The S/PDIF Port – Explained

The S/PDIF Port - Explained

S/PDIF port is a digital audio transport protocol that is used in consumer and professional audio electronics.

S/PDIF transmits high-quality digital audio by RCA connectors or a fiber-optic cable with a TOSLINK connector. The latter could be found in CD players and video games consoles and RCA connectors are common in DVD players, flat-screen TVs, and home theatres. Although S/PDIF ports have their disadvantages, they are still widely used in consumer electronics instead of, or alongside with HDMI connectors. Audio producers can implement S/PDIF ports professionally to track the input of digital preamps, bypass the DACs of their audio interfaces or track the digital output of guitar modeling amps.

The S/PDIF Port: explained

S/PDIF port stands for Sony/Philips Digital Interface or, more accurately, Sony/Philips Digital Interconnect Format. It is a digital audio transfer protocol that is commonly used in consumer and professional audio electronics. This protocol is capable of transmitting high-quality digital audio to a receiver, and there are two ways to transmit a signal.

One of them is a coaxial cable with RCA connectors which, chances are, you are the most familiar with. Most of the modern DVD players, flat-screen TVs, and home theatres use this type of connection. It consists of 3 color-coded cables, white and yellow usually stream video data, and the orange one is an audio cable capable of using S/PDIF protocol.

The second way to transmit the S/PDIF signal is a TOSLINK connectors with a fiber-optic cable. Although this type of connection is also very common and is used in a lot of consumer audio equipment such as CD players and video game consoles, TOSLINK connectors are very fragile and could be permanently damaged if bent.

When the S/PDIF format was firstly implemented, standard XLR connectors were used, but because XLRs were considerably more expensive at that time, the connection was changed to RCA and TOSLINK. Both connectors are identical in the content that they transmit, but TOSLINK fiber-optic cables are not susceptible to ground loops and radio-frequency interference. 

Capabilities and advantages

S/PDIF ports are capable of transmitting stereo uncompressed PCM audio or compressed surround sounds with 5 or 7 channels. Unfortunately for audiophiles, lossless surround formats are not supported. S/PDIF is most commonly used in consumer audio electronics such as DVD players or computers to transfer compressed digital audio surround sound to a home theatre receiver. Also, it is widely used to carry uncompressed digital audio from a CD player to an amplifier.

S/PDIF is capable of transmitting the digital signals in different formats, but two of the most used ones are the 48 kHz sample rate and 44.1 kHz format, which is a standard for CD audio. S/PDI format has no defined bit rate. Instead, the original work clock is extracted from the carrying signal.

By default, S/PDIF uses 20-bit audio data streams, and in order to successfully transmit a signal with a lesser bit rate, additional bits are set to 0. S/PDIF is also capable of transmitting 24-bit audio, but it strictly relies on the equipment. If an audio device does not support a 24-bit rate, those 4 extra bits would be ignored.

Limitations and disadvantages

Since S/PDIF ports do not control the bit rate, it relies very heavily on the ability of the receiver to recover the source clock. Depending on the equipment, there’s a chance that a bit slip might occur. Bit slip is a loss of one or several bits that occurs if receiver and source clocks vary. And unfortunately, this is not the only downside of using S/PDIF ports.

Every time when the clock recovery is happening, and it is desynchronized, there may occur an effect which is called jitter. This is an effect that occurs when a periodic signal loses its constancy. Specifically, in clock recovery, this process is called timing jitter. In digital audio, jitter is more often than not presented by clicks and other undesirable artifacts.

Unlike TOSLINK optical fiber cables, coaxial cables that use RCA connectors are very susceptible to ground loops and radio frequency interference. But despite some very clear disadvantages, S/PDIF ports are still widely used in various applications, both consumer and professional.

Consumer usage

Even considering that most modern consumer audio applications have built-in digital audio converters, they still, more often than not, could be connected to outboard devices with DAC. Let’s presume for a second that you have a flat-screen TV that you want to connect to an external acoustic system with a 5.1 surround sound. To this day, the best way to do so is to use an S/PDIF port. But you should be aware that if your flat-screen TV has, for example, an RCA connector and acoustic surround system uses optical TOSLINK, those two systems are completely incompatible. So make sure that the systems you are using have the same type of connection.

You also have the ability to bypass DAC in your PC or laptop and output your audio through a device that has converters of a higher quality. But it should be said though that nowadays, some computer manufacturers do not include an S/PDIF output to the backplate of the motherboard. That being said, even though the output itself is not included, every motherboard has an S/PDIF port on it. This could be considered a disadvantage, but to think about it, this gives you an opportunity to purchase the right adaptor that is compatible with an external device and connect it straight to the motherboard.

It should also be said that all of this makes sense only if you are interested in a digital audio output. If you add a video to the mix, it is rather more beneficial to use an HDMI cable. This type of connection supports both digital video and audio outputs and is more convenient since it uses only one connector, unlike S/PDIF ports that need at least 3 different connectors for audio and video. Also, unlike S/PDIF ports, HDMI supports lossless uncompressed surround audio both with 5 and 7 channels.

Overall, it is useful to have S/PDIF cables at your disposal only if you, on occasion, interact with older digital equipment or modern video game consoles in order to separate video and audio outputs. Also, it could be done with aforementioned flat-screen TV.

Professional application

Generally speaking, we, audio producers, have very little to no interest in consumer audio electronics. We like top-shelf professional stuff that would help us to create exceptional audio production. And since the equipment that we use already has DAC of the highest quality possible, how could we benefit from S/PDIF ports?

At first glance, we actually could not, since our audio interfaces and digital mixes have USB and Firewire connectivity both of which have greater capabilities than S/PDIF ports. But things are not as simple as it seems. First of all, some audio interfaces offer you some sort of digital audio output which, in turn, will let you to output your audio digitally to the speakers that have built-in DAC. This could seem like a bit of an overkill, but if your setup, for example, struggles with a ground loop noise, using S/PDIF optic fiber cables could help you eliminate this problem.

Some modern digital outboard gear also offers S/PDIF connectivity which could be rather beneficial for a workflow since you could easily track your recordings in stereo. Some of the modern dual-channel preamps can output a digital S/PDIF signal through an RCA cable which means that you can bypass XLR inputs of your audio interface if you choose to do so. The reason why you might want to do this is if you for some reason do not want to use DACs of your audio interface or simply do not have enough analog inputs.

Having an S/PDIF port could also be rather beneficial to guitar players who prefer to use digital emulations. If you’re a guitar player who uses a modeling amp of some sort, chances are that it has an S/PDIF connectivity means that you could easily record or monitor your guitar in stereo.