AQA A-Level Computer Science AS Paper 2, June 2025: Question 4
4 marks · Easy difficulty · Short Answer
Explain how sound samples are represented as bit patterns, the role and necessity of a DAC in playing sound, and determine the highest frequency in an analogue sound sampled at 2200 Hz using the Nyquist theorem.
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Question text
04.1 Each time a sample of an analogue sound is taken a bit pattern is stored.
Describe what each bit pattern represents.
[1 mark]
04.2 A sound file is being played on a laptop computer. The laptop contains a
digital to analogue converter (DAC).
Explain the role of the DAC and why it is necessary.
[2 marks]
04.3 A particular analogue sound must be sampled at a rate of at least 2200 Hz to be
accurately represented.
State the highest frequency present in the analogue sound.
[1 mark]
Mark scheme
Show the mark scheme
Qu Pt Marking Guidance Marks
04 1 Mark is for AO1 (understanding) 1
A measurement of the amplitude of the (analogue) sound wave (at a point in time);
A. Height for amplitude.
NE. Each pattern is a measurement of the sound wave.
Qu Pt Marking Guidance Marks
04 2 Marks are for AO1 (understanding) 2
Role of DAC:
DAC transforms digital data/information into analogue (audio) signal/
/wave/voltage;
DAC converts PCM pulses back to PAM (and smooths the output);
Max 1
Why necessary:
The laptop speakers/headphones can only play back an analogue signal;
Qu Pt Marking Guidance Marks
04 3 Mark is for AO2 (application) 1
1100 (Hz) // 1.1 kHz;
How to answer it
Analogue Audio, DACs & Nyquist’s Theorem
This question examines core concepts in Fundamentals of Data Representation (Sound) from the AQA specification:
- Sampling representation: What stored binary values correspond to in digitised sound waves.
- Hardware conversion: The explicit role and purpose of a Digital-to-Analogue Converter (DAC) in audio playback.
- The Nyquist Theorem: Calculating the maximum sound frequency that can be faithfully reproduced at a given sampling frequency.
Bit Patterns in Sound Sampling
Describe what each bit pattern represents when an analogue sound is sampled.
✅ Model Answer
A measurement of the amplitude (or height) of the analogue sound wave at a specific point in time.
🧠 Exam Technique
Always name the physical property being measured! Saying "the amplitude at a point in time" ensures you hit the key descriptor required by the mark scheme.
💡 Key Knowledge
- Amplitude: Relates to the volume or height of the sound wave.
- Sample Resolution: The number of bits allocated to store each amplitude reading (e.g. 16-bit sound gives 65,536 discrete amplitude levels).
❌ Common Errors
- Vague answers: "A measurement of the sound" is explicitly rejected with NE (Not Enough).
- Confusing properties: Writing frequency or pitch instead of amplitude. Bit patterns record vertical wave height, not frequency.
• 1 mark: Amplitude (or height) of the (analogue) sound wave (at a point in time).
Role and Necessity of a DAC
Explain the role of the DAC and why it is necessary.
✅ Model Answer
Role: Converts digital binary data (discrete numeric values) into a continuous analogue electrical signal/voltage.
Necessity: Output hardware like laptop speakers or headphones are analogue devices that can only produce sound from continuous analogue signals.
🧠 Two Distinct Points
Notice the question has two prompts: "the role" AND "why it is necessary". Break your answer cleanly into two sentences to secure both marks without ambiguity.
💡 Key Knowledge
- Computers manipulate discrete digital binary patterns ( 0 s and 1 s).
- The physical world and our ears perceive continuous vibrations. Transducers like speaker cones require a fluctuating analogue voltage to vibrate.
❌ Common Errors
- Circular definitions: "The DAC converts digital to analogue because we need to make it analogue" fails to mention the speaker/headphone requirement.
- Confusing ADC and DAC: Describing recording from a microphone (ADC) instead of playback through speakers (DAC).
• Role (1 mark): Transforms digital data/information into analogue signal/wave/voltage (or converts PCM pulses to PAM and smooths output).
• Necessity (1 mark): Laptop speakers or headphones can only play back an analogue signal.
Highest Frequency & The Nyquist Theorem
A particular analogue sound must be sampled at a rate of at least 2200 Hz to be accurately represented. State the highest frequency present in the analogue sound.
📐 Step-by-Step Calculation
- State the formula: According to the Nyquist Sampling Theorem:
Sampling Rate ≥ 2 × Highest Frequency (fmax) - Rearrange for maximum frequency:
fmax = Sampling Rate / 2 - Substitute the values:
fmax = 2200 Hz / 2 = 1100 Hz
✅ Model Answer
1100 Hz (or 1.1 kHz)
Both standard units (Hz) or kilo-units (kHz) are fully accepted by examiners.
💡 Nyquist Theorem Core Rule
To faithfully digitise and reproduce an analogue signal without aliasing (distortion), the sampling frequency must be at least twice the maximum frequency component present in the signal.
❌ Common Errors
- Multiplying instead of dividing: Calculating 2200 × 2 = 4400 Hz (doing the inverse of what is required).
- Missing or incorrect units: Always write Hz or ensure prefix conversion is correct ( 1.1 kHz , not 1100 kHz ).
• 1 mark: 1100 (Hz) or 1.1 kHz.
Topics
4.5 Fundamentals of data representation · 4.5.6 Representing images, sound and other data
Question and mark scheme from the AQA A-Level Computer Science examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.