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”, 2nd Edition, Pearson
Education, 2009
• B.P.Lathi, “Modern Digital and Analog
Communication Systems” 3rd Edition, Oxford
University Press 2007

18. Thank You