Representing
Sound

Analogue vs Digital · Sampling · Sample Rate · Bit Depth · File Size

CSZoneAQA GCSE Computer Science 8525

Analogue to Digital

How Sound Is Captured

Sound is an analogue wave — it is continuous. Computers can only store digital data, so sound must be sampled at regular intervals and each sample stored as a binary number.

Sample Rate — how many samples are taken per second (measured in Hz or kHz). CD quality = 44,100 Hz (44.1 kHz)

Bit Depth (Sample Resolution) — how many bits used per sample. More bits = more accurate amplitude. CD quality = 16 bits

Quality vs File Size

The Trade-off

HIGHER QUALITY

Higher sample rate → captures more detail → closer to original analogue sound

More bit depth → more precise amplitude → less distortion

LARGER FILE

More samples per second → more data to store

More bits per sample → more data per sample

Calculating File Size

Sound File Size Formula

File size = Sample rate × Bit depth × Duration (seconds)
(result in bits)

Example: 3-minute song at CD quality
Sample rate: 44,100 Hz
Bit depth: 16 bits
Duration: 3 × 60 = 180 seconds
Channels: stereo (×2)

File size = 44,100 × 16 × 180 × 2
= 254,016,000 bits
÷ 8 = 31,752,000 bytes
÷ 1,024,000 ≈ 30.3 MB

Exam Practice

Have a go at this question

AQA-style question

A recording uses a sample rate of 8,000 Hz, 8-bit depth, for 10 seconds (mono).
(a) Calculate the file size in bytes.
(b) State how increasing the sample rate affects sound quality and file size.

4 marks

(a) 8,000 × 8 × 10 = 640,000 bits ÷ 8 = 80,000 bytes [2]
(b) Higher sample rate → better quality (more detail captured) [1] but larger file size (more data stored) [1]

Key Takeaways

What to Remember

Sound is sampled at regular intervals to convert analogue → digital

Sample rate (Hz) = samples per second · Bit depth = bits per sample

File size = Sample rate × Bit depth × Duration (in bits)

Higher sample rate/bit depth = better quality + larger file