Dell 2135CN, Service Manual

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Chapter 6 Principle of Operation (Fax)
Version 1 2008.02.01
1.2 Analog and Digital Signals
An electrical signal generated by the telephone's microphone is an analog signal. The waveform of this
analog signal fluctuates responsive to the voice volume. When the voice is loud, the amplitude
(voltage) increases; when soft, the amplitude decreases. When the voice is high-pitched, the frequency
(number of vibrations) increases; when low-pitched, the frequency decreases.
A signal whose values change in a continuous manner with time like this is called an analog signal. In
contrast, a digital signal is a set of values that change with time in a discrete instead of continuous
manner. In other words, an analog signal is like a hill. A digital signal is like stairs.
A digital signal is a series of values obtained by sampling a continuous analog signal at a certain
required rate. For example, when sampling is by time, the rate is once a second, millisecond, etc.
Because the sampling reduces the amount of data along the time axis, the converted signal is
compressed and smaller in data size. Thus, once digitized, the signal information is thinned out
compared to the original analog signal.
Moreover, digital signal transmission is performed by dividing a continuously changing electrical signal
according to a certain rate of time, then converting each division to a value of 1 or 0, depending on
whether it is greater or less than a specified threshold value. Compared to an analog signal, a digital
signal offers precise data exchange because the only change that must be handled is that between 1
(high voltage) and 0 (low voltage) with respect to a standard value (the threshold value).