For desktop computer users, it should be clear that headphones/speakers should be plugged into the 3.5mm audio interface on the I/O panel at the back of the motherboard. However, many motherboards also have a square interface in the audio interface area, but many people do not know its function. Even professional DIY users believe that few have used it. It is SPDIF optical output.
SPDIF (Sony/Philips Digital Interface Format, SONY /Philips Digital Interface Format) is an interface standard for transmitting digital audio signals, jointly developed by SONY and Philips, aiming to achieve digital audio data transmission between different audio devices.
We know that when a 3.5mm interface is plugged in with headphones or speakers, it will directly produce sound. This is because it outputs analog signals. SPDIF, on the other hand, transmits unconverted digital audio signals, reducing signal loss during the digital-to-analog conversion process and theoretically preserving purer sound quality.

SPDIF has two interface forms. One is Coaxial, which transmits through the RCA interface (Coaxial cable). This interface was often seen on early motherboards and its appearance is similar to that of a regular audio lotus head.
Another type is Optical fiber, which is the square interface we mentioned at the beginning. It is basically equipped on mid-to-high-end motherboards. It is transmitted through optical fiber cables, using optical signals to convey data. It has a stronger ability to resist electromagnetic interference and is suitable for long-distance transmission.

Compared with the 3.5mm interface, SPDIF can transmit many lossless audio formats. For example, audio formats such as Dolby Digital and DTS can output 5.1 and 7.1 channel surround audio. Therefore, it is often used to connect AV amplifiers and home theater systems.
For music lovers who have strict requirements for sound quality, it can also be used to output PCM format to a dedicated power amplifier. Therefore, in addition to desktop motherboards, Blu-ray players, XBOX/PS game consoles, professional sound cards, TV boxes and even some televisions are also equipped with them.
Take a close look at the optical fiber audio interface. There is also a movable protective door. When you open it, you can see red light inside. This is because the motherboard is the output end, which transmits the light loaded with digital audio signals to the receiving device through the optical fiber cable. Therefore, the optical fiber interface of the receiving device does not emit light.
Of course, to use this interface, you naturally need to purchase an optical fiber audio cable. It is not expensive but rather delicate. Since it is made of glass fiber inside, it cannot be bent. Also, pay attention to keeping both ends clean; otherwise, it will affect the quality of signal transmission and reception.
So, if this interface is so useful, why do most people basically not use it? On the one hand, the inherent nature of a desktop computer is not a mobile usage scenario. Isn't it tiring to just think about moving it from the study to the living room to connect a power amplifier?
On the other hand, this interface is too niche. Apart from audio enthusiasts, almost no one uses it. In addition, SPDIF's support for high-specification formats and high-sampling-rate audio has reached a bottleneck. Moreover, digital audio can also be output together with video signals from DP and HDMI. This true one-wire function is more convenient to use, so SPDIF has become even less popular.