What is the primary difference between Sample Rate and Bit Depth?

Sample Rate refers to how often measurements are taken per second (frequency), while Bit Depth refers to the precision of each measurement (amplitude levels).

How does the file size change if you switch from Mono to Stereo recording?

The file size doubles because stereo recording requires two separate audio channels to be sampled and stored simultaneously.

What is the relationship between Bit Depth and the number of available amplitude levels?

The number of levels is $2^n$, where $n$ is the bit depth. For example, an 8-bit depth provides $256$ possible amplitude levels.

What is a common mistake when calculating file size from a sample rate given in kHz?

Forgetting to convert kilohertz (kHz) to Hertz (Hz) by multiplying by $1,000$. Calculations must use the base unit of Hz.

Why might a student get a file size that is 8 times too large?

They likely provided the answer in bits instead of bytes. To get bytes, the total number of bits must be divided by $8$.

What happens to the digital representation if the sampling interval is too large?

The digital wave becomes a poor approximation of the analogue wave, leading to loss of high-frequency detail and potential distortion (aliasing).

Define 'Sampling' in the context of audio.

Sampling is the process of measuring the amplitude of an analogue sound wave at regular, discrete intervals of time.

What is 'Hertz' (Hz) a measure of in digital audio?

Hertz is the unit for Sample Rate, representing the number of samples captured per second of audio.

What is an Analogue-to-Digital Converter (ADC)?

An ADC is a hardware device that converts continuous analogue electrical signals into a stream of discrete binary numbers.

Why does increasing bit depth improve sound quality?

It reduces quantization error by providing more precise values for the amplitude, resulting in a sound that is closer to the original analogue signal.

Representing Sound | OCR GCSE Computer Science

Representing Sound

Summary

Sound digitization is the process of converting continuous analogue sound waves into discrete digital data that computers can process and store. This is achieved through sampling, where the amplitude of the wave is measured at regular intervals and converted into binary values based on defined sample rates and bit depths.

1. Definition & Core Concepts

Analogue Sound: Naturally occurring sound is an analogue signal, meaning it is continuous in both time and amplitude. It travels as waves of varying pressure through a medium like air.
Digital Sound: Computers cannot process continuous waves; they require discrete, numerical data. Digital sound is a sequence of binary numbers that approximate the original analogue wave.
Analogue-to-Digital Conversion (ADC): This is the hardware process (often performed by a sound card) that captures analogue signals from a microphone and converts them into digital bits for storage.

A diagram showing a continuous red analogue wave being sampled at regular intervals, represented by green dots (discrete data points).

2. Underlying Principles: Sampling and Quantization

Sampling: The process of taking 'snapshots' of the sound wave's amplitude at specific points in time. The frequency of these snapshots determines how accurately the wave's shape is captured.
Quantization: Once a sample is taken, its amplitude must be mapped to the nearest available numerical value. This value is then stored as a binary integer.
Reconstruction: During playback, a Digital-to-Analogue Converter (DAC) uses these stored binary values to recreate the electrical signals that drive speakers or headphones.

3. Methods & Techniques: Key Parameters

Sample Rate: Measured in Hertz (Hz), this is the number of samples taken per second. A higher sample rate captures higher frequencies and results in a digital wave that more closely resembles the original.
Bit Depth (Sample Resolution): The number of bits used to store each sample. This determines the number of possible amplitude levels that can be recorded, affecting the dynamic range and precision of the sound.
Channels: Sound can be recorded in Mono (one channel) or Stereo (two channels). Stereo recording effectively doubles the amount of data stored as it captures two distinct audio streams.

4. Mathematical Foundations: File Size Calculation

5. Key Distinctions

6. Exam Strategy & Tips