S/PDIF vs. Optical: What’s the Difference?

Digital audio transmission is the norm and is used across the board, including in professional audio equipment, home theater systems, and personal computers.

There are various digital audio transfer formats. HDMI (high-definition multimedia interface) is the one with the most street cred. Bluetooth, albeit wireless, is another commonly used format for sending audio files and more.

Then, there are S/PDIF and TOSLINK audio interfaces. They don’t get discussed within the general public domain as often as some of the above formats do. However, musicians, audio professionals, etc., know their capabilities.

Through this article, we’d like to democratize S/PDIF and optical (TOSLINK) and make knowledge about the two formats more widespread. We’ll discuss how the two are similar and different, which suits better for specific needs, etc.

If you’d like to know your audio interfaces beyond HDMI, USB (Universal Serial Bus), and Bluetooth, keep reading.

What Is S/PDIF?

S/PDIF (Sony/Philips Digital Interface) is an interface that helps transmit digital audio signals or information between devices in both professional and consumer audio equipment setups.

Employing the AES3 standard (a more professional standard), S/PDIF can be transported over optical or coaxial cables, supporting up to 192 kHz sampling rates and 24-bit resolutions.

The digital audio interface is widely used to connect digital audio players, CD/DVD players, home theater receivers, and other digital audio devices. Digital audio workstations and mixing consoles are some of the professional audio equipment where S/PDIF plays a role.

S/PDIF uses an RCA (Radio Corporation of America) or BNC (Bayonet Neill-Concelman) connector, commonly referred to as “phono connectors,” because they are widely accepted, relatively simple to work with, secure, and found on various audio equipment.

Note that S/PDIF could also use TOSLINK (optical cable) to send audio.

What Is “Optical?”

The term “optical” denotes anything with light or light properties.

The word could be related to or used in the context of optical equipment and devices (cameras, microscopes, telescopes, etc.). It could also be brought up when discussing reflection, diffraction, refraction, etc.

In the “cable” context, “optical” denotes a cable that employs one or multiple optical fibers to send information as light. Optical fibers are thin, transparent strands of plastic or glass guiding light along their length.

An optical fiber cable carries light signals that a computer interprets as data. It carries much more information than standard copper cables and is immune to radio frequency interference (RFI) and electromagnetic interference (EMI).

Optical cables are used for varied purposes, including ISPs (internet service providers), telecommunications, cable TV, and industrial automation. Because the cable is immune to EMI and RFI, it is particularly handy in environments with considerable electrical noise.

There are different optical cable types: single-mode fiber, multi-mode fiber, plastic optical fiber, flexible optical fiber, graded-index fiber, etc. Single-mode and multimode are two broad categories under which the other types fall.

By the way, TOSLINK (Toshiba Link) is an optical fiber connector commonly found on audio equipment optical cables. TOSLINK employs fiber optic tech to transmit digital audio signals.

The connector is commonly used to connect home theater receivers and CD/DVD players, and other digital audio devices. Optical cables could also be referred to as TOSLINK because the latter only uses optical cords.

S/PDIF vs. Optical

S/PDIF is a digital interface, whereas the term “optical” in this context denotes the fiber-optic cable with the TOSLINK connector. The two don’t compete but complement each other.

But a comparison could be made between S/PDIF and TOSLINK. They both are digital audio interfaces, with TOSLINK exclusively using optical cables, unlike S/PDIF, which can switch between coaxial and optical.

Mode of Transmission

S/PDIF employs electricity to transmit information. Optical/TOSLINK uses light-based fiber optics, which doesn’t use electrical signals.

The light travels via a thin plastic or glass core covered by cladding (plastic or glass).

The cladding material has a smaller refractive index at its core, which means less bending of light near the cladding or trapping of light inside the core without escaping from the cable’s sides.

The electrical signals in a S/PDIF coaxial connection pass through the cable’s center conductor, which runs through the cable’s center. During transit, the electrical current creates a magnetic field surrounding the cable.

Because S/PDIF can also use optical cables, they are also capable of delivering light-based audio signals.

Connectors

TOSLINK connectors, also called optical or fiber-optic audio connectors, are rectangular or square and use a removable plug. S/PDIF connectors, as mentioned earlier, are usually RCA connectors.

S/PDIF’s phono connectors are more widely used and can be seen on several professional and consumer audio devices. TOSLINK’s square-shaped connectors, on the contrary, are more niche and don’t work with all devices.

TOSLINK doesn’t employ phono connectors (RCA) because they aren’t cut out to send light-based digital data but are more suited for analog audio.

Cable Length

Since S/PDIF cable length could employ coaxial or optical cables, the length of the cord varies accordingly.

It could be up to 50 feet (15 meters) in length if it’s a coax. Coaxial cables can get much longer than that based on use. For Ethernet, the cable can extend up to 328 feet (100 meters).

If the cable is longer than 15 meters for S/PDIF, signal distortion or loss is quite likely. If the S/PDIF uses an optical cord, the cable length could be as high as 100 feet (30 meters).

TOSLINK cables can afford to be longer than S/PDIF coax cords. The maximum length is usually more than 15 meters. Some can even be over 100 feet (30 meters) long.

Note that these cable length numbers are not set in stone and could vary based on the particular implementation, cable quality, and the connector used. Electromagnetic interference and other similar factors could also determine the maximum cable length.

If using lengthier cables, it’s advised to use signal repeaters or amplifiers to preserve signal quality.

Applications

Although S/PDIF can be seen paired with digital audio workstations (DAWs) and mixing consoles, it is mostly used with consumer audio equipment.

TOSLINK suits professional setups better, primarily due to its increased electromagnetic interference immunity. Toshiba manufactured TOSLINK optical in 1983 to link receivers to CD players.

The lower bit depth and sample rate support deem S/PDIF not ideal for use with professional applications. S/PDIF’s unbalanced RCA connectors’ susceptibility to external noise is another reason professionals steer clear.

Most importantly, S/PDIF is one-way and not bi-directional like TOSLINK. This inability to send data back and forth between two devices limits S/PDIF’s chances in a professional environment.

S/PDIF ports can also be found on some motherboards, especially high-end ones, but that’s rare. The interface could be seen on certain desktop and laptop gaming computers, sometimes integrated into the headphone jack.

Resistance to Externalities

Because optical cables are less vulnerable to external radio interference than electrical cables, TOSLINK can send signals much longer without signal degradation concerns.

Optical cables, however, are more susceptible to physical damage, bending, temperature changes, etc. Coaxial cables, on the other hand, are much more resistant as they are made to be compliant and withstand some bending.

Optical cables aren’t as flexible as they use plastic or glass fibers as signal transmission mediums. Those are more delicate and sensitive than copper and, therefore, more prone to damage.

To better understand the respective cables’ rigidness, go over their specifications in the manual.

Below is a quick run-through of the differences between S/PDIF and optical/TOSLINK:

Although S/PDIF and TOSLINK have pros and cons, the latter is usually considered more capable and advanced due to improved immunity to interference and noise and data transfer capabilities.

Note: S/PDIF and optical/TOSLINK do not support Dolby TrueHD or DTS-HD lossless surround formats, as their data transfer requirements are greater than what the two audio interfaces can chew.

Conclusion

To conclude, S/PDIF is a digital audio interface, and “optical” denotes a physical cable.

But because a comparable audio interface called TOSLINK uses optical lines only, it’s not incorrect to position this as a S/PDIF and TOSLINK battle instead.

Both S/PDIF and optical have their respective use cases. For example, although TOSLINK is more advanced, musicians who prioritize low-latency audio will prefer S/PDIF.

If you’re keen on high-quality audio transfer over long distances with little to zero interference, TOSLINK suits the role better.

HDMI is better than the two at supporting several advanced audio formats. But if the focus is on audio only, S/PDIF is better.

Audio professionals will prefer TOSLINK over HDMI and S/PDIF since the latter two are more consumer-focused.