What is Coaxial? (Digital Audio Interface)

In the consumer audio world, "Coaxial" refers to the digital coaxial interface – a connection method that uses a 75-ohm impedance coaxial cable to transmit digital audio signals.

It is a widely used physical implementation of the S/PDIF (Sony/Philips Digital Interface) standard. Because it follows a standard, different audio manufacturers can design products that work interchangeably.

Core Principle: The S/PDIF Standard Co-developed by Sony and Philips

The S/PDIF standard was jointly defined by Sony and Philips to transmit high-quality stereo digital audio signals over a single cable. S/PDIF is a protocol – a common language. Coaxial cable and optical fiber are two different physical carriers for this language.

To ensure accurate transmission, the coaxial cable must meet strict technical specifications:

  • 75-ohm characteristic impedance: This is the key physical standard for digital coaxial transmission. Characteristic impedance is like the design specification of an "information highway". Any deviation causes signal reflections and distortion.
  • Special cable construction: A standard coaxial cable differs from ordinary analog RCA cables. It has a center conductor, dielectric insulator, shield, and outer jacket all sharing the same axis, which helps maintain signal integrity and reject interference.
  • Common connector types:
    • RCA connector (phono plug): Most common – looks identical to analog audio connectors, but the cable and purpose are different. Do not mix them.
    • BNC connector: Has a bayonet lock for a more secure connection and better interference rejection; commonly found on professional equipment.
  • What audio signals can it carry? It can carry uncompressed PCM stereo (up to 24-bit/192kHz) as well as compressed multi-channel surround sound formats like Dolby Digital and DTS.

Comparison: Coaxial vs. Optical

Instead of a table, here is a descriptive comparison between Coaxial and Optical (Toslink):

Coaxial

  • Transmission medium: Copper cable carrying electrical signals
  • Signal conversion: Direct electrical signal – no conversion needed
  • Susceptibility to interference: Weaker; the cable itself can pick up electromagnetic interference (EMI)
  • Jitter performance: Low jitter – more precise sound
  • Transmission distance: Not suitable for long distances (under 1.5 meters recommended)
  • Typical use cases: High-quality stereo, home theater

Optical

  • Transmission medium: Fiber optic cable carrying light signals
  • Signal conversion: Requires electrical-to-optical and optical-to-electrical conversions
  • Susceptibility to interference: Stronger; light signals are immune to EMI
  • Jitter performance: The extra conversions can introduce jitter
  • Transmission distance: Suitable for long distances with minimal signal loss
  • Typical use cases: Long runs, electrically noisy environments, some portable devices

In simple terms, Coaxial transmits electrical signals directly through the cable, avoiding the extra electro-optical conversions. Theoretically, it has lower jitter (a type of distortion affecting timing precision), which provides an advantage when playing high-resolution audio, giving richer detail. However, it is sensitive to cable quality and shielding, and is more susceptible to electromagnetic interference. It is not ideal for long distances.

Important reminder: Do not mistake a digital coaxial audio cable for a standard red/white analog RCA cable. Although the connectors look the same, ordinary audio cables are not designed to meet the 75-ohm standard. Using one can cause unstable signal transmission and degraded sound quality.

Common Applications in Daily Life

Because of its reliability and high-quality transmission, the coaxial interface is very common in audio-visual equipment. For example:

  • Home theater systems: Connect a Blu-ray player to an AV receiver with a coaxial cable to transmit high-quality Dolby Digital or DTS multi-channel surround sound for a cinematic experience.
  • Hi-Fi audio setups: If you have a separate digital-to-analog converter (DAC) , use a coaxial cable to connect a CD transport or computer sound card to enjoy cleaner sound than analog connections.
  • Connecting a TV to a sound system: Use a coaxial cable to connect your TV's digital audio output to a soundbar or receiver to significantly improve the audio quality of TV shows.

Common Issues and Solutions

Here are some practical tips if you encounter problems:

  • No sound or popping noises: First, check that your source device has S/PDIF or coaxial output enabled. Next, ensure the cable is securely connected. Then, verify that the audio format is supported by your DAC/receiver. Finally, try replacing the cable with a high-quality 75-ohm coaxial cable.
  • Intermittent sound: This is usually caused by impedance mismatch or poor contact. Make sure you are using a dedicated 75-ohm coaxial cable.
  • Only noise (hissing/static): This often happens because your source (e.g., a computer) is outputting a surround sound format like Dolby Digital, but your DAC only supports PCM. Change the source device's output format to PCM.

Summary and Buying Tips

In summary, digital coaxial (Coaxial) is a reliable choice for connecting high-quality audio equipment, especially for hi-fi music listening and home theater surround sound.

When shopping, keep these points in mind:

  1. Check the connector type: Determine whether your equipment uses RCA (phono) or BNC connectors.
  2. Buy the right cable: Always purchase a cable clearly marked as "digital coaxial cable" or "75-ohm". Do not use an ordinary analog audio cable.
  3. Consider your budget: If your budget allows, choose a cable with dual shielding for better immunity to interference.