Understand how analogue sound is sampled to be stored digitally, the effect of sample rate and bit depth on quality and file size, and calculate sound file sizes.

What this dot point is asking

AQA wants you to explain how analogue sound is sampled to be stored digitally, describe how sample rate and bit depth affect quality and file size, and calculate the size of a sound file.

Sampling

The smaller the gap between samples, the closer the reconstructed wave is to the original. If samples are taken too infrequently, rapid changes in the wave fall between samples and are lost, which is heard as a loss of high frequencies or distortion. This is why music is sampled at 44100 Hz, fast enough to capture the highest frequencies humans can hear.

Sample rate and bit depth

The two settings control different things: sample rate is about how often you measure (horizontal detail, capturing fast changes), while bit depth is about how finely you measure each value (vertical detail, the range of loudness levels). CD-quality audio uses a 44100 Hz sample rate and a 16-bit depth.

Calculating file size

How sampling reconstructs the wave

Picture the analogue wave as a smooth curve and the samples as dots placed on it at equal time intervals. To play the sound back, the computer joins the dots to recreate the wave. If the dots are close together (a high sample rate), the rebuilt wave closely follows the original; if they are far apart (a low sample rate), fast changes in the wave fall between samples and are lost, so the playback is distorted or muffled and high frequencies disappear. This is why a higher sample rate gives better quality: more dots leave less room for the wave to change unseen between measurements.

Quality versus file size

As with images, better quality costs storage. Doubling the sample rate doubles the number of samples per second, so the file size doubles; doubling the bit depth doubles the bits per sample, so again the size doubles. CD-quality audio (44100 Hz, 16-bit) is chosen because it captures the full range of human hearing without wasting space on detail the ear cannot detect. For speech or a phone call a much lower sample rate is acceptable, producing a far smaller file, because speech does not contain the high frequencies that music does. Matching the settings to the use keeps files no larger than they need to be.

Try this

Q1. State what is meant by the sample rate. [1 mark]

• Cue. The number of samples taken per second (in hertz).

Q2. Calculate the size in bits of a 5-second sound clip sampled at 2000 Hz with a bit depth of 16. [2 marks]

• Cue. $2000 \times 16 \times 5 = 160000$ bits.