Pulse code modulation

Pulse code modulation

1. PCM – PULSE CODE
MODULATION
R.Ramalakshmi
Assistant Professor
Ramco Institute of Technology
Rajapalayam

2. •PCM is an important method of analog-to-
digital conversion.

•In this modulation the analog signal is
converted into an electrical waveform of two
or more levels.
A simple binary PCM waveform

3. •The essential operations in the transmitter of
a PCM system are

Sampling
Quantizing
Coding

•The quantizing and encoding operations are
usually performed by the same circuit,
normally referred to as analog to digital
converter.

4. •The essential operations in the receiver are

Regeneration
Decoding
Demodulation of the quantized samples

•Regenerative repeaters are used to
reconstruct the transmitted sequence of
coded pulses.

5. PCM TRANSMITTER

•Basic blocks:

1.Anti-aliasing filter
2.Sampler
3.Quantizer
4.Encoder

6. BASIC BLOCK DIAGRAM OF PCM
SYSTEM

8. •An anti-aliasing filter is basically a filter used to
ensure that the input signal to sampler is free
from the unwanted frequency components.

•For most of the applications these are low pass
filters. It removes the frequency components of
the signal which are above the cut-off frequency
of the filter.

•The cut-off frequency of the filter is chosen such
that it is very close to the highest frequency
component of the signal.

9. •Sampler unit samples the input signal and
these samples are then fed to the quantizer
which outputs the quantized values for each
of the samples.

•The quantizer output is fed to an encoder
which generates the binary code for every
sample. The quantizer and encoder together is
called as analog-to-digital converter.

10. REGENERATIVE REPEATER
•Regeneration: The feature of the PCM system
lies in the ability to control the effects of
distortion and noise produced by transmitting
a PCM wave by means of regenerative
repeaters.

•Three basic functions:
Equalization
Timing
Decision Making

11. BLOCK DIAGRAM OF REGENERATIVE
REPEATER

12. •The equalizer shapes the received pulses so as
to compensate for the effects of amplitude
and phase ditortions produced by the
transmission characteristics of the channel.

•The timing circuit provides a periodic pulse
train, derived from the received pulses, for
sampling the equalized pulses at the instants
of time where the signal to noise ratio is
maximum.

13. •The decision device is enabled at the sampling
time determined by the timing circuit.

•It makes it’s decision based on whether the
amplitude of the quantized pulse plus noise
exceeds a predetermined voltage level.

14. TRANSMISSION BANDWIDTH IN PCM
q=2^v
Where q-total no. of digital levels of q- level
quantizer.
V-no. of bits per sample.
Signalling rate in PCM = r = v.fs; fs≥2w

Transmission BW of PCM,
BW ≥ (½)r = (1/2)vfs = vw

15. ADVANTAGES OF PCM
•Effect of channel noise and interference is
reduced.
•PCM permits the regeneration of pulses along
the transmission path. This reduces noise
interference.
•The bandwidth and SNR are related by
exponential law.
•Multiplexing of various PCM signals is easily
possible.
•Encryption and decryption can be easily
incorporated for security purpose.

16. DISADVANTAGES OF PCM
•PCM systems are complex compared to analog
pulse modulation methods.

•The channel bandwidth is also increased
because of digital coding of analog pulses.

17. REFERENCES
•S.Haykin, “Digital Communications”, John
Wiley, 2005
•B.Sklar, “Digital Communication Fundamentals
and Applications”, 2
nd
Edition, Pearson
Education, 2009
•B.P.Lathi, “Modern Digital and Analog
Communication Systems” 3
rd
Edition, Oxford
University Press 2007

18. Thank You