Common audio interface introduction (2): digital audio interface

In terms of digital audio interfaces, we are actually talking more about transmission protocols or standards. In terms of the physical appearance of the interface, it is difficult to see what type of interface it is.

1. AES/EBU interface

AES/EBU is an abbreviation of Audio Engineering Society/European Broadcast Union, and is now a more popular professional digital audio standard. It is a serial bit transmission protocol for transmitting digital audio data based on a single twisted pair. Without equalization, data can be transmitted over a distance of up to 100 meters. If equalized, data can be transmitted over longer distances.

AES/EBU provides two channels of audio data (up to 24-bit quantization). The channels are automatically timed and self-synchronized. It also provides transmission control methods and channel status bits (channel status bit) and some error detection capabilities. Its clock information is controlled by the transmitting end and comes from the bit stream of AES/EBU. Its three standard sampling rates are 32kHz, 44.1kHz, and 48kHz. Of course, many interfaces can work at different sampling rates.
There are various physical interfaces for AES/EBU. The most common is the three-core XLR interface, which is used for balanced or differential connection. In addition, there are audio coaxial interfaces that use RCA plugs to be discussed later, for single-ended unbalanced Connection; and use fiber optic connectors for optical connections.

2. S/PDIF interface

S/PDIF is the abbreviation of Sony/Philips Digital Interconnect Format. It is a civilian digital audio interface protocol developed by Sony and Philips. Due to its widespread adoption, it has become the de facto standard for civilian digital audio formats. S/PDIF and AES/EBU have slightly different structures. Audio information occupies the same position in the data stream, making the two formats compatible in principle. In some cases, the professional equipment of AES/EBU and the user equipment of S/PDIF can be directly connected, but this method is not recommended, because there are very important differences in electrical specifications and channel status bits. When mixing protocols May have unpredictable consequences.

There are generally three types of S/PDIF interfaces, one is RCA coaxial interface, the other is BNC coaxial interface, and the other is TOSLINK optical fiber interface. In international standards, S/PDIF requires BNC interface 75 ohm cable transmission. However, due to various reasons, many manufacturers frequently use RCA interface or even use 3.5mm small stereo interface for S/PDIF transmission. Over time, RCA and 3.5mm interface Became a “civilian standard”. Later we will talk about the coaxial interface and fiber interface.

3. coaxial interface

There are two types of coaxial interface, one is RCA coaxial interface, and the other is BNC coaxial interface. The appearance of the former is no different from the analog RCA interface, while the latter is a bit similar to the signal interface that we commonly see on TVs, and it has a locking design. The coaxial cable connector has two concentric conductors, the conductor and the shield share the same axis, and the impedance of the wire is 75 ohms.

The coaxial transmission impedance is constant and the transmission bandwidth is high, so the audio quality can be guaranteed. However, although the appearance of the RCA coaxial interface is the same as the RCA analog interface, it is best not to mix the cables. Because the RCA coaxial cable is fixed at 75 ohm impedance, the mixed cable will cause unstable sound transmission and degrade the sound quality.

4. Optical fiber interface

The English name of the optical fiber interface is TOSLINK, which is derived from the technical standards formulated by Toshiba (TOSHIBA) and is generally marked as “Optical” on the equipment. Its physical interface is divided into two types, one is a standard square head, and the other is a round head that is similar to 3.5mm TRS connectors on portable devices. Because it transmits digital signals in the form of optical pulses, from a technical point of view, it is the fastest transmission speed.

The optical fiber connection can achieve electrical isolation, prevent digital noise from being transmitted through the ground wire, and help to improve the signal-to-noise ratio of the DAC. However, because it requires a light emitting port and a receiving port, and the photoelectric conversion of these two ports requires a photodiode, there can be no close contact between the fiber and the photodiode, which will cause digital jitter-like distortion, and this distortion is superimposed of. Coupled with the distortion in the photoelectric conversion process, it is much worse than coaxial in terms of digital jitter. As a result, fiber optic interfaces are beginning to fade out of sight.

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