PULSE CODE MODULATION (PCM)

It is a digital representation of an analog signal that takes samples of the amplitude of the analog signal at regular intervals. The sampled analog data is changed to, and then represented by, binary data.

1. PONDICHERRY UNIVERSITYPONDICHERRY UNIVERSITY
DEPARTMENT OF ELCTRONICS
ENGINNERING
BY:BY:
VISHNUDHARAN.B VISHNUDHARAN.B
M.TECH 1 M.TECH 1
STST
YEAR YEAR

2. AGENDAAGENDA
Introduction
Block diagram of PCM
PCM processes
PCM standards
Bit rate and bandwidth requirements of PCM
Advantages
Disadvantages
Application

3. PULSE CODE MODULATION PULSE CODE MODULATION
Analog voice data must be translated into a series of
binary digits before they can be transmitted.
With Pulse Code Modulation (PCM), the amplitude of
the sound wave is sampled at regular intervals and
translated into a binary number.
The difference between the original analog signal and
the translated digital signal is called quantizing error.

4. BLOCK DIAGRAM OF PCMBLOCK DIAGRAM OF PCM

5. PCM PROCESSESPCM PROCESSES
Filtering
Sampling
Quantization
Encoding

6. SAMPLINGSAMPLING
Analog signal is sampled every T
S
sec.
T
s
is referred to as the sampling interval.
f
s
= 1/T
s
is called the sampling rate or sampling
frequency.
There are 3 sampling methods:
Ideal - an impulse at each sampling instant
Natural - a pulse of short width with varying amplitude
Flattop - sample and hold, like natural but with single amplitude
value

7. DIFFERENT TYPES OF SAMPLING METHODS DIFFERENT TYPES OF SAMPLING METHODS

8. SAMPLINGSAMPLING

9. QUANTIZINGQUANTIZING
The process of measuring the numerical values of
the samples and giving them a table value in a
suitable scale
The finite number of amplitude intervals is called
the ‘quantizing interval’ like quantizing interval
no.1 is 10-20mV; 2 is 20-30mV etc. in a case of
1V signal.
Linear quantizing is where the quantizing
intervals are of the same size

10. QUANTIZINGQUANTIZING
Quantization intervals are coded in binary
form, and so the quantization intervals will be
in powers of 2.
In PCM, 8 bit code is used and so we have 256
intervals for quantizing (128 levels in the
positive direction and 128 levels in negative
direction)

11. QUANTIZATION DISTORTIONQUANTIZATION DISTORTION
The deviation between the amplitude of
samples at the transmitter and receiving ends
In linear quantization, the distortion is more
and to decrease the distortion, the no. of steps
in the given amplitude range has to be
increased.
Due to BW limitations, more quantum levels in
small amplitude region are planned results to
Non linear (uniform) quantization

12. PCM StandardsPCM Standards
There are two standards of pcm namely
1) The European Standard
2 ) The American Standard
They differ slightly in the detail of their working but
the principles are the same.
European pcm = 30 channels
North american pcm = 24 channels
Japanese pcm = 24 channels
In india we follow the european pcm of 30 channels
system working.

13. BIT RATE AND BANDWIDTH BIT RATE AND BANDWIDTH
REQUIREMENTS OF PCMREQUIREMENTS OF PCM
The bit rate of a PCM signal can be calculated form the
number of bits per sample x the sampling rate
Bit rate = n
b
x f
s
The bandwidth required to transmit this signal depends on
the type of line encoding used. Refer to previous section
for discussion and formulas.
A digitized signal will always need more bandwidth than
the original analog signal. Price we pay for robustness and
other features of digital transmission.

14. ADVANTAGES OF PCMADVANTAGES OF PCM
Uniform Transmission Quality
Compatibility of different classes of Traffic in
the Network
Integrated Digital Network
Increased utilization of Existing Circuit
Low Manufacturing Cost
Good Performance Over Very poor
Transmission Paths

15. DISADVANTAGES OF PCMDISADVANTAGES OF PCM
Large Bandwidth required for Transmission
Noise and crosstalk leaves low but rises
attenuation
An integrated Digital network can only be
realized be a gradual extension of Noise

16. APPLICATIONAPPLICATION
In compact disk
Digital telephony
Digital audio applications