Technical information

it was developed by them, but the name above seems to have come into common
currency.
Dolby HX Dolby Headroom eXtensioin. Not to be confused with the various sorts
of Dolby noise reduction, Dolby HX is a process that improves the high frequency,
high level recording performance of magnetic tape, particularly compact
cassettes.
Dolby noise reduction It was the invention of an effective noise reduction circuit by
Ray Dolby in the 1960s that got Dolby Laboratories onto the road to where it is
today. This circuit was developed into Dolby Type A noise reduction, which
became very widely used for professional analogue recording onto tape, prior to
the advent of digital recording. Most analogue recording media produce
background noise, typically like white noise. The simple solution would be to boost
the middle and upper frequencies during recording, then cut them on playback,
thus also cutting the hiss. But this causes overloading problems. Dolby noise
reduction systems rely on the masking effect of sounds. If the signal was loud,
the hiss would be largely inaudible anyway. So Dolby's system tracked the level of
the signal, leaving it unaltered when it was strong, but introducing the boost/cut
system when the signal was low. Dolby noise reduction later made it onto
consumer equipment in the form of Dolby B, C and S noise reduction systems
which all work along similar lines. It was Dolby B, in particular, that allowed the
compact cassette to become an established media. Dolby HX, which also appears
on recent cassette decks, is not to be confused with the noise reduction systems.
Dolby Pro Logic An improved version of Dolby Surround which decodes two
channel Dolby Stereo sound tracks to four channels. In addition to the front left,
front right and mono surround channels, it also extracts a front centre channel
signal. The use of a centre channel improves the localisation of sound, particularly
dialogue, for those viewers not seated directly in front of the screen.
Dolby Pro Logic II A further enhancement of the two-channel based Dolby Pro
Logic, this cleverly decodes separate left and right surround channels from the
original signal and eliminates the 100 to 7,000 hertz bandwidth limitations of the
older system.
Dolby Stereo The original name for Dolby Surround, as it was used in cinemas. In
cinema usage 'stereo' tended to mean some form of surround sound, although it
was usurped by the home entertainment industry to mean two channels at the
front of the room.
Dolby Surround The home equivalent of Dolby Stereo. This was the original
system for decoding sound tracks for surround sound in the home, usually from
video cassette. Dolby Surround sound tracks carry three channels of sound,