Compare analogue and digital signals, state the advantages of digital, and describe converting between the two

What this dot point is asking

SEAB wants you to compare analogue and digital signals, to state the advantages of digital, and to describe how a signal is converted between the two forms. The central insight is that an analogue signal varies smoothly and can take any value, while a digital signal is restricted to two levels, and that the two-level form, though it loses some detail, is far more robust and easier to process and store.

The answer

Analogue signals

An analogue signal varies continuously and can take any value within its range. The voltage from a microphone, a temperature sensor, or the mains supply are analogue: they change smoothly over time and carry their information in the exact size of the voltage at each instant. This makes them a natural match for real-world quantities, which also vary smoothly.

Digital signals

A digital signal is allowed only two levels: logic 0 (low) and logic 1 (high). It changes in steps, not smoothly, and carries information as patterns of these two states (binary). Computers, logic circuits and memory all work with digital signals.

Comparing the two

The key differences:

• Values: analogue takes any value; digital takes only two.
• Form: analogue is smooth and continuous; digital changes in steps.
• Noise: an analogue signal is easily corrupted, because any added noise changes the exact value; a digital signal is robust, because small noise does not push a clear 0 or 1 into the wrong state.

Advantages of digital

Digital signals are widely preferred for processing and storing information because:

• They are robust against noise, so they can be copied and transmitted many times without the information degrading.
• They are easy to store and process with logic circuits and memory, and can be checked and corrected for errors.
• They allow reliable, repeatable behaviour that does not drift with temperature or age the way analogue circuits can.

The trade-off is that converting a smooth analogue quantity to digital loses a little detail, because the value is rounded to the nearest available step.

Converting between the two

Because the real world is analogue but processing is digital, conversion is needed in both directions:

• An analogue-to-digital converter (ADC) measures an analogue voltage at regular instants and represents each measurement as a binary number.
• A digital-to-analogue converter (DAC) turns a stream of binary numbers back into a smoothly varying analogue voltage, for example to drive a loudspeaker.

This conversion is why a sound can be captured by a microphone (analogue), stored and processed digitally, and then played back through a speaker (analogue again).

Examples in context

Example 1. Music: vinyl versus streaming. A vinyl record stores sound as a continuous analogue groove, which can pick up scratches and hiss. A streamed file stores the same sound digitally as 0s and 1s, which can be copied endlessly and sent across the world without degrading. This is the everyday triumph of digital robustness over analogue fragility.

Example 2. A digital thermometer. A thermistor gives a smoothly varying analogue voltage, which an analogue-to-digital converter turns into a number shown on the display. Working digitally lets the thermometer store readings, sound alarms at set values and avoid the drift of a purely analogue meter, showing why sensing is analogue but processing is digital.

Try this

• Cue. State the defining feature of a digital signal. It has only two levels (logic 0 and 1) and changes in steps, rather than varying continuously through any value.

• Cue. Give one advantage of digital over analogue for storing information. It is robust against noise, so it can be copied and transmitted many times without the information degrading.

• Cue. Name the device that converts an analogue sensor voltage into a binary number. An analogue-to-digital converter (ADC).