Represent sound as analogue and digital information
Trace a microphone signal, sample and quantise supplied values, then reconstruct sound at a receiver.
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
- Distinguish analogue and digital signals.
- Trace a microphone-to-transmitter-to-receiver path.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the microphone's role in a voice communication system?
What you will learn
- Explain that sound can be transmitted as analogue or digital information.
- Distinguish sampling, discrete coding and reproduced sound.
Represent the same sound in two ways
A microphone produces an electrical representation of sound variations. An analogue communication system can transmit information following those continuous variations. A digital system measures sample values at successive times and represents them using discrete codes.
The transmitted electromagnetic carrier is not a sound wave in air. A receiver recovers the representation, and a loudspeaker creates a local sound disturbance. The original air particles have not travelled along the communication route.
Read a supplied simple coding rule
For an illustrative four-level model, round each supplied sample to the nearest allowed level 0, 1, 2 or 3, then code these as 00, 01, 10 and 11. Values 0.2, 1.7, 2.8 and 1.2 become levels 0, 2, 3 and 1, giving 00 10 11 01.
Four samples using two bits each need eight data bits before any additional framing. Sampling and quantisation lose some original detail; the reconstructed sound is not guaranteed to match every continuous value exactly. More suitable sampling and level resolution can improve representation without changing the sound's carrier into air vibration.
Pause and explain
Using nearest levels 0–3, how is a sample of 1.7 represented?
Worked example
Encode samples 0.2, 1.7, 2.8 and 1.2 using nearest levels 0–3 and their two-bit codes.
Show the worked solution
- Nearest levels are 0, 2, 3 and 1.
- Codes are 00, 10, 11 and 01.
- Four two-bit samples use eight bits.
Answer 00 10 11 01; eight sample-data bits.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
Describe the analogue and digital routes for transmitting sound information.
Give me a hint
Start with the microphone, then compare representations.
Compare my reasoning
- The microphone represents sound electrically.
- Analogue follows continuous variations; digital uses sampled discrete codes.
- A receiver converts recovered information into local sound.
Both routes represent sound; continuous or coded signals are recovered and reproduced locally.
Look for these in your work
- Describe both routes.
- Separate carrier and sound.
Encode sample values 0.8 and 2.6 using nearest levels 0–3 and two-bit codes.
Give me a hint
Round to an allowed level before using the code table.
Compare my reasoning
- 0.8 rounds to level 1.
- 2.6 rounds to level 3.
- Their codes are 01 and 11.
01 11; four sample-data bits.
Look for these in your work
- Apply nearest-level coding.
- Count two bits per sample.
Why may reconstructed digital sound differ from the original continuous sound even if every transmitted bit is recovered?
Give me a hint
Separate transmission errors from sampling and level resolution.
Compare my reasoning
- Sampling records selected times.
- Quantisation replaces amplitudes with allowed levels.
- Correct bit recovery cannot restore all discarded original detail.
Sampling and quantisation can lose detail before transmission.
Look for these in your work
- Identify both restrictions.
- Do not equate correct bits with exact original waveform.
Common mistakes
- A sample value is not an individual bit.
- Quantisation can lose amplitude detail.
- Reproduced sound is generated locally by the receiver.
What can you explain now?
Can the same original sound be represented by analogue or digital signals?
Compare with the explanation
Yes.
The representations differ even though they can describe the same sound information.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain both sound representations and use a supplied sample-to-level-to-code rule.
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.