2. Pulse Code Modulation (PCM)
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.
3. PCM
PCM uses a sampling rate of 8000 samples per
second.
Each sample is an 8 bit sample resulting in a
digital rate of 64,000 bps (8 x 8000).
4. Converting Samples to Bits
Quantizing
Similar concept to pixelization
Breaks wave into pieces, assigns a value in a
particular range
8-bit range allows for 256 possible sample
levels
More bits means greater detail, fewer bits
means less detail
7. FDM
Is the process of translating
individual speech circuits (300-
3400Hz) into pre assigned slots
within the bandwith of transmission
medium. and, the preassigned slots
are always available to each user
11. TDM
The process where a transmission
medium is shared by a number of circuits
in time domain by establishing a
sequence of time slots during which
individual channels can be transmitted…
Thus the entire bandwidth is periodically
available to each channel
16. SAMPLING THEOREM
“ If a band limited signal is sampled at regular
intervals of time and at a rate equal to or more
than twice the highest signal frequency in the
band, then the sample contains all the
information of the original signal”
Fs= >2fH
17. PULSE CODE MODULATION
• Voice Frequency range 0- 4 Khz
• Sampling the Voice Signal @ 8 Khz
(Double the Max. Frequency as per
sampling theorem) i.e. 8000s/sec
• Sampling time period Ts=1sec/8000
• Ts= 125 microsec
• Time available for sampling each channel,
when we have N total channels=125/N
• In PCM, Time frame=125microsec ;time
available per chl=125/32 =3.9microsec.
18. QUANTIZING
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
19. QUANTIZING
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)
20. QUANTIZATION 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
21. COMPANDING
Is the process where non uniform quantization
is achieved using segmented quantization
In companding, to specify the location of
sample value, the following are necessary…
sign of the sample, the segment no., the
quantum level within the segment.
23. FRAME STRUCTURE
In PCM we have 32 Ts and Ts 0 (FAW)
carries the synchronization signals and FAW
digit value is X 0 0 1 1 0 1 1 . FAW
transmitted in alternate frame. In FAW unused
frames, supervisory and alarm signals are
transmitted
Ts 16 carries the signalling information (for 2
channels)
24. FRAME STRUCTURE
For carrying the signalling for all 30 chls and
for carrying sync. Data for all frames, in PCM
16 frame pattern is used and it is known as
multi frame
Duration of multi frame is 2msecs.
25. PCM Standards
THERE ARE TWO STANDARDS OF PCM
NAMELY
1) THE EUROPEAN 2 ) THE AMERICAN.
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.
26. JUSTIFICATION TYPES
Positive justification: Common
synchronization bit rate offered at each
tributary is higher than the bit rate of
individual tributary.
Positive-negative justification
Negative justification
