Sound Representation: Sampling, Sample Rate, Bit Depth, and File Size

Computers store sound by taking samples of an analogue sound wave and converting them to digital data. This lesson covers how sound is digitised and how to calculate audio file sizes, as required by OCR J277 Section 2.6.

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Analogue vs Digital Sound

Analogue sound is a continuous wave — the air vibrates smoothly, producing a wave with infinite detail. Digital sound is a series of discrete numerical values that approximate the original wave.

To convert analogue sound to digital, a computer uses an ADC (Analogue-to-Digital Converter). The ADC measures the amplitude (height) of the sound wave at regular intervals. Each measurement is called a sample.

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Sampling

Sampling is the process of measuring the amplitude of a sound wave at regular time intervals. Each sample is stored as a binary number.

The quality of the digital recording depends on two factors:

1. Sample rate — how often samples are taken
2. Bit depth — how many bits are used to store each sample

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Sample Rate

The sample rate is the number of samples taken per second, measured in Hertz (Hz).

Sample rate	Samples per second	Use
8,000 Hz (8 kHz)	8,000	Telephone quality
22,050 Hz	22,050	Radio quality
44,100 Hz (44.1 kHz)	44,100	CD quality
48,000 Hz (48 kHz)	48,000	DVD/professional audio
96,000 Hz (96 kHz)	96,000	Studio quality

A higher sample rate captures the sound wave more accurately because more measurements are taken, producing a closer approximation to the original analogue wave.

OCR Exam Tip: CD quality audio uses a sample rate of 44,100 Hz. This number appears frequently in exam questions — memorise it.

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Bit Depth

Bit depth (also called sample resolution) is the number of bits used to store each individual sample. More bits means a wider range of amplitude values can be recorded.

Bit depth	Number of levels	Use
8 bits	2^8 = 256	Low quality (telephone)
16 bits	2^16 = 65,536	CD quality
24 bits	2^24 = 16,777,216	Professional studio

A higher bit depth means:

• More possible amplitude values per sample
• Better representation of quiet and loud sounds
• Less quantisation error (the difference between the actual amplitude and the stored value)

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How Sampling Works: A Visual Explanation

Imagine a smooth sound wave. The ADC takes readings at regular intervals:

Amplitude
    |     *  *
    |   *      *
    |  *   |    *   |    Samples taken at marked intervals
    | *    |     *  |
    |*     |      * |
    +------+-------+-------> Time