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第二章第一篇
To say that we
live in an age of electronics is an
understatement. From the omnipresent integrated
circuit to the equally omnipresent
digital computer, we encounter electronic devices
and systems
on a daily basis. In every
aspect of our increasingly technological
society
—
whether it is
science,
engineering,
medicine,
music,
maintenance,
or
even
espionage
—
the
role
of
electronics
is
large,
and it is growing.
谈论关于我们生活在一个电子学时代的论调是一种空泛的论调。
从无
处不在的集成电路到同
样无处不在的数字计算机，
我们在日常活
动中总会遇到电子设备和电子系统。
在我们日益发
展的科技社会
的方方面面
——
无论是在科学、
工程、
医药、音乐、维修方面甚至是在谍报方
面
——
< br>电子学的作用是巨大的，而且还将不断增强。
In
general,
all
of
the
tasks
with
which
we
shall
be
concerned
can
be
classified
as
“
tasks. Let us
explore the meaning of this term
一般说来，<
/p>
我们将要涉及到的工作被归结为
“
信号<
/p>
——
处理
”
工作
，
让我们来探究这个术语的
含义吧。
A
signal
is
any
physical
variable
whose
magnitude
or
variation
with
time
contains
information.
This
information
might
involve
speech
and
music,
as
in
radio broadcasting,
a physical
quantity
such as the temperature of the air in a
room, or numerical data, such as the record of
stock market
transactions. The physical
variables that can carry information in an
electrical system are voltage
and
current.
When
we
speak
of
therefore,
we
refer
implicitly
to
voltages
or
currents.
However,
most
of
the
concepts
we
discuss
can
be
applied
directly
to
systems
with
different
information-
carrying
variables.
Thus,
the
behavior
of
a
mechanical
system
(in
which
force
and
velocity are the variables) or a
hydraulic system (in which pressure and flow rate
are the variables)
can often be modeled
or represented by an equivalent electrical system.
An understanding of the
behavior of
electrical systems, therefore, provides a basis
for understanding a much broader range
of phenomena.
信号就是其与时间有关的量值或
变化包含信息的任何物理变量。这种信息或
许像无线电广播的演讲和音乐，
或许是像室内温度的物理量，
或许像股市交易记录的数字数
< br>据。在电气系统中能够载有信息的物理变量是电压和电流。因此当我们谈到
“
信号
”
，
我们不<
/p>
言而喻指的是电压和电流，
然而，
我们要
讨论的大多数概念是可以被直接应用于载有不同信
息的变量的系统，
因此，
一个机械系统
（在这个系统中力和速度是其变量）<
/p>
或者液压系统
（在
这个系统中压力和流速
是其变量）
的性能通常可以用一个等效的电气系统来模拟或表示。
因
此，我们对于电气系统性能的理解为理解更宽领域的现象打下了一个基础。
A signal can carry information
in two different forms. In an analog signal the
continuous variation
of the voltage or
current with time carries the information. An
example, in Fig.2-l, is the voltage
produced
by
a
thermocouple
pair
when
the
two
junctions
are
at
different
temperatures.
As
the
temperature difference between the two
junctions varies, the magnitude of the voltage
across the
thermocouple
pair
also
varies.
The
voltage
thus
provides
an
analog
representation
of
the
temperature difference.
一个信号可以以两种形式来承载信息。
在一个模拟信号中电压或电流随
时间而产生的连续变
化载有信息。
在图
2-1
中，
当一对热电偶的接头处于不同的温度时由热电偶所产
生的电压就
是一个例子。
当两个接头之间的温度差改变时，
一对热电偶两端的电压也将改变。
于是电压
就提供了温度差的模拟表现形式
The other
kind of signal is a digital signal. A digital
signal is one that can take on values within
two
discrete
ranges.
Such
signals
are
used
to
represent
ON-
OFF
or
YES-NO
information.
An
ordinary household
thermostat
delivers
a
digital
signal
tocontrol
the furnace. When the
1
room temperature drops below a preset
value, the thermostat switch closes turning on the
furnace.
Once
the
room
temperature
rises
high
enough,
the
switch
opens
turning
off
the
furnace.
The
current
through
the
switch
provides
a
digital
representation
of
the
temperature
variation:
ON
equals
另一种的信号是数字信号。
数字信号是在两个离散的范围内能够呈现一定数值
的信号。
这种
信号常用以表示
“
开
—
关
”
或
“
是
—
不是
”
信息。
一个普通的家用
恒温器传递一种数字信号来控
制炉子当房间的温度下降到预定温度以下时，
恒温器的开关合上使炉子开始加热；
一旦房间
的温度
上升到足够高，
开关就断开使炉子关闭。
流过开关的电流提供了
温度变化的数字表示：
ON
即为
“
太冷
”
而
OFF<
/p>
即为
“
不太冷
”
A signal-processing system
is an interconnection of components and
devices that
can accept an
input signal or a group of input
signals, operate on the signals in some fashion
either to extract or
improve the
quality of the information, and present the
information as an output in the proper form
at the proper time.
一个信号处理系统是某些元件或设备之间的相互连接，
这些元件和设备能够接收
一个输入信
号或一组输入信号，
信号处理系统以某种方式来处理
这些信号即提取这些信号或提高这些信
号的品质，然后在适当的时间以适当的形式把这个
信号表示为输出量。
Fig.2-2
illustrates the components in such a system. The
central circles represent the two types of
signal
processing
(digital
and
analog),
while
theblock
between
the
two
signal-
processing
blocks
represents
the
conversion
of
an
analog
signal
to
equivalent
digital
form
(A
/
D=Analog-to-
Digital)
and
the
reverse
conversion
of
a
digital
signal
to
the
corresponding
analog form
(D
/
A=Digital-to-Analog). The
remaining blocks involve inputs and
outputs
—
getting
signals into and out of the processing
system.
图
2-2
显示了这样一个系统的组成部分。
中间的圆圈代表了两种类型的信号处理
（数字和模
拟）
，而处于信号处理框之
间的方框表示模拟信号向等效数字形式（
A/D
即模拟到数字）
的
转换，以及从数字信号向相应的模拟形式（
D/A
即数字到模拟）的逆转换。剩下的方框涉及
输入和输出
——
取得信号以及从处理系统输出信号。
Many
electrical
signals
derived
from
physical
systems
are
obtained
from
devices
called
transducers. We have already
encountered an example of an analog transducer,
the thermocouple
pair. It converts
temperature difference (the physical variable) to
a voltage (the electrical variable).
Generally, a transducer is a device
that converts a physical or mechanical variable to
an equivalent
voltage
or
current
signal.
Unlike
the
thermocouple
example,
however,
most
transducers
require
some form of electrical excitation to
operate
从物理系统获得的很多电气信号是从被称为传感器的器件中输入的。<
/p>
我们已经碰到了一个模
拟传感器的例子。即热电偶。它把温度的变
化（物理变量）转换成电压（电气变量）
。通常，
传感器是一种
将物理或机械变量转换成等效电压或电流信号的器件。
然而，
不
同于热电偶例
子，大多数传感器需要一些形式的电激励以驱动传感器
The
output
from
a
system
can
be
in
many
forms,
depending
on
the
use
to
be
made
of
the
information
contained
in
the
input
signals.
One
can
seek
to
display
the
information,
either
in
analog
form
(using
a
meter,
for
example,
in
which
the
needle
position
indicates
the
size
of
the
variable of interest) or
in digital form (using a set of digital display
elements that are lit up with a
number
corresponding to the variable of interest). Other
possibilities are to convert the output to
sound energy (with a loudspeaker),
or to use the output asan
input signal to another system, or to
use the output as a control signal to
initiate some action.
个系统的输
出可以有多种形式，
这取决于包含在输入信号中的信息所起的作用。
我们可以选
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