SPDIF (Sony/Philips Digital Interface) is the abbreviation of SONY, PHILIPS digital audio interface. In terms of transmission, SPDIF is divided into two types of output (SPDIF OUT) and input (SPDIF IN). Most of the sound card chips are able to support SPDIF OUT, but we need to note that not every product will provide a digital interface. There are relatively few sound card chips that support SPDIF IN, such as EMU10K1, YMF-744 and FM801-AU, CMI8738, etc. The typical application of SPDIF IN on a sound card is CD SPDIF, but not every sound card that supports SPDIF IN provides this interface either. In terms of transmission carrier, SPDIF is divided into coaxial and optical fiber two, in fact, they can transmit the same signal, only the carrier is different, the interface and the appearance of the cable also has differences. But the optical signal transmission is a popular trend in the future, its main advantage is that there is no need to consider the interface level and impedance issues, the interface is flexible and more resistant to interference. Transmission of digital sound signals through the SPDIF interface has become a common feature of the new generation of PCI sound cards. In the past, when CD-ROMs played records, music in digital format first had to go through D/A processing inside the optical drive. After being converted into an analog signal, it was transmitted to the sound card via the four-pin analog signal cable that we often use, and then a series of processing was performed. The key to the problem is that the quality of D/A chips used in different CD-ROMs varies, and there is a lot of distortion in the output analog signal after poor quality D/A conversion. So it also causes the effect of different models of CD-ROM drives in playing records to vary, and the better reputation in CD decoding quality belongs to SONY and CREATIVE products, while some miscellaneous CD-ROM drives are unbearable to listen to. In order to avoid this problem, most optical drives have a digital signal output (Audio Digital) next to the analog signal output pins. Through this two-pin interface, the record sound signal can be transmitted directly to the sound card in a digital way, the D / A conversion to the audio processing chip to complete. The premise is that the sound card chip can complete the relevant conversion work and support SPDIF IN, to receive digital signals. the signal-to-noise ratio of CD playback will then be significantly improved. S/PDIF is often used to transmit compressed audio signals, and it is customized by the IEC 61937 standard. It is often used in home DVD theaters that support Dolby technology or DTS surround effects. The other is the transmission of the original audio signal from the CD player to the audio receiver. Of course, some home computers and laptops that support Dolby or DTS technology are also loaded with S/PDIF. it is necessary to explain that you should not think that using SPDIF to transmit AC-3 signal is AC-3 decoding, there is no product in civilian sound card that can support hardware-grade Dolby Digital decoding, and the function of SPDIF at this time is mainly to transmit digital AC-3 signal from sound card to decoder. And those six-channel products are the products of analog 5.1 and software decoding.