Technical information

form. Rather than replacing the analogue soundtrack (which is left on for broad
compatibility), the Dolby Digital data is optically recorded between the sprocket
holes on the edge of the film.
OSD On Screen Display. The menus and information screens shown by TVs, DVD
players, VCRs and some home theatre receivers on the TV screen. These make
setting up these devices somewhat easier than those devices that lack them.
Overtone See harmonic.
P
Passive loudspeaker A loudspeaker without built-in amplification for all the drivers.
Most domestic Hi-Fi loudspeakers are passive, not active.
Passive subwoofer A subwoofer without built-in amplification. Most subwoofers
are active, not passive.
PCM Pulse Code Modulation. This is standard, uncompressed digital audio of the
kind that is on compact discs. However while normal CDs are always encoded
with 16 bits and 44,100 hertz sampling rate, on DVD bit depths of up to 24 and
sampling rates of up to 96,000 are permitted.
Perceptual encoding A compression technique for digitally recorded sound.
Digital audio does not compress at all well using conventional 'non-lossy' schemes
because there is very little redundancy in the data stream. Perceptual encoding
techniques rely on extensive psycho-acoustic analyses of how the human ear and
brain detect and interpret sound. In essence they manage to achieve high levels
of compression by discarding elements of the sound that such analyses suggest
cannot be heard. In this way they can reduce the data volume of the sound by a
factor of 10 or more without obviously degrading the sound quality. Examples of
perceptual encoding compression systems are Dolby Digital, DTS (although DTS
claims that it first uses non-lossy compression techniques to reduce or eliminate
the need for perceptual encoding), MP3, SDDS, Windows Media, RealAudio and
ATRAC.
Phase The alignment or otherwise of two signals in time. 'In phase' means that
they are aligned 'Out of phase' means that one or the other is shifted in time by
one half the period (inverse of frequency) of the tone in question. If an out of
phase pair are played together, they will cancel each other out. Phase is usually
measured in degrees. A zero degree shift is in phase, while a 180 degree shift is