Sound Representation

Updated Specification based on Examiner Mark Schemes & Reports

1. The Sampling Process (Analogue to Digital)

Standard Spec: Analogue sounds must be stored in binary.

Updated Detail: To convert analogue to digital, the computer does not just "record" the sound; it performs sampling.

  • The amplitude (height) of the sound wave is measured[cite: 14].
  • Measurements are taken at set / regular intervals (not random).
  • Each amplitude measurement is converted into a unique binary number[cite: 11].
⚠️ CRITICAL EXAMINER NOTE: Do not say "The frequency is measured." This is a common error. The amplitude (height) is the value being measured and stored. Frequency refers to how often the samples are taken, not the value of the sample itself.
Exam Question Example

Q: Describe how an analogue sound wave is converted into digital form. [3 marks]

Reveal Mark Scheme Answer
  • The analogue sound wave is sampled.
  • The amplitude / height of the wave is measured[cite: 42, 145].
  • At set / regular time intervals[cite: 145].
  • Each measurement is stored as a binary number[cite: 145].

2. Sample Rate (Frequency)

Standard Spec: Sample rate – measured in Hertz (Hz).

Updated Detail:

  • Definition: The number of times per second the sound wave is measured[cite: 15].
  • Unit: 1 Hz = 1 sample per second. 44.1 KHz = 44,100 samples per second.
  • Effect of Increasing Sample Rate:
    • File Size: Increases (more data points to store)[cite: 104].
    • Quality: Improves (the digital wave is closer/truer to the original analogue wave)[cite: 105, 171].
Exam Question Example

Q: A radio station records an interview. Before the second interview, the sampling frequency is increased. Explain two effects this will have on the recording.

Reveal Mark Scheme Answer

Effect 1 (File Size): The file size will be larger[cite: 104].

Effect 2 (Quality): The quality of playback will be better / more accurate to the original sound[cite: 105].

3. Bit Depth (Resolution)

Standard Spec: Bit depth – number of bits available to store each sample.

Updated Detail:

  • Definition: The number of bits allocated to each specific sample[cite: 16].
  • Effect on Range: The higher the number of bits, the wider the number of amplitudes (dynamic range) that can be measured/recorded.
  • Effect on Accuracy: Higher bit depth results in greater accuracy of the sampled wave[cite: 140].
⚠️ EXAMINER NOTE: Do not confuse Bit Depth with Sample Rate. Increasing Bit Depth does not mean "more samples per second." It means "more detailed information for each sample"[cite: 140].
Exam Question Example

Q: Explain how changing the bit depth will affect the sound file.

Reveal Mark Scheme Answer
  • File Size: The file size will change (e.g., higher bit depth = larger file)[cite: 140].
  • Quality/Accuracy: There will be a change in the accuracy of each sample. A wider range of amplitudes can be recorded[cite: 140].