Page
115
NewS
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User Guide and Technical Manual
Term
Defi nition
NTSC
National Television System Committee
Color television standard used in the US. Provides 525 horizontal lines of
resolution. Not compatible with PAL or SECAM.
OFDM
Orthogonal frequency-division multiplexing [OFDM] is a method of encoding digital
data on multiple carrier frequencies. OFDM has developed into a popular scheme
for wideband digital communication, whether wireless or over copper wires, used
in applications such as digital television and audio broadcasting, DSL Internet
access, wireless networks, powerline networks, and 4G mobile communications.
OFDM is a frequency-division multiplexing [FDM] scheme used as a digital
multi-carrier modulation method. A large number of closely spaced orthogonal
sub-carrier signals are used to carry data on several parallel data streams or
channels. Each sub-carrier is modulated with a conventional modulation scheme
[such as quadrature amplitude modulation or phase-shift keying] at a low symbol
rate, maintaining total data rates similar to conventional single-carrier modulation
schemes in the same bandwidth.
The primary advantage of OFDM over single-carrier schemes is its ability to cope
with severe channel conditions [for example, attenuation of high frequencies in a
long copper wire, narrowband interference and frequency-selective fading due to
multipath] without complex equalization
fi
lters. Channel equalization is simpli
fi
ed
because OFDM may be viewed as using many slowly modulated narrowband
signals rather than one rapidly modulated wideband signal. The low symbol rate
makes the use of a guard interval between symbols affordable, making it possible
to eliminate intersymbol interference [ISI] and utilize echoes and time-spreading
[on analogue TV these are visible as ghosting and blurring, respectively] to
achieve a diversity gain, i.e. a signal-to-noise ratio improvement. This mechanism
also facilitates the design of single frequency networks [SFNs], where several
adjacent transmitters send the same signal simultaneously at the same frequency,
as the signals from multiple distant transmitters may be combined constructively,
rather than interfering as would typically occur in a traditional single-carrier
system.
PAL
Phase Alternation Line
Color television standard used in many European countries. Provides 625
horizontal lines of resolution. Not compatible with NTSC or SECAM.
PCM
Pulse-Code Modulation
PCM is a method used to digitally represent sampled analog signals. It is the
standard form of audio in digital applications. The amplitude of the analog signal
is sampled regularly at uniform intervals; each sample is quantized to the nearest
value within a range of digital steps. PCM streams have two basic properties that
determine their
fi
delity to the original analog signal:
•
the sampling rate,
•
the number of times per second that samples are taken;
•
the bit depth, which determines the number of possible digital values that
each sample can take.
