Know that a sound can be transmitted as a digital or analogue signal

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3.3 Electromagnetic Spectrum – Sound Transmission

Objective

Know that a sound can be transmitted as either an analogue or a digital signal, and be able to describe the main features, advantages and disadvantages of each method.

1. Sound – Fundamental Concepts

  • Production of sound: Produced by a vibrating source that creates alternating compressions and rarefactions in a material medium.
  • Nature of the wave: Longitudinal – particle displacement is parallel to the direction of wave travel.
  • Frequency range: 20 Hz – 20 kHz for the human ear.
  • Need for a medium: Sound cannot travel in vacuum; it requires air, water, or solids.
  • Speed of sound:
    • Air (20 °C): ≈ 340 m s⁻¹
    • Liquids (e.g., water): ≈ 1500 m s⁻¹
    • Solids (e.g., steel): ≈ 5000 m s⁻¹
  • Compression & rarefaction: The alternating high‑pressure (compression) and low‑pressure (rarefaction) regions transport the acoustic energy.
  • Ultrasound (frequencies > 20 kHz):
    • Medical imaging (sonography)
    • Industrial non‑destructive testing
    • Sonar for navigation and depth finding

2. Electromagnetic (EM) Spectrum – Overview

All EM waves travel at the same speed in vacuum:

c = 3.0 × 10⁸ m s⁻¹

Region Typical Frequency / Wavelength Common Uses (including an example for radio waves) Potential Harmful Effects (high intensity)
Radio waves 3 kHz – 300 GHz (> 1 mm) AM/FM broadcasting, TV, mobile phones, Wi‑Fi, radar, RFID tags Thermal heating of tissue (very high‑power transmitters)
Microwaves 300 MHz – 300 GHz (1 mm – 1 mm) Microwave ovens, satellite communication, GPS Burns and cataracts from strong exposure
Infrared (IR) 300 MHz – 430 THz (1 mm – 700 nm) Remote controls, thermal cameras, night‑vision, heating Skin burns with intense IR sources
Visible light 430 THz – 770 THz (700 nm – 400 nm) Illumination, displays, fibre‑optic communication Eye damage (photochemical or thermal) from very bright sources
Ultraviolet (UV) 770 THz – 30 PHz (400 nm – 10 nm) Sun‑lamps, sterilisation, black lights, photolithography Skin burns, premature ageing, increased cancer risk
X‑rays 30 PHz – 30 EHz (10 nm – 0.01 nm) Medical imaging, airport security, material analysis Ionising radiation – DNA damage, cancer risk
Gamma rays > 30 EHz (< 0.01 nm) Radiotherapy, industrial radiography, astrophysics Highly penetrating ionising radiation – severe health hazard

3. Transmitting Sound over the EM Spectrum

Sound is first converted into an electrical signal, then used to modulate an EM carrier. Two distinct approaches are used:

  • Analogue transmission – the electrical signal varies continuously, directly mirroring the pressure variations of the original sound.
  • Digital transmission – the sound waveform is sampled, quantised and encoded into a series of binary numbers before modulation.

4. Difference Between Analogue and Digital Signals (Syllabus wording)

Analogue Signal

  • How it works: The audio voltage directly modulates an EM carrier.
    • Amplitude Modulation (AM) – carrier amplitude varies with the audio signal.
    • Frequency Modulation (FM) – carrier frequency varies with the audio signal.
  • Typical carriers: AM and FM radio, analogue television, traditional land‑line telephone.
  • Advantages
    • Simple transmitters and receivers.
    • Real‑time transmission – virtually no latency.
  • Disadvantages
    • Noise and distortion are added directly to the signal.
    • Signal quality degrades with distance.
    • Limited ability to correct errors.

Digital Signal

  • How it works: The continuous sound wave is converted into a stream of binary data.
    1. Sampling – the waveform is measured at regular intervals (sample rate).
    2. Quantisation – each sample is assigned a numeric value from a finite set of levels (bit depth).
    3. Encoding – the binary numbers are packaged for transmission (e.g., PCM, MP3, AAC).
  • Common digital carriers: CDs, MP3 files, Digital Audio Broadcasting (DAB), Bluetooth audio, internet streaming, digital TV.
  • Advantages
    • High resistance to noise – errors can be detected and corrected.
    • Data can be compressed, giving higher effective data‑rates.
    • Signal can be regenerated accurately at repeaters, allowing long‑range transmission.
    • Easy storage, editing and duplication.
  • Disadvantages
    • Requires analogue‑to‑digital and digital‑to‑analogue conversion equipment.
    • Introduces a small processing delay (latency).
    • Higher bandwidth requirement compared with simple analogue carriers (though compression mitigates this).

5. Benefits of Digital Signalling (linked to AO2)

These benefits are examined in Assessment Objective 2 (handling information and problem‑solving) when students compare analogue and digital transmission:

  • Higher data‑rates – many bits per unit time; compression further increases efficiency.
  • Long‑range transmission – repeaters regenerate a perfect digital signal, preventing cumulative distortion.
  • Accurate regeneration – the original binary pattern can be reproduced exactly at each stage.
  • Built‑in error‑checking and correction – reduces the impact of noise.

6. Process of Digitising Sound

  1. Sampling – the analogue waveform is measured N times per second.
    • Nyquist theorem: \(f_s \ge 2\,f_{\text{max}}\).
      For the audible range (\(f_{\text{max}} \approx 20\) kHz) the minimum is 40 kHz; CD audio uses 44.1 kHz.
  2. Quantisation – each sample is rounded to the nearest of \(2^b\) levels, where b is the number of bits per sample.
    • 8‑bit audio → 256 levels.
    • 16‑bit audio (CD quality) → 65 536 levels.
  3. Encoding – quantised values are expressed in binary; optional compression (MP3, AAC, FLAC) reduces the amount of data to be transmitted.

7. Comparison of Analogue and Digital Signals

Aspect Analogue Digital
Signal form Continuous waveform that directly follows the sound pressure variation Discrete binary numbers representing sampled values
Transmission medium Radio waves, telephone lines (analogue modulation) Radio waves, optical fibre, Ethernet (digital modulation)
Noise sensitivity High – noise adds directly to the signal Low – errors can be detected/corrected; noise must exceed a threshold to alter bits
Bandwidth requirement Usually lower (depends on carrier) Higher – set by sampling rate & bit depth, but can be reduced by compression
Quality over distance Degrades gradually (cumulative distortion) Remains essentially constant – repeaters regenerate the exact digital data
Typical uses FM/AM radio, analogue telephone, old TV broadcasts CDs, MP3/streaming, DAB radio, digital TV, Bluetooth audio

8. Practical Applications

  • Radio broadcasting – AM/FM (analogue) versus DAB (digital).
  • Telecommunications – traditional analogue land‑line phones versus digital mobile networks (2G, 3G, 4G, 5G).
  • Audio storage – vinyl records and cassette tapes (analogue) versus CDs, DVDs, Blu‑ray, MP3/FLAC files (digital).
  • Streaming services – rely entirely on digital encoding, compression and packet‑based transmission over the internet.
  • Wireless audio – Bluetooth and Wi‑Fi use digital modulation to send sampled audio to headphones, speakers, or cars.
  • Ultrasound imaging – uses high‑frequency digital signals to produce medical pictures.

9. Sample Experimental Question (AO3 – Practical Skills)

Design an experiment to compare the quality of sound received from an analogue FM broadcast and a digital DAB broadcast of the same music piece.

  1. Set up a standard FM receiver and a DAB receiver in the same location.
  2. Use identical loudspeakers and keep the volume setting constant.
  3. Record a 30‑second excerpt from each source using a calibrated sound‑level meter and a computer‑based audio analyser.
  4. Analyse the recordings for:
    • Signal‑to‑noise ratio (SNR)
    • Total harmonic distortion (THD)
    • Frequency response (± 3 dB bandwidth)
  5. Discuss which transmission method provides higher fidelity and why, linking the results to the advantages and disadvantages listed above.

10. Suggested Diagram

Flowchart showing (a) conversion of a sound wave into an analogue signal (modulation) and (b) conversion into a digital signal (sampling → quantisation → encoding → modulation).

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