Technical information

some DVDs. This permits up to 7.1 channels of audio to be encoded, although
only 5.1 channels is ever seen in practice and 7.1 is now obsolete. MP3 is one
particular form of MPEG stereo.
Multichannel sound Often called surround sound. Any system designed to deliver
more than two channels (stereo) of sound to the consumer. The most common
forms these days deliver 5.1 channels of audio. Some systems can process two
channels, or even one channel, of sound to produce the effect of multi-channel
sound.
N, O
Noise shaping A system of adding dither noise to a digital audio signal so that it is
biased towards high frequency noise. This permits the noise floor to be lower in
the range of frequencies to which the ear is most sensitive, at the cost of poorer
noise performance in the higher frequencies where the ear is less sensitive.
Nominal impedance A specification of the load a loudspeaker places on an
amplifier, measured in ohms. It tends to oversimplify the actual situation because
the impedance of a loudspeaker varies according to frequency. So a speaker with
a nominal impedance of eight ohms may actually present a load of six ohms at
some frequencies and thirty ohms at others. Common loudspeaker impedances
are four, six and eight ohms. Back in the 1960s, higher impedance speakers --
16 and 32 ohms -- were fairly common.
Octave A range of frequencies of some form of repetitive wave, where the highest
frequency is precisely twice the lowest frequency. In music, for example, the
fundamental frequency of A below Middle C is 220 hertz. The A above Middle C is
440 hertz.
Ohm A unit of electrical resistance or impedance. In a DC circuit, the number of
ohms of resistance offered by a component can be calculated by dividing the
voltage across the component by the current (in amps) flowing through it.
Optical digital A method of communicating digital audio between components
using light carried on optical fibre. See TOSLink. The data format accords with
the S/PDIF specification.
Optical soundtrack The traditional medium for carrying a film's soundtrack. The
sound was converted to markings which were developed onto the edge of the film.
The majority of optical soundtracks were analogue (in the form of squiggly lines,
like the track on an LP). However Dolby Digital is also carried on the film in optical