PCM MUX Hierarchy New

R.T.T.C. HYDERABAD 1

Concepts of

PCM, Digital MUX


Pulse Code Modulation System

• PCM systems use TDM technique to provide a number of circuits on the same transmission medium viz open wire or underground cable pair or a channel provided by carrier, coaxial, microwave or satellite system.

• To develop a PCM signal from several analogue signals, the following processing steps are required

• Filtering• Sampling• Quantization• Encoding• Line Coding

(i). Filtering: Filters are used to limit the speech signal to the frequency band 300-3400 Hz.


• (ii). Sampling: The amplitude of the sample depends upon the amplitude of the input signal at the instant of sampling. The duration of these sampled pulses is equal to the duration for which the switch S is closed.

• Minimum number of samples are to be sent for any band limited signal to get a good approximation of the original analogue signal and the same is defined by the Nyquist sampling Theorem.

• Mathematically, if fH is the highest frequency in the signal to be sampled then the sampling frequency Fs needs to be atleast greater than or equal to 2 fH.i.e. Fs>=2fH

• Since the voice signals are band limited to 4 KHz the sampling frequency be 8 KHz.

• Time period of sampling Ts = 1 sec or Ts = 125 micro seconds

8000

Pulse Code Modulation


• (iii). Quantizing: The process of measuring the numerical values of the samples and giving them a table value in a suitable scale is called "Quantizing". Of course, the scales and the number of points should be so chosen that the signal could be effectively reconstructed after demodulation.

• In practice, non-uniform quantization is achieved using segmented quantization (also called companding).

• The ‘A’-law companding rules for the 30 chl. E1 PCM system used in India & Europe and ‘µ’-law used for 24 chl. T1 PCM in USA/Canada and Japan.

Pulse Code Modulation contd..


• (iv). Encoding:Conversion of quantized analogue levels to binary signal is called encoding. To represent 256 steps, 8 level code is required. The eight bit code is also called an eight bit "word".

• The 8 bit word appears in the form P ABC WXYZPolarity bit ‘1’ Segment Code Linear encodingfor + ve & ‘0' for - ve. in the segment

• The first bit gives the sign of the voltage to be coded. Next 3 bits gives the segment number. There are 8 segments for the positive voltages and 8 for negative voltages. Last 4 bits give the position in the segment. Each segment contains 16 positions.



• (v). Line Coding: For distortion free transmission, the PCM output signal should be converted into a suitable line code which will match the characteristics of the medium



Synchronization & Signaling in PCM

• In PCM System, a frame of 125 microseconds duration has 32 time slots. These slots are numbered Ts 0 to Ts 31.

• Information for providing synchronization between trans and receive ends is passed through a separate time slot. Usually the slot Ts 0 caries the synchronization signals. This slot is also called Frame Alignment Word (FAW).

• The FAW is transmitted in the Ts 0 of every alternate frame. Frames which do not contain the FAW, are used for transmitting supervisory and alarm signals.

• The signaling information is transmitted through time slot Ts 16.• Ts 1 to Ts 15 are utilized for voltage signal of channels 1 to 15

respectively.• Ts 17 to Ts 31 are utilized for voltage signal of channels 16 to 30

respectively.


• The time slot 16 of each frame carries the signaling data corresponding to two VF channels only.

• Therefore, to cater for 30 channels, we must transmit 15 frames, each having 125 microseconds duration. For carrying synchronization data for all frames, one additional frame is used.

• Thus a group of 16 frames (each of 125 microseconds) is formed to make a "multiframe". The duration of a multiframe is 2 milliseconds.

• The multiframe has 16 major time slots of 125 microseconds duration. Each of these (slots) frames has 32 time slots carrying the encoded samples of all channels plus the signaling and synchronization data.

Pulse Code Modulation System contd..


Concept of Digital Multiplexing


In analogue system, multiplex equipment uses F.D.M. to assemble individual channels into groups, super group etc. Similarly, in digital systems, hierarchical levels have been defined using T.D.M. and are identified by their digit rate

measured in No. of bits/sec.

PCM MUX Hierarchy New

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