KNOW A THING ABOUT MOBILE COMMS – PCM MADE EASY

The analog signal is firstly sampled and then converted into an n-bit binary sequence called code. This code is helpful in transmission. The basic operations that signal undergo are sampling, quantization, and encoding.


ANALOG SIGNAL-->  SAMPLING--> QUANTIZATION--> ENCODING--> DIGITALLY CODED SIGNAL (PCM)

In pulse code modulation, time and amplitude are represented as discrete values.
PCM mainly consists of three steps:
• Sampling
• Quantising
• Encoding
Pulse code modulation is not only a modulation technique, it is also an encoding technique with which the signals are assigned with a definite digital value.
In pulse code modulation each code has an identical number of bits and run-length for transmission.
Basically, it is the only digitally encoded modulation technique for transmission. In the binary system in which the pulse within a prescribed time period represents logic 1 and the lack of pulse within a time period represents logic 0. Which is for specific time intervals.
• If pulse arrives then it is represented as logic 1
• If a pulse does not arrive then it is represented as logic 0

From the above diagram let us see the appliance and its functions

Bandpass filter
To limit the analog input frequency up to standard voice band ranges from 300Hz to 3000Hz. It is used to band restrict the input signal.
Sample and hold circuit
It converts the continuous-time analog signal into a discrete-time signal
Analog to digital
This block converts the discrete-time signal into a digital bitstream
Parallel to serial converter
Converts n-bit parallel binary sequence into the n-bit binary frame. Serial n-bit digital serial pulse code modulated code
Transmission path
Physical media like wires, co-axial cables, twisted pairs,s, etc. In the case of the transmission path, regenerative repeaters will be there.
• Regenerative repeaters
These receivers receive, amplify or boost the signals and retransmit them. They are used here to reduce signal attenuation.
Serial to parallel converters
Converts the received serial PCM code frame into the n-bit parallel stream.

Digital to analog
Digital to analog conversion is done by successive approximation method. That results in distant time pulses.
• Hold circuit
Reconstruction of the analog signal at the output.
Low pass filter
A low pass filter helps in transmitting the noise and quantization noise which results in error-free output.

ADVANTAGES OF PULSE CODE MODULATION (PCM)


• PCM is convenient for long-distance communication
• PCM has higher transmitter efficiency than other modulation
• PCM has higher noise immunity than other modulation

DISADVANTAGES OF PULSE CODE MODULATION (PCM)


•The PCM requires more bandwidth while compared to other modulation
• In PCM the following are complex to perform:
(i)encoding
(ii)Decoding
(iii)Quantizing
• PCM is an integrated digital network that can only be realized by a gradual extension of noise.
• PCM is not used in wireless-based communication systems.
• More crosstalk occurs.

APPLICATIONS OF PULSE CODE MODULATION


• The pulse code modulation is most commonly used in the satellite transmission system
• PCM is used in space communication.
• PCM is used in telephony communication.
• The compact disc (CD) is a recent application in the field of pulse code modulation.

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