Home | Contact | Site Map

Home / Sample Questions

2.1 Time Division Multiplexing (TDM) is deployed in the PCM system in
order to maximize the utilization of the transmission media. Explain briefly the TDM
systems deployed in the Europe and America / Japan.

Digital Transmission is governed by sampling theorem. According to the sampling
theorem voice band (20KHz) is limited to a telephone channel of 4kHz and this
limited 4 kHz signal is sampled at 8 kHz, the resultant samples has to be reproduced
over the other end of the transmission media by low-pass filtering. This process
involves 8,000 samples per second for a given voice. This has resulted in sending one
sample in every 125us. In other words 125us period is being used by the transmission
media if we use to transmit only for one telephone channel. This idling period can be
further utilized to send some other similar telephone channels sampling information
which will follow the same destination. The concept of Time Division Multiplexing
derive how to utilize the transmission media more efficiently. In the Europe system 32
channel samples are being sent during this 125us while in the American and Japanese
system 24 channel samples are used. If we assume 8 bits per sample for 125us the
Europe system will send 256 bits and the American system will send 192 bits
resulting 2.048 Mbps and 1.542 Mbps respectively.

Data rates used under Europe and America/Japan



North American:




2.2 In practice, non-linear quantizing is deployed as against linear quantizing, explain the
reasons behind.

The quality factor of any transmission media is measured by Signal to Noise Ratio. In
linear quantizing SNR is good only high valued samples where as in the practical
application 90 percent of the samples are lying around + or – Vmax/2 where Vmax is the
highest voltage generated by a telephone
. Hence linear quantizing will not be suitable
for the practical applications for the transmission of human voice. Hence non-linear
quantizing is used. In the non-linear quantizing out of 256 quantized levels only 16
quantizing levels are used for a range of Vmax/2 to Vmax to suit practically.

2.3 256 levels are used for non-linear quantizing and there are 8 segments for each
positive and negative sample. Explain the basic characteristics of non-linear
quantizing used for European system (the A-law signal compression of 13 linear

Y axis: Number of levels - 8 segments, each with 16 sub-levels (i.e. +127 to -127)
X axis: Voltage in mV - From +Vm to -Vm, where Vm= 3,072mV
Gradients of upper 0 and 1 segments and lower 0 and 1 segments are same. Therefore,
15 total number of segments are reduced to 13 segments.This makes it a 13 segment graph.

Segment No. Voltage Range (mV) Level Range Increment/Level
7 1,536 – 3,072 111 – 127 96
6 768 – 1,536 95 – 111 48
5 384 – 768 79 – 95 24
4 192 – 384 63 – 79 12
3 96 – 192 47 – 63 6
2 48 – 96 31 – 47 3
1 24 – 48 15 – 31 1.5
0 0 – 24 0 – 15 1.5

According to the voltage of the sampled signal, a segment number and a level number
is determined.
This creates a 8-bit representation of the sampled signal as shown below;


Sign: + or – (1-bit)
Segment No.: 3-bits
Level No.: 4-bits