What is the audio interface?

The point to note here is that the interface we're talking about actually includes a discussion of the two aspects of "interface" (interface) and "connectors" (connector), which we often call "connectors" or "plugs." "Interface" defines the physical characteristics of the connection between electronic devices, including the frequency and intensity of the transmitted signal, the type and quantity of the corresponding connection, and the structural design of the plugs and sockets, and the connector is the device that physically connects the devices.

TRS interface of analog audio interface

When it comes to the TRS interface, the average person may not know what it is, but as long as the object is in front of you, everyone knows what it is. In fact, the most we see in daily life is the TRS interface, its joint appearance is cylindrical shape, usually has three kinds of sizes 1/4 "(6.3mm), 1/8" (3.5mm), 3/32 (2.5mm), we are most common is the 3.5mm size of the connector.

TRS Connectors of different sizes

2.5mm TRS connectors in the phone before the more popular, but now is rare, the headset interface is basically 3.5mm interface Unified Lake. and 6.3mm joints in many professional equipment and high-end headphones are more common, but now there are many high-end headphones also gradually began to switch to 3.5mm connectors. The meaning of TRS is tip (signal), ring (signal), Sleeve (ground), representing the 3 contact points of the joint, we see three metal columns separated by two sections of insulating material. Therefore, 3.5mm connectors and 6.3mm connectors are also known as "the core" and "Junior".

The construction of "junior core"

The TRS interface is a circular hole that corresponds to the connector and has three contacts, separated from each other by insulating material. Some people say there are no four-pin plugs? Yes, the four-pin plugs we see on headphones or Walkman are used to transmit voice signals or control signals. In addition, a four-pin 3.5mm plug for headphones is used to transmit the balance signal. 6.3mm "Junior" plug can be used to transfer the balance signal or unbalanced stereo signal, that is, it can and we will talk about the XLR balance interface, can transmit the balance signal, but because the production of such a balance line cost is higher, so generally only used in high-end professional audio equipment.

Two-core 6.3mm TRS electric guitar Wire

Of course, since can add core, that also can reduce core. Two-core TRS connectors can be used to transmit unbalanced mono audio signals, such as electric guitars with lines that are two-core TRS lines. Therefore, from the TRS interface look, we do not know whether it supports the balanced transmission; from the core number, we can not determine whether the four-core and above TRS connectors support the balanced transmission, the specific situation needs to see equipment.

RCA interface for analog audio interface

RCA interface in our daily life is also very common, speakers, television, amplifier, DVD players and other equipment on the basic. It derives its name from the English abbreviation of ABC (Radio Corporation of America), which, in the 40 's, introduced the interface to the market and used it to connect the phonograph and the speakers, and therefore it is also known as the Phono interface in Europe. We are more familiar with the joint call is "Lotus head."

RCA interface is very common in our daily life

The RCA connector known as the "Lotus Head"

The RCA interface adopts coaxial transmission signal, the middle axis is used to transmit the signal, and the contact layer on the outer rim is used for grounding. Each RCA cable is responsible for transmitting the audio signal of a channel, so you can use the RCA cable matching the number of channels according to the actual needs of the channel. For example, to group two-channel stereo requires two RCA cables.

XLR interface of analog audio interface

The XLR interface is also known as the "card farm" because the Cannon Electric Company, which was founded by James H. Cannon, was its original producer. Their earliest product was the "Cannon X" series, later, the improved product added a locking device (Latch), then added an "L" after "X", and then added a rubber seal around the metal contact of the joint (rubber compound), and then the "L" A "R" is added to the back. People put three capital letters together, calling the joint "XLR connector".

More common three-core XLR interface

Some ear-top will provide four-core balanced XLR headphone connector

We usually see the XLR plug is 3 feet, of course, there are 2 feet, 4 feet, 5 feet, 6 feet, such as in some high-end headphone line, we will see four core XLR balance connector. The XLR interface, like the "Junior core" TRS interface, can be used to transmit audio balance signals. Here we simply say the balance signal and the unbalanced signal. When sound waves are converted into electrical signals, if the direct transmission is an unbalanced signal, if the original signal is reversed 180 degrees, and then transmitted both the original signal and the counter trust number, this is the balance signal. Balanced transmission is the use of phase cancellation principle, the audio signal transmission process to minimize other interference. Of course, the XLR interface is the same as the "Junior" TRS interface, can transmit the unbalanced signal, so the light from the interface, we can't see what kind of signal it is transmitting.

Aes/ebu interface of digital audio interface

In terms of digital audio interface, we actually speak more about transmission protocols or standards. In the physical appearance of an interface, it is difficult to see what type of interface it is. Let's talk about Aes/ebu first.

Aes/ebu is the abbreviation of audio Engineering Society/european Broadcast Union (the Association of Audio Engineers/European Broadcasting Union) and is now a popular professional digital audio standard. It is a serial bit transmission protocol based on single strand pair to transmit digital audio data. The data can be transmitted over a distance of up to 100 meters without a balance, and if it is balanced, it transmits farther distances.

The most common Aes/ebu physical interface using a three-core XLR interface

The AES/EBU provides two channels of audio data (up to 24 bit quantization), and the channel is automatically timed and synchronized. It also provides the method of transmission control and representation of state information (channel status bit) and some error detection capabilities. Its clock information is controlled by the transmission end, from the AES/EBU bit stream. Its three standard sampling rates are 32kHz, 44.1kHz, 48kHz, and of course many interfaces can work at different sample rates.

There are many physical interfaces of Aes/ebu, the most common is the three-core XLR interface, which is used for balancing or differential connection; In addition to the audio coaxial interface using an RCA plug for a single end unbalanced connection, and optical connection using a fiber optic connector.

S/pdif interface of digital audio interface

S/PDIF is the abbreviation of sony/philips Digital interconnect format, which is a civil digital audio interface protocol developed by Sony and Philips. Because it is widely used, it becomes the de facto standard for civil 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 Aes/ebu professional equipment and S/PDIF user devices can be directly connected, but this practice is not recommended, because there are very important differences between the electrical specifications and the channel state bits, which can have unpredictable consequences when mixing protocols.

S/PDIF interface with RCA coaxial and fiber interface

S/pdif interface generally has three kinds, one is RCA coaxial interface, the other is BNC coaxial interface, there is a Toslink fiber interface. In the international standard, S/PDIF need BNC interface 75 Ohm cable transmission, but many manufacturers for various reasons, frequent use of RCA interface even using 3.5mm small stereo interface for S/PDIF transmission, over time, RCA and 3.5mm interface became a "folk standard." We will refer to the coaxial interface and the fiber interface in the following detail.

Coaxial interface of digital audio interface

Coaxial interface is divided into two types, one is the RCA coaxial interface, the other is the BNC coaxial interface. The appearance of the former is not any different from the analog RCA interface, and the latter is somewhat similar to our common signal interface on the TV, and has a locking design. Coaxial cable connector has two concentric conductors, the conductor and shielding layer share the same axis, the line impedance is 75 ohms.

Coaxial axis of BNC coaxial interface

Coaxial transmission impedance is constant, high transmission bandwidth, so can guarantee the quality of audio. However, although the appearance of the RCA coaxial interface with the RCA analog interface is the same, but the line is best not to mix, because the RCA coaxial axis is fixed 75 ohm impedance, so the mixed line will cause sound transmission instability, so that the sound quality down.

Optical interface of digital audio interface

The English name of the optical interface is Toslink, which originates from the technical standard developed by Toshiba (TOSHIBA) and is generally labeled "optical" on the equipment. Its physical interface is divided into two types, one is the standard square head and the other is a round head similar to the 3.5mm TRS connector on a portable device. Because it transmits the digital signal in the form of the light pulse, it is the fastest transmission from the technical point of view.

Fiber Connector for square head and round head

The optical fiber connection can achieve electrical isolation, prevent digital noise transmission through the ground, and improve the DAC signal-to-noise ratio. However, since it requires a light emitting port and a receiving port, and the photoelectric conversion of these two ports requires a photodiode, there is no close contact between the optical fiber and the photodiode, resulting in the distortion of the digital dithering class, and the distortion is superimposed. In addition to the distortion in the photoelectric conversion process, it is much worse than the coaxial axis in the digital jitter. As a result, the optical interface is now gradually fading away from people's horizons.

PostScript language:

This article, small series and you share some of the more common types of audio interface, in fact, with the continuous progress of technology, there are new audio standards and protocols developed, such as I2S, CobraNet, EtherSound, Dante, AVB and so on, but these are many manufacturers of the agreement standards, In the physical interface is also a variety of implementations can not be unified, useful HDMI interface, useful coaxial interface, useful RJ45 interface and so on. Avoid installation excuses the best way to create a risk is to read the manual a few times, this must have.