Coaxial and Optical Cables: A Comparative Overview

Coaxial and optical cables are two fundamentally different physical transmission media. In digital audio, they most commonly serve as the physical layer for the S/PDIF protocol, carrying digital audio signals between devices. While they transmit the same underlying data, their distinct physical properties result in different characteristics, advantages, and disadvantages.

1. Coaxial Cable

Physical Structure:
A coaxial cable features a central conductor surrounded by concentric layers:

  • Center Conductor: A single copper or copper-clad steel wire carrying the electrical signal.
  • Dielectric Insulator: Surrounds the conductor, maintaining spacing and electrical isolation.
  • Shielding: A braided copper mesh and/or aluminum foil that protects against Electromagnetic Interference (EMI) and acts as a ground return.
  • Jacket: The protective outer coating.

Transmission & Connection:

  • Connector: Typically uses an RCA connector in consumer audio.
  • Signal Type: Transmits an electrical voltage signal (a 0.5V pulse).
  • Key Specification: Requires a characteristic impedance of 75Ω for proper signal transmission and to prevent reflections.

Advantages:

  • High Bandwidth: High-quality coaxial cables can easily support all high-resolution audio formats, including PCM up to 192kHz/24-bit and higher DSD rates (via DoP).
  • Potentially Lower Jitter: As a direct electrical connection, it can, with a well-designed receiver, result in lower jitter during clock recovery, potentially benefiting sound quality.
  • Durability: Robust connectors and cables that are resistant to physical damage.

Disadvantages:

  • Susceptible to EMI: While shielded, it can still be vulnerable to electromagnetic interference in noisy environments.
  • Ground Loops: The electrical connection can create ground loops between devices, introducing a audible hum if they are at different ground potentials.
  • Impedance Matching: Requires specific 75Ω cables for optimal performance.

2. Optical Cable (TOSLINK)

Physical Structure:
An optical cable uses thin, flexible fibers made of glass or plastic.

  • Core: The central light-carrying fiber.
  • Cladding: A layer surrounding the core with a lower refractive index, trapping light inside the core via total internal reflection.
  • Jacket: The protective outer layer.

Transmission & Connection:

  • Connector: Known as a TOSLINK connector.
  • Signal Type: Transmits pulses of light. An LED or laser diode in the transmitter converts electrical signals to light ("on" for 1, "off" for 0), which a photodetector in the receiver converts back.

Advantages:

  • Total Immunity to EMI: Since it carries light, it is completely unaffected by electromagnetic interference.
  • Galvanic Isolation: The non-conductive fiber completely eliminates electrical ground loops, solving hum problems.
  • Safety: Ideal for connecting devices in different power domains.

Disadvantages:

  • Bandwidth Limitations: Particularly older TOSLINK hardware and fibers may have insufficient bandwidth for very high sample rates (e.g., PCM > 192kHz or DSD128), potentially causing dropouts.
  • Higher Inherent Jitter: The electro-optical conversion process and modal dispersion (light scattering in the fiber) can introduce significant jitter, potentially degrading audio quality.
  • Physical Fragility: The cables are more susceptible to damage from sharp bends or kinks.

3. Summary Comparison

Characteristic

Coaxial

Optical (TOSLINK)

Signal Type

Electrical

Light

Connector

RCA

TOSLINK

EMI Immunity

Good

Excellent

Ground Loop Prevention

No

Yes

Bandwidth Potential

High

Limited (esp. old hardware)

Typical Jitter

Lower

Higher

Max Distance

Longer (>10m)

Shorter (varies with quality)

Durability

High

Moderate (avoids sharp bends)

 4. How to Choose

The choice depends on your specific system and environment:

  • Choose Optical if:
    • You have a persistent ground loop hum between devices.
    • The cable run is through an area with strong EMI.
  • Choose Coaxial if:
    • You are pursuing the best possible sound quality and your equipment is designed for low jitter.
    • You need to transmit very high-resolution formats and your gear supports it.
    • There are no ground loop issues in your setup.

Key Consideration: In most home environments without audible noise, a high-quality coaxial connection often provides marginally superior audio performance due to lower inherent jitter. However, optical is the definitive solution for eliminating ground loops and EMI.