Introduction
to Pulse Code Modulation (PCM)

Pulse-code modulation
(PCM):

is a digital
representation of an analog signal where the magnitude of the signal is sampled
regularly at uniform intervals, then quantized to a series of symbols in a
numeric (usually binary) code

Pulse code
modulation (to be discussed in depth later) is the heart of technology in
communications in today’s digital world. It’s a process in which analog signals
are converted to digital form. The analog signal is represented by a series of
pulses and non-pulses (1 or 0 respectively). At this stage to fully understand
we can refer back to the notes on signals.

Why PCM?

The stream of pulses
and non-pulse streams of 1’s and 0’s are not easily affected by interference
and noise. Even in the presence of noise, the presence or absence of a pulse
can be easily determined. Since PCM is digital, a more general reason would be
that digital signals are easy to process by cheap standard techniques. This
makes it easier to implement complicated communication systems such as
telephone networks (covered later in this course

The practical
implementation of PCM makes use of other processes. The processes are carried
out in the order in which they appear below:

Filtering

Sampling

Quantizing

Encoding

The filtering stage
removes frequencies above the highest signal frequency. These frequencies if
not removed, may cause problems when the signal is going through the stage of
sampling. Sampling of a waveform means determining instantaneous amplitudes of
a signal at fixed intervals. You may have a problem in understanding what this
sentence means, but if you take time to look at Figure 7 on the following
Slide, you should be able to understand

Figure 7a shows the
determination of instantaneous amplitude at uniform intervals. 7b shows the
samples corresponding to instantaneous amplitudes of the input signal, and 7c
shows the output which represents the reconstructed input signal. Earlier we
talked about PCM as the process of changing/converting signals from one form
(analog) to the other (digital). Sampling is the first part of the answer as to
how the signal changes from one form to the other. Quantization is the process
of allocating levels to the infinite range of amplitudes of sample values of
the analog signal. This may not be clear now, but it should be after looking at
the example below.

In this example the maximum amplitude
value is +8 and the minimum is –8. The amplitude values are quantized into four
levels. The full range (from –8 to + of values is 16. Therefore the width of
each level is 16 divided by 4 which gives 4 volts. Quantization is done such
that half the steps are at the top and half at the bottom.

We now move onto the last process which
is encoding. In this process each step level is assigned a number. The numbers
start with zero at the lowest level. These assigned numbers are then expressed
in binary form (in terms of 0’s and 1’s). This will be the last part of the
conversion and the PCM signal will be transmitted/sent.

One disadvantage of PCM is that the
signal accuracy is reduced because of the quantizing of the samples

Digital Transmission

Advantages

Digital information is better
transmitted in its digital form because converting the signal to analog and
sending it through an analog network can be costly. Digital data is easily
compressed; therefore it can be transmitted using a small bandwidth. Because of
the nature of devices used to boost the signal strength during transmission,
error performance is much improved when compared with analog. It is also better
to transmit information in digital form because computer components used in the
transmission process are very reliable.

Disadvantages:

Information is widely available as
analog. Conversion of analog signal to digital bit stream requires special
equipment. This adds cost and additional equipment may also contribute
additional points of failure. Another disadvantage is that during the
conversion process the quality of the signal is negatively affected.

Signal Conversion

Conversion is the changing of a signal
from one class type to the other. Analog signals are readily available. With
this in mind, we can safely say the natural world is analog in nature. The
modern world of computers is digital and because we live in this modern world,
everything around us appears to be going digital, hence the need for
conversion. Conversion from Digital-to-Analog and Analog-to- Digital is
achieved with the assistance of a device called a Modem.

Without the use of a modem, it would be
impossible to accomplish computer communication across analog circuits. A modem
enables us to send digital information across an analog circuit.

Modem

A modem (modulator/demodulator) is
connected between a computer and a telephone network. It connects computers by
a telephone line. This device converts a digital signal from one computer into
an audio signal (tones) suitable for transmission over a telephone line. The
tones can then be passed as normal analog speech. At the receiving end, the
tones are converted back to the digital before being applied to another
computer. A modem is one device that makes it possible for us to surf the
internet, to ‘chat’ with our friend and send them text based messages called
electronic mail or E-mail for short.

The fact that computers can processes
information in digital form has had a deep effect on the modern world of
communication and Information processing.

Codecs

Codecs (coder/decoder) make the reverse
process to that of a modem. It enables us to send analog information across a
digital circuit. In short we can say it helps us to achieve Analog- to -Digital
and Digital-to- Analog conversion.