23-02-2013, 12:28 PM
NOISE AND DISTORTION
NOISE AND DISTORTION.pdf (Size: 171.49 KB / Downloads: 340)
noise can be defined as an unwanted signal that interferes with the
communication or measurement of another signal. A noise itself is a
signal that conveys information regarding the source of the noise.
For example, the noise from a car engine conveys information regarding the
state of the engine. The sources of noise are many, and vary from audio
frequency acoustic noise emanating from moving, vibrating or colliding
sources such as revolving machines, moving vehicles, computer fans,
keyboard clicks, wind, rain, etc. to radio-frequency electromagnetic noise
that can interfere with the transmission and reception of voice, image and
data over the radio-frequency spectrum. Signal distortion is the term often
used to describe a systematic undesirable change in a signal and refers to
changes in a signal due to the non–ideal characteristics of the transmission
channel, reverberations, echo and missing samples.
Noise and distortion are the main limiting factors in communication and
measurement systems. Therefore the modelling and removal of the effects of
noise and distortion have been at the core of the theory and practice of
communications and signal processing. Noise reduction and distortion
removal are important problems in applications such as cellular mobile
communication, speech recognition, image processing, medical signal
processing, radar, sonar, and in any application where the signals cannot be
isolated from noise and distortion. In this chapter, we study the
characteristics and modelling of several different forms of noise.
Introduction
Noise may be defined as any unwanted signal that interferes with the
communication, measurement or processing of an information-bearing
signal. Noise is present in various degrees in almost all environments. For
example, in a digital cellular mobile telephone system, there may be several
variety of noise that could degrade the quality of communication, such as
acoustic background noise, thermal noise, electromagnetic radio-frequency
noise, co-channel interference, radio-channel distortion, echo and processing
noise. Noise can cause transmission errors and may even disrupt a
communication process; hence noise processing is an important part of
modern telecommunication and signal processing systems. The success of
noise processing method depends on its ability to characterise and model the
noise process, and to use the noise characteristics advantageously to
differentiate the signal from the noise. Depending on its source, a noise can
be classified into a number of categories, indicating the broad physical
nature of the noise, as follows:
(a) Acoustic noise: emanates from moving, vibrating, or colliding
sources and is the most familiar type of noise present in various
degrees in everyday environments. Acoustic noise is generated by
such sources as moving cars, air-conditioners, computer fans, traffic,
people talking in the background, wind, rain, etc.
(b) Electromagnetic noise: present at all frequencies and in particular at
the radio frequencies. All electric devices, such as radio and
television transmitters and receivers, generate electromagnetic noise.
© Electrostatic noise: generated by the presence of a voltage with or
without current flow. Fluorescent lighting is one of the more
common sources of electrostatic noise.
(d) Channel distortions, echo, and fading: due to non-ideal
characteristics of communication channels. Radio channels, such as
those at microwave frequencies used by cellular mobile phone
operators, are particularly sensitive to the propagation characteristics
of the channel environment.
(e) Processing noise: the noise that results from the digital/analog
processing of signals, e.g. quantisation noise in digital coding of
speech or image signals, or lost data packets in digital data
communication systems.
White Noise 31
Depending on its frequency or time characteristics, a noise process can
be classified into one of several categories as follows:
(a) Narrowband noise: a noise process with a narrow bandwidth such as
a 50/60 Hz ‘hum’ from the electricity supply.
(b) White noise: purely random noise that has a flat power spectrum.
White noise theoretically contains all frequencies in equal intensity.
© Band-limited white noise: a noise with a flat spectrum and a limited
bandwidth that usually covers the limited spectrum of the device or
the signal of interest.
(d) Coloured noise: non-white noise or any wideband noise whose
spectrum has a non-flat shape; examples are pink noise, brown noise
and autoregressive noise.
(e) Impulsive noise: consists of short-duration pulses of random
amplitude and random duration.
(f) Transient noise pulses: consists of relatively long duration noise
pulses.
White Noise
White noise is defined as an uncorrelated noise process with equal power at
all frequencies (Figure 2.1). A noise that has the same power at all
frequencies in the range of ±∞ would necessarily need to have infinite
power, and is therefore only a theoretical concept. However a band-limited
noise process, with a flat spectrum covering the frequency range of a bandlimited
communication system, is to all intents and purposes from the point
of view of the system a white noise process. For example, for an audio
system with a bandwidth of 10 kHz, any flat-spectrum audio noise with a
bandwidth greater than 10 kHz looks like a white noise.