An electronic signal is a channel that carries information from one system to another. For example from your phone via a USB cable to your laptop. Signals can be carried between devices in two ways—analog or digital. Let’s start by defining these terms.

Analog Systems

An analog is something that is a representation of another; an output that is a representation of some or other input.

Think of a clock with hands. The hands are driven by some mechanism and show the passing of time—or an analog of the passage of time. A moving coil voltmeter points to numbers on a dial face representing an analog of the applied voltage. The amount of water in a rain gauge is an analog of how much rain fell that day.

So in a communication system, we can say that an analog signal represents some sort of information that has been impressed on the system, usually by an alternating current waveform.

For example, a microphone generates a voltage on a cable that is an analog of the singer’s voice and carries this to the amplifier. Analog signals then are infinitely varying in level and as a result, are capable of great accuracy. AM and FM radio signals are an example of an analog communication system. When used in a communications system, these varying voltages might be seen as changes or modulations of either the frequency, phase, or amplitude of an alternating carrier.

Digital Systems

On the other hand, a digital system is one where the representation of the original source is transmitted or displayed in a binary format. Only ones and zeros are used, and no intermediate value.

Going back to the clock example, in a digital clock, there is a crystal square wave oscillator running at 32768 times a second. If we use digital dividers to divide by two for 15 times, we get 1Hz or a one-second tick. This tick gets counted in multiples of 60 to give us minutes and hours. These units are then multiplexed onto LED or LCD displays to give us our conventional digital clock.

But how would digital information like this be transmitted down a wire or through the air?

Binary words are made up of a sequence of ones and zeros sent one after the other. The word size is typically 8 bits long but can be larger. A group of bits can be called a byte and half a byte is called a nibble. Some additional bits will be added to enable error checking such as parity. The words may be grouped into packets with destination addresses, for example, in networking and the Internet.

If the distance is short, the binary words representing this data are fed one bit at a time, one after the other into the wire. We might say if the bit representing the least important part of the data is sent first, then this is LSB or the least significant bit first. Do it the other way around and you have MSB or most significant bit first.

Modulation and Modems

What if the wire is very long or it needs to go on a telephone line? Or it needs to be transmitted wireless?

Digital on/off bit transitions fed into a wire will soon degrade and be lost due to the capacitance and inductance of the wire. So the answer is to use a modem, which gets its name from modulator-demodulator. The modem (if it is FSK or frequency shift keying) has two stable frequency states.

Let’s say it is stable at 1000Hz with a zero on the input and 1 comes along. It shifts frequency to say 1500Hz for the duration of the 1. At the other end of the wire or radio link, a demodulator responds to the changing frequencies and outputs ones and zeros. But there are many types of modulation; FSK, ASK. PSK etc.

You might argue that the information on the wire is now an analog signal. But because it has no amplitude information, it can easily be amplified and any amplitude type noise removed.

Twenty years ago, most phone, radio, and TV systems were largely analog-based but are now being replaced with digital ones.

The size of a binary byte is governed by the type of accuracy required. As there are only two states—on or off—we can say that for an 8-bit word (or byte), there are 28 or 256 possible states. Expressed as accuracy, this would be 1/256 * 100 or 0.4%, which is very suitable for most needs. But doubling this up to 16 bits, we would have 65536 possibilities or accuracy of 1/65536 * 100 or 0.0015%.

A to D Conversion

D to A conversion

Advantages and Disadvantages of Analog vs Digital

AnalogDigital
Subject to degradation and noiseFairly immune to noise
Easy to processMore difficult
Best suitable for audio and videoCan be used for either
Limited number of simultaneous channelsCan have large number of simultaneous channels
High error probabilityLow error probability
Encryption and security is difficultEncryption and security is easy
Frequency division multiplexing used for multiple channelsTime division multiplexing used for multiple channels
Synchronization is difficultSynchronization is easier
Denoted by sine wavesDenoted by square waves
Low bandwidthHigh bandwidth
Has higher density and can represent more accurate definition, almost infinite limited by noise floor onlyInformation density is totally dependent on byte size

These days it would be unusual to see a pure analog or digital system. Both technologies are inevitably combined and each system has its place.

Permission to use images from https://www.monolithicpower.com/en/analog-vs-digital-signal.