Understand that analogue signals are sampled to create the digital version…

The idea

Sound travels as a continuously changing analogue wave: the air pressure rises and falls smoothly, so the signal has a value at every instant. A computer cannot store an infinite number of values, so it takes samples at regular intervals set by the sample rate, measured in hertz (Hz), where 1 Hz means one sample per second. Each sample records the wave's amplitude at that instant, and that value is converted into a binary number using the sample resolution, measured in bits. The list of binary values, plus the sample rate and resolution, is the digital version of the sound. A higher sample rate captures rapid changes more faithfully; a higher resolution records each amplitude more precisely. Both increase file size.

For example: Sampling at 44 100 Hz for 10 seconds produces 441 000 samples, each stored as a binary number.

What earns the marks

• Analogue sound is a continuous signal with a value at every instant, whereas digital data is discrete.
• Sampling means measuring the signal at regular intervals determined by the sample rate in Hz.
• Each sample's amplitude is converted to a binary number using the sample resolution in bits.
• The stored sequence of binary values, with sample rate and resolution, forms the digital representation.
• Increasing sample rate or resolution improves accuracy but increases file size.

Common mistakes and how to put them right

• Thinking sampling captures the whole continuous wave at once.

  Put it right: Sampling takes discrete measurements at set time intervals.

• Confusing sample rate with resolution.

  Put it right: Sample rate is how often (Hz), resolution is how precisely each value is stored (bits).

• Believing a higher sample rate reduces file size.

  Put it right: More samples per second means a larger file.

Exam tips

• Link each term to its unit: sample rate in Hz, resolution in bits, amplitude as the measured value.
• When asked why sampling is used, state that computers store discrete binary values, not continuous signals.
• Use a short worked example, such as samples per second multiplied by duration, to show understanding of sample rate.