Distinguish analogue and digital signals
Compare continuously varying representations with discrete allowed values, without confusing amplitude levels and bits.
Extended only (Supplement); Physics 0625, 2026–2028. Follow the selected spectrum or sound section. Diagrams illustrate stated ideal models and supplied data. Written practice is self-checked.
Before you begin
- Read a signal-value against time graph.
- Distinguish information from its carrier.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a signal-value versus time trace describe?
What you will learn
- Describe the difference between analogue and digital signals.
- Read discrete level representations without claiming every digital signal is a square wave.
Describe the representation
An analogue signal varies continuously to represent a changing quantity, such as a microphone voltage following sound-pressure variations. In the ideal model it can take any value within a range. A digital representation uses discrete allowed values or symbols rather than every intermediate value.
Binary digital information uses two logical values, 0 and 1. Physical voltage or light levels represent those symbols. Real transitions take time and received levels can contain noise, so a drawn square pulse is a simplified representation rather than a claim of physically instantaneous changes.
Separate levels from the thing represented
A digital system can encode several amplitude levels using groups of bits. Four allowed amplitude values can be labelled 00, 01, 10 and 11 using two bits per sample. These discrete values remain digital even though there are more than two represented amplitudes.
An analogue waveform is not automatically sound, and a digital waveform is not automatically light. Both describe the information representation; the physical transmission may use an electrical or electromagnetic system. Read the labelled axes before interpreting a graph.
Pause and explain
Which description identifies a digital representation?
Worked example
An amplitude is restricted to 0, 1, 2 or 3 and each value is sent as 00, 01, 10 or 11. Classify the representation.
Show the worked solution
- There are four allowed amplitude values.
- They are discrete rather than a continuous range.
- Each is represented by a two-bit group.
Answer Digital; four discrete amplitude levels encoded by two bits each.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define the difference between analogue and digital signals.
Give me a hint
Compare continuous values with allowed discrete states.
Compare my reasoning
- Analogue varies continuously.
- Digital uses discrete allowed states.
- Both may represent the same original quantity.
Analogue: continuous variation; digital: discrete allowed values or symbols.
Look for these in your work
- State both descriptions.
- Keep carrier separate.
How many bit patterns are available using two bits, and how can they label four amplitude levels?
Give me a hint
List the ordered pairs of binary symbols.
Compare my reasoning
- The patterns are 00, 01, 10 and 11.
- Each labels one of four levels.
- The representation is discrete.
Four patterns, one for each of four allowed levels.
Look for these in your work
- Include all four groups.
- Distinguish levels from individual bits.
A physical pulse has slightly rounded transitions. Does that alone prove its information is analogue?
Give me a hint
Ask whether the receiver interprets discrete states.
Compare my reasoning
- Real transitions need finite time.
- A receiver can still distinguish allowed symbols.
- Rounded edges alone do not determine the encoding category.
No; discrete interpreted symbols can still carry digital information.
Look for these in your work
- Separate physical shape and coding.
- Use the stated interpretation.
Common mistakes
- Digital means discrete, not necessarily only two measured amplitudes.
- A graph's axis may be voltage rather than air displacement.
- Carrier and representation are separate descriptions.
What can you explain now?
Can an analogue signal take intermediate values in its ideal range?
Compare with the explanation
Yes.
Continuous variation distinguishes it from a restricted set of digital values.
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Make it stick
Define analogue and digital representations and explain discrete amplitude coding with groups of bits.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.