Signal Characteristics

Telecommunication is mainly concerned with the transmission of messages between two distant points. The signal that contains the messages is usually converted into electrical waves before transmission. Our voice is an analog signal which has amplitude and frequency characteristic.

Voice frequencies. The range of frequencies used by a communication device determines the communication channel, communicating devices, bandwidth or information carrying capacity. The most commonly used parameter that characterizes an electrical signal is its bandwidth of analog signal or bit rate if it is a digital signal. In telephone system, the frequencies it passes are restricted to between 300 to 3400 Hz. Thus the network bandwidth is 3100 Hz. The bandwidth and bit rate for various types of system are shown in Table 1.1.

Table 1.1. Bandwidth requirements of various applications

Type

Bandwidth

Bit Rate

Telephone (speech)

300—3400 Hz

Music

50 Hz—16 kHz

Facsimile

40 kHz

Broadcast televison

0—55 MHz

Personal communication

300 to 9600 bits/sec

E—Mail transmission

2400 to 9600 bits/sec

Digitized voice phone call

6400 bits/sec

Digital audio

1 to 2 M bits/sec

Compressed video

2 to 10 M bits/sec

Document imaging

10 to 100 M bits/sec

Full motion video

1 to 2 G bits/sec

Speech spectrum. The telephone channel over which we wish to send data are designed to transmit electrical oscillations (microphone converts sound into equivalent number of electrical oscillation) of voice. Fig. 1.2 is described as a speech spectrum diagram. It illustrates human speech strength variations at various frequencies. Most of the energy is concentrated between 300 Hz to 3400 Hz.

Voice bandwith ^ 3100 Hz

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