Data representation and security · GCSE Computer Science

Sound representation

GCSE Computer Science sound: sampling rate, bit depth, file size calculation, and analogue-to-digital conversion.

UNDERSTAND→RETRIEVE→REMEMBER
THE MEMORY HOOK
Sample often, measure finely — rate × depth × time × channels = bits.

The important bits

What you need to know

  1. 1

    Sound is analogue; computers sample amplitude at intervals → discrete digital values.

  2. 2

    Sample rate (Hz): samples per second. CD quality 44.1 kHz — Nyquist: rate at least twice highest frequency needed.

  3. 3

    Bit depth: bits per sample — resolution of amplitude; higher = more dynamic range, less quantisation error.

  4. 4

    Monophonic vs stereo: multiply by channels (1 or 2).

  5. 5

    File size (bits) ≈ rate × depth × duration (s) × channels. Convert to bytes (÷8), KiB/MiB per question convention.

  6. 6

    Higher rate/depth = better fidelity, larger file. Compression (MP3 lossy, FLAC lossless).

  7. 7

    Quantisation error: rounding between levels — audible as noise if depth too low.

  8. 8

    Metadata: sample rate, bit depth in file header so player knows how to reproduce.

Quotations worth analysing

Short evidence. Real method.

“Nyquist: sample at least twice the highest frequency you want to capture.”
Sampling theory GCSE summary

Explains 44.1 kHz for human hearing ~20 kHz. Under-sampling causes aliasing — higher tier on some specs.

Go deeper

Work a file size

30 s mono, 16-bit, 44.1 kHz: 44100 × 16 × 30 × 1 bits. Show arithmetic. Examiners award method.

WORKED EXAMPLE

See the idea in action

Calculate 10 s stereo 16-bit 48 kHz: 48000 × 16 × 10 × 2 = 15,360,000 bits = 1,920,000 bytes. Note doubling for stereo.

Exam technique

Turn knowledge into marks

Write formula before numbers. Check channels and seconds units.

Common mistakes

Do not give these marks away

  1. 01

    Forgetting to multiply by channels for stereo.

  2. 02

    Confusing bit depth with sample rate.

  3. 03

    Using 1000 vs 1024 when question specifies.

QUICK RETRIEVAL

What does sample rate measure?

AHow many samples per second

BHow many colours per pixel

CCPU clock speed

DHard disk RPM

Show the answer

How many samples per second. Hz = samples per second. Bit depth is amplitude resolution per sample.

Quick questions

If this is the bit you searched

Why 44.1 kHz?

Covers human hearing with Nyquist margin; CD standard.