S/PDIF can carry two channels of uncompressed PCM audio or compressed 5.1/7.1 surround sound such as Dolby Digital and DTS audio; however, it is not advanced enough to support lossless audio formats due to higher bandwidth requirements.[1] Nevertheless, it’s able to carry digital audio signals in high resolution from playback sources like CD, DVD, and Blu-Ray players and computers to home theater systems, amplifiers, receivers, and other types of sound systems.
There are two types of S/PDIF ports that are easily identifiable through their shapes. In some devices, you may see a small, square port with a S/PDIF Out label, while in others, the port is small and circular. This is because S/PDIF refers to the interface or the data link protocol and not the port per se, and it can transmit data signals over an optical or electrical cable. Depending on the type of implementation used by the device, the port may either be optical (square) or electrical (circular), but some devices contain two kinds of ports for flexibility.
Electrical Implementation
S/PDIF can transmit digital data using electricity; this is implemented using coaxial cables and uses the small, circular S/PDIF ports. Coaxial cables use multi-core wires and are terminated by an RCA connector. Shielded coaxial cables are highly recommended for excellent performance and to avoid quality loss.
Coaxial cables transmit data at a bandwidth of 24bit/192KHz; higher than their optical counterpart resulting in better quality audio and higher resolution. One downside to coaxial cables is that it picks up electrical noise and interferences as signals travel from the source to the destination; thus, it is not suitable for longer distances.
It’s also important to note that S/PDIF requires the use of a 75-Ohm RCA phono connector for the interface to function properly. Otherwise, audio jitters and drop-outs will be experienced, or there will be no audio in the worst cases. This type of cable is easily distinguishable by its orange color on the outside or inside the housing, although not all coaxial ports have this orange coding.
Coaxial cables are normally found in both legacy and modern devices such as DVD or Blu-Ray players, DACs, FM tuners, and stereo amplifiers.
Optical Implementation
S/PDIF can also transmit data using fiber optics, and the most common connector is the TOSLINK (Toshiba Link) or Mini Optical connectors. These cables have small, square connectors that fit the square S/PDIF ports. Fiber optic cables are either made of glass, plastic, or silica. The cables are usually thin cables with a small square connector at the end that emits light. Optical S/PDIF ports are found in flat-screen TVs, DVD/Bluray players, soundbars, AV receivers, video game consoles, integrated amplifiers, and other sound systems.
Optical cables operate at a lower bandwidth than coaxial cables operating at 48bit/96KHz. However, unlike coaxial cables, optical cables use light to emit signals and are therefore not susceptible to circuit noise, ground loops, and RF interference. This makes optical cables ideal for longer distances. In fact, good-quality optical cables can run as far as 50 meters. However, one downside to optical cables is that it is fragile and can easily be damaged, so it is not advisable to run them through bends.
Limitations
S/PDIF delivers quality digital audio, but it comes with certain limitations. Because it does not control bit rate, the receiver must catch up with the source’s clock for sychronization. If the source and the receiver have the same clock, this does not pose any issue. However, if the opposite is the case, bit slip, which is a loss of bits or bits of data, is likely to occur. Bit slip usually occurs when the source and the receiver have different clocks resulting in audio artifacts such as jitters.
Another limitation of S/PDIF is that it only works with up to two channels of audio, while some optical ports can work with four to eight channels. In addition, it can only carry audio signals, while modern digital connections like HDMI can transmit both video and audio signals in a single interface.
Converting Signals
Modern devices only embed digital audio interfaces such as S/PDIF, for audio connections eliminating the traditional analog connections. This is not a problem if the device you’re connecting to has a S/PDIF port, but how will you connect your legacy analog devices that are still working? You may not be able to directly connect your old devices to newer ones, but that doesn’t mean you will no longer be able to use them.
You can still connect your legacy analog-only devices to the digital audio interface using a DAC (Digital to Analogue Converter). Similarly, if your audio source only uses an analog interface, you can use an analog to Digital Converter to connect the older device to your newer, digital device.
Conclusion
S/PDIF is a reliable format for transmitting digital audio over relatively short distances, supporting both electrical and optical transmission. Although there are disadvantages to using S/PDIF, you can still find it in a wide range of consumer electronics and audio systems such as in-game consoles, flat-screen TVs, DVD/Blu Ray players, AV receivers, and amplifiers.
Many audio interfaces have already emerged since S/PDIF, which offer more advanced features than S/PDIF. This has caused S/PDIF’s popularity to decline, but it is still not phased out because it supports both legacy and modern audio devices.
Sources:
[1] Wikipedia.S/PDIF. https://en.wikipedia.org/wiki/S/PDIF Accessed March 6, 2022
