Many of the signals used in modern communication are digital (for example, the codes for alphanumeric characters and the binary data used in computer programs). In addition, digital techniques are often used in the transmission of analog signals. Digitizing a signal often results in improved transmission quality with a reduction in distortion and an improvement in signal-to-noise ratio. This topic looks mainly at the digital transmission of analog signals such as voice and video signals.
In Figure, the following Digital Transmission System Block Diagrams are shown.
(a) Analog Signal and Base Band Transmission
(b) Analog Modulation using Modulation and Demodulation
(c) Digital Signal Transmitted on Digital Channel
(d) Digital Signal Transmitted by Modem
(e) Analog Signal Transmitted Digitally
(f) Analog Signal Digitized and Transmitted by Modem
The functions of Digital Transmission System with examples are defined below.
In Fig (a) an analog signal is sent over a channel with no modulation. This is an example in an ordinary public-address system consisting of a microphone (converts sound energy into electrical signal), an amplifier (amplifies electrical signal) and a loud speaker (which converts electrical signal into sound energy) using twisted pair wire as a channel.
Fig (b) shows analog transmission using modulation and demodulation. Broadcast radio and television are good examples.
Fig (c) and (d) start with a digital signal (for example, a data file from a computer)
In Fig (c) the link can handle some kind of digital pulse signal directly.
In Fig (d) the channel cannot transmit pulses directly. In these cases the digital signal has to be modulated into a carried at One end and demodulated at the other. The modem is a combination, of modulator-demodulator.
Fig (e) and Fig (f) show an analog signal that is digitized at the transmitter and converted back to analog form at the receiver. The difference between these systems is that in Fig (e) the transmission is digital, while in the transmission channel cannot carry pulses, so modulation and demodulation are required.
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Digital Transmission