Analog and Digital Transmission

To be transmitted data must be transformed into electromagnetic signals. Data can be analog or digital. Analog data takes continues values on some interval. For example, voice and video (are continuously varying patterns of intensity). Digital data take on discrete values, for example, data stored in a computer memory.

An analog signal is a continuously varying electromagnetic wave that may be propagated over a media. A digital signal is a sequence of voltage pulses that may be transmitted over a media.

Periodic and Non-Periodic Signals

A signal is periodic if it completes a pattern within a measurable time frame, called a period, and repeats that pattern over identical subsequent periods. The completion of one full pattern is called a cycle. A period is defined, as the amount of time (in seconds) required to complete one full cycle. Period is represented by T. A sine wave is the simple periodic signal.

A non-periodic or an aperiodic signal changes continuously without exhibiting a pattern or cycle that repeats over time.

Both analog and digital signals can be periodic or non-periodic. We commonly use periodic analog signals (because they needs less bandwidth) and non-periodic digital signals (because, they can represent variation in data).

Period refers to the amount of time in seconds, a signal needs to complete one cycle. Frequency refers to the number of periods in one second. Frequency is expressed in Hertz (Hz) which is cycle per second.

Period is the inverse of frequency.

If a signal does not change at all, it never completes a cycle, so its frequency is zero. When a signal change instantaneously, its period is zero, since frequency is the inverse of period, in this case, the frequency is 1/0 or infinite.

A composite signal is a signal made of many simple signals. The bandwidth of a composite signal is the difference between the highest and lowest frequencies contained in that signal.

Digital Signal

Most digital signals are aperiodic and thus period or frequency is not appropriate. We use bit interval (instead of period) and bit rate (instead of frequency) to describe digital signals. The bit interval is the time required to send one single bit. The bit rate is the number of bits sent in one second, usually expressed in bits per second (bps).

If a digital signal has L levels then each bit level needs log2L bits, as shown below.

Two level digital signal

Four level digital signal

We can transmit a digital signal either by base band transmission or by broadband transmission. In base band transmission, the raw signal is transmit without any modification. It is possible only if we have a low-pass channel with an infinite or very high bandwidth. In broadband transmission we modulate the original signal before transmit.

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