First avail- able from NTSC laser discs and DVDs, it depends on the more gener- ous capacity of digital media. Similar in this respect is the MPEG2- audio/5.1 audio standard.
HOME CINEMA
The main components of a home cinema set-up are a large viewing screen and a surround-sound system. The screen may be a direct- view or projection type of the sorts described in Chapter 5, with large widescreen and projection types favoured best. Signal sources, as well as terrestrial and satellite broadcasts, are videotape (whose drawback is that movies are only available in relatively low-quality standard- VHS format) and video disc, laser or DVD type. The latter gives best possible sound and vision reproduction, especially with a wide screen and a good audio system because it is the only domestic recording medium which is designed for widescreen and surround sound, without compromises.
Other ‘widescreen’ TV picture sources, excepting true 16:9 aspect- ratio digital broadcasts, really consist of ‘letterbox’ pictures, in which a standard 4:3 aspect-ratio system is used, with black bands above and below the picture. It has the disadvantages of wasting up to a third of the available screen area on a conventional picture tube, and of impairing the vertical definition on a wide screen, because the vertical scan amplitude has to be increased to fit the picture to the screen; the result is a coarse line structure. Cinema films recorded on VHS videotape are likewise in letterbox format, and the same limitations apply. Those VHS videorecorders sold with ‘widescreen compatibility’ have no special format or processing features beyond an ability to detect an on-tape flag and pass it to the TV as a midpoint voltage (6–8 V) at Scart pin 8: a suitably equipped widescreen TV or monitor auto-switches to widescreen scanning on receipt of this. The way in which 4:3 aspect-ratio pictures are adapted to wide screens was shown on page 213.