Describe how analogue sound is sampled to produce digital data, and explain how sample rate and bit depth affect sound quality and file size.

What this dot point is asking

WJEC asks you to explain how a real, continuous sound, such as a voice or music, is turned into binary numbers a computer can store, and how the two recording settings, sample rate and bit depth, affect both quality and file size. As with images, the file-size calculation is a frequent exam question, so you need the process and the numbers.

Analogue versus digital

The starting point is the difference between the real sound and what a computer can hold.

Sampling

The conversion is done by sampling.

Because samples are taken only at intervals, the digital version is a series of steps that approximate the smooth original. Taking samples more often, and storing each one more precisely, makes the approximation closer to the real wave.

Sample rate

Sample rate controls how often the wave is measured.

Bit depth

Bit depth controls how precisely each sample is stored.

Calculating sound file size

This is the headline exam skill.

Sample rate and bit depth together

The two settings control different things, and a good answer keeps them apart. Sample rate is about how often the wave is measured, so it governs how well rapid changes and high-pitched sounds are captured: too low a sample rate and high frequencies are lost, making the sound dull or distorted. Bit depth is about how precisely each individual measurement is stored, so it governs the accuracy of each sample's amplitude: too low a bit depth and the steps between possible values are coarse, adding a background "graininess" to quiet passages. Raising either improves quality, and raising either increases the file size, because both multiply into the size formula. In an exam, name which setting you are changing and state its specific effect rather than saying "the quality goes up".

Mono and stereo

A real recording may store more than one channel of sound. A mono recording has a single channel, while a stereo recording has two (left and right), which is what gives a sense of direction through headphones. Because a stereo file stores two channels, its data is roughly double that of the equivalent mono file at the same sample rate and bit depth. If a question states a recording is stereo, the size formula result is multiplied by the number of channels, so a stereo clip doubles the figure from sample rate times bit depth times seconds. Knowing this lets you explain why a music track is larger than a spoken-word recording even at the same settings.

Why this matters

Sound files can be large: CD-quality audio uses a high sample rate and bit depth, so a few minutes is tens of megabytes. This is why music is usually compressed before streaming or storing on a phone. Knowing the trade-off lets you justify settings: a podcast of speech can use a low sample rate and bit depth to keep files small, while a music recording needs high values for fidelity.