pcm principles
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pcmTRANSCRIPT
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PCM PRINCIPLES
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Need for PCM
To cater to the increased demand of traffic between two stations or between two subscribers at the same station.
Increasing number of pairs to the under ground cable is uneconomical and leads to maintenance problems.
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Multiplexing
The technique used to provide a number of circuits using a single transmission link is called Multiplexing.
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Multiplexing techniques Frequency Division Multiplexing (FDM).
Time Division Multiplexing (TDM).
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Frequency Division Multiplexing
It is the process of translating individual speech circuits(300-3400 Hz) into pre assigned frequency slots within the transmission bandwidth.
At input, Amplitude Modulation of the audio frequency with an appropriate carrier frequency.
At the output of the modulator a filter network is connected to select either a lower or upper side band.
Find their applications in the analogue transmission systems.
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Frequency division multiplexing
carrier
carrier
carrier
VF I/P
VF I/P
VF I/P
CHL
CHL
CHL
F1
F2
F3
F1 +VF or F1-VF
F2 +VF or F2-VF
F3 +VF or F3-VF
Transmission
link
BPF
BPF
BPF
FDM PRINCIPLE 6
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Time division multiplexing Sharing a transmission medium by a number
of circuits in time domain.Establishing a sequence of time slots during
which individual channels (circuits) can be transmitted.
Entire bandwidth is completely available to each channel.
Each channel is assigned a time slot with a specific common repetition period called a frame interval.
Each channel is sampled at a specified rate and transmitted for a fixed duration.
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Time division multiplexing
CH1
CH2
CH3
CH1
CH2
CH3
medium
TRANS RECEIVEGATE
GATE
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In 1938 , Mr. A. H. Reaves (USA) developed a Pulse Code Modulation (PCM) system to transmit the spoken words in the digital form.
PCM systems use TDM technique to provide a number of circuits on the same transmission medium.
Pulse code modulation systems
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Basic requirements for PCM systems
To develop a PCM signal from several analogue signals, the following processing steps are required.
1.Filtering 2.Sampling3.Quantization4.Encoding 5.Line coding
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Filtering
Filters are used to limit speech signal to the frequency band of 300-3400 Hz.
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Sampling process
SWITCH S
RV1 V2
W m
(a)
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MAKE(S)
BREAK(S)
V1
V1
V2
V2
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When the switch S is closed , an output appears across R.
The rate at which switch S is closed is called the SAMPLING FREQUENCY.
The amplitude of the sample is dependent upon the amplitude of the input signal at the instant of sampling.
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Sampling theorem If a band limited signal is sampled at a 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
If our voice signals are band limited to the 4 KHz and let the sampling frequency be 8 KHz then the time period of the sampling, Ts = 1sec/8000.
or Ts = 125 µsecs. If we are to sample N channels one by one at the
rate specified by the SAMPLING THEOREM, then the time available for sampling each channel would be equal to Ts/N microseconds.
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Sampling & Combining channels
CH1 CH1
CH2 CH2
CH3 CH3
TRANSMISSION PATH
a
b
c
a
b
c
a
b
c 16
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Sampling & Combining channelsThe channels gates (a , b, c…,n) correspond to the
switch S.These gates are opened by series of pulses called
“CLOCK PULSES”.These are called gates because they connect the
channels to the transmission medium during clock period and isolate them during the OFF periods.
The time interval during which the common transmission medium is allocated to a particular channel is called the TIME SLOT for that Channel.
The width of the Time Slot depends on number of channels to be combined and the clock pulse frequency i.e. sampling frequency.
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30 CHANNEL PCM SYSTEM
125 microseconds are divided into 32 parts (Time Slots).
30 Time Slots are used for 30 speech channels.One time slot for signalling of all the 30 channels.One time slot for synchronization between
transmitter and receiver.Time available per channel would be 125/32=3.9 µ seconds.Time period of sampling or the interval between
two consecutive samples of a channel is 125 µ sec this duration is called TIME FRAME.
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PAM output signals
A
B
C
CHANNEL 1
CHANNEL 2
CHANNEL 3
CONTINUE ->
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D
E
F
MULTIPLEX SIGNAL
PULSE TRAIN
SEPARATING CHANNEL 2
CHANNEL 2 SEPARATED
RECONSTRUCTION OF ORIGINAL SIGNAL20
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Quantization
• The process of measuring the numerical values of the samples and giving them a table value in a suitable scale is called Quantising.
• In other words, Process of breaking continuous amplitude range into a finite number of amplitude values or steps.
• Used for converting PAM signals into digital form to avoid distortion.
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Quantization signal with +Ve
and –Ve values
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DECIMAL BINARY
65432
2
1
1
00
3456
7
7
1111111O110111001011101010011000000000010010
01000011
010101100111
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Illustration of Quantization Distortion
Analog Signal Amplitude Range
Quantizing Interval (Mid Value)
Quantizing Level
Binary Code
0-10 mV 5mV 0 1000
10-20 mV 15mV 1 1001
20-30 mV 25mV 2 1010
30-40 mV 35mV 3 1011
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Quantization Distortion
As in the Quantization in one level (step size)
there are several amplitudes (one range) which
are given one binary Code . Hence
The process of quantization leads to approximation of the input signals.
The detected signal have some deviations from the actual values.
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• If v represents the step size & e represents the difference in amplitude between the actual signal level and its quantized equivalent, then it can be proved that Mean square quantizing error is equal to v²/12. i.e. the error depends upon the size of the step.
• In linear quantization , equal step means equal degree of error for all input amplitudes.
• The S/N ratio for weaker signals will be poor.
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.To reduce quantization distortionWe need to reduce step size or in
other words, increase the number of steps in the given amplitude range .
B = Fm log L, where L is the number of quantum steps hence when we increase the no. of steps, bandwidth (B) increases.
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Non uniform quantizationProbability of occurrence of small
amplitude is more than the probability of occurrence of large one.
Providing more quantum levels in the small amplitude range.
Providing few quantum levels in higher amplitudes.
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Companding
Also called segmented quantization.Provides non uniform quantization.Equal no. of segments for both +Ve and –Ve excursions.To specify the location of the sample it is
necessary to know the following: 1. Sign of the sample. 2. The segment number. 3. The quantum level within the segment.
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Segmented coding curve
1/64 1/32 1/16
1/8 1/4 1/2 1
16
32
48
64
80
96
112
128
QU
AN
TIZ
ATIO
N L
EV
ELS
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Encoding
Conversion of quantized analogue levels to binary signal is called Encoding.
To represent 256 steps ,8 level code is used & is called an eight bit “word”.
8 bit word appears in the form P ABC WXYZPolarity bit Segment code linear encoding
in the segment
‘1’ for +Ve ‘0’ for -Ve
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Encoding Curve with Compression 8 Bit Code
7
7
6
6
5
5
4
4
16
1632
32
64
6448
48
80
80
96
96112
112
128
128
VcVm/4-Vm/4-Vm/2
Vm/2 +Vm-Vm
VMAXIMUM VOLTAGE
HERE Vc IS ENCODED AS
1 111 0101
SIGN SEGMENT POSITION
N
BIN
AR
Y N
UM
BE
R
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The quantization and encoding are done by a circuit called CODER .
CODER converts PAM signals to 8 bit binary signal. The curve has following characteristics The function N=f(V) is non linear. It is symmetrical about origin. Zero level corresponds to zero voltage to encoded. It is logarithmatic function. Have 13 straight segments 0 to 7 in +Ve direction 0 to 7 in –Ve direction 4 segments 0,1(+) and 0,1(-) lying between
+Vm/64 to –Vm/64 being collinear are taken as one segment
From previous figure
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Time division multiplexing
EN
CO
DE
R
DE
CO
DE
R
CO
MB
INE
R
SA
PA
RA
TO
R
SIGNALLING
SYNC
TRANS SAMPLING GATES
REC SAMPLING GATES
PAM PAMCH1
CH1
CH2CH2
CH3
CH3
aa
bb
nn
Ts
a
b
c
d
SAMPLING PULSES FROM CHANNEL A TO N
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Concepts of frameTs in a 30 channel PCM system is 125 µ Secs. Signalling information of all the channels is
transmitted through a separate time slot.To maintain synchronization between receive
end and transmit end synchronization data is transmitted through another Time Slot.
For a 30 channel PCM system, we have 32 time slots.
Time available per channel is 3.9 µ secs.Frame duration is 125 µ seconds.
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Structure of frame
A frame of 125 µ seconds duration has 32 time slots. These time slots are numbered Ts 0 to Ts 31.
Time slot Ts 0 carries synchronization signal. This slot is also called Frame Alignment Word (FAW).
Time slot Ts 16 carries signalling information.
Rest of all time slots carry speech signals.
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Synchronization
The receiver looks for FAW and once it is detected it knows that the information for channel 1 will be there and so on.
The FAW is transmitted in the Ts 0 of every alternate frame.
Frame not containing FAW, are used for carrying supervisory and alarm signals.
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Signalling in PCM systems
Signalling used for:Proper routing of call between two
subscribers.Providing certain information like dial
tone ,busy tone, ring back , NU tone, metering pulse, trunk offering signal etc.
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Characteristics of signalling information
Are in the form of DC pulses or multifrequency pulses.
Signalling pulses retain their amplitude for a much longer period than the pulses carrying speech information.
Signalling channel can be digitized with less number of bits than voice channel.
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Multi Frame Formation
----------------- ----------------------------
Tb
1 2
2
2
3 4 5 6 7 8
8-------------- -----------------------------
0
0
1
1
15
15
16 29 30 31
141312
Tb=0.488µs
1 time slot(8 bits) 3.9µs
1 frame(32 time slots)=125 µs
One multi frame(16 frames)
2mSecs
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MultiframeTime slots 16 of each frame carries the
signalling data corresponding to two VF channels only.
For 30 channels we transmit 15 frames, each having 125 µ seconds duration.
For synchronization one extra frame is used.These group of 16 frames is called
“multiframe”The duration of multiframe is 2 msecs. Each of these frame has 32 time slots .Each time slot carrying the encoded samples of
all the channels plus the signalling and synchronization data.
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------------------------------------------------
TIME SLOTS
FRAME
FRAME
FRAME
FRAME
0
1
2
15
0 1 2 3 4
FA
FA
FA
FA
-----------------16 17 --------------------------- 31
MFA
------------------------
------------------------
------------------------
FA : Frame alignment MFA : Multiframe alignment Sn =Signalling for channel(4 bits)
Frame duration 125us
2.048 Mb/s PCM MULTIFRAME
S1/S16
S2/S17
S15/S30
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PCM multiframeWe have 32 time slots in a frame, each slot
carries a 8 bit word.The total number of bits per frame 32 x 8 = 256 bits.The total number of frames per seconds is
8000.The total number of bits per second are
256 x 8000 = 2048 Kbps.Thus, a 30 channels PCM systems has
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Multiframe Structure In the time slot 16 of F0, the first four bits
(positions 1 to 4) contain the multiframe alignment signal which enables the receiver to identify a multiframe.
The other four bits (no. 5 to 8) are spare. These may be used for carrying alarm signals.Time slots 16 of frames F1 to F15 are used for
carrying the signalling information.Each frame carry signalling data for 2 VF channels.As each multiframe includes 16 frames, each with a
sampling rate 8000 per second, the signalling of each channel will occur at a rate of 500 per second.
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Thanks
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