According to the different electrical structure and physical shape, HDMI interfaces can be divided into three types: A, B and C, and each type of interface consists of a socket at the device end and a plug at the wire end. It is driven by a low voltage of 5V, and the impedance is100Ohm. These three plugs can provide reliable TMDS connection, of which Type A is the standard 19 pin HDMI interface, with the highest penetration rate; Class B interface is slightly larger in size, but has 29 pins, which can provide dual TMDS transmission channels, so it supports higher data transmission rate and dual-link DVI connection. The C-type interface has the same performance as the A-type interface, but the size is smaller, which is more suitable for compact portable devices.

Physical specifications of type a

Type A HDMI socket is a flat "D" with a wide top and a narrow bottom. There is a circle of 0.5 mm thick metal shielding layer outside the interface to prevent various external interference signals. Among them, the socket used for the equipment end has the widest inner diameter, which is 14 mm, and the height is 4.55 mm 19 and the pins are arranged in two layers in the center. Each pin has a width of 0.45 mm and a length of 4. 1 mm. ..

The widest outer diameter of type A plug is 13.9 mm and the height is 4.45 mm. The inner pins are arranged in a ring shape. However, the HDMI standard stipulates that the error of these dimensions should be controlled within a fairly small range (about 0.05 mm) to ensure good contact.

Compared with the A-type interface, the physical structure of the B-type HDMI interface has not changed much, and they are all "D" types. However, the maximum width of the socket end reaches 2 1.3mm, which is a whole circle larger than that of type A14 mm.

The C-type HDMI interface is designed for compact portable devices, so the size of the C-type socket is only 10.5× 2.5mm, and the plug is only 10.42× 2.4mm ... very small.

These three HDMI interfaces are not completely compatible, which means that the A-type header cannot be connected to the B-type header through the switching device, and the B-type header cannot be switched to the C-type header. However, since the A-type connector and the C-type connector are only different in physical dimensions, they can be compatible through switching equipment.

Due to the adoption of the same TMDS transmission mechanism as DVI, HDMI is very compatible with DVI interface. At present, there are many HDMI-DVI adapter products on the market, which are very suitable for old devices without HDMI. The HDMI-DVI adapter is essentially a physical conversion tool between two connectors, which only involves the shape, size and pin definition of the interface, and the circuit part has not changed. Compatibility with DVI devices is also considered in the HDMI standard: as long as the HDMI device detects that the signal sent by the other party does not contain special control data (VSDB signal, which is specially used to confirm the mutual identity between two devices), it will regard the other party as a DVI device and switch the transmission specification to DVI format, thus ensuring good compatibility.

[Edit this paragraph ]HDMI audio function analysis

Analysis of audio function of 1 High Definition Multimedia Interface

Before HDMI appeared, the transmission of digital audio signals mainly depended on two ways: digital coaxial cable with standard RCA interface and SPDIF optical fiber transmission. From the quality and characteristics of transmission, both of them have their own advantages, but they can both achieve the purpose of transmitting digital audio signals well.

Coaxial cable has a relatively perfect shielding layer, and the copper core in the middle is the channel for signal transmission.

Using 75 ohm coaxial cable to transmit digital audio signals is a very mature and high-quality way. This interface standard does not require much hardware, but it is easy to cause great attenuation when transmitting high-frequency signals, which will affect the final sound quality.

Compared with coaxial transmission, optical fiber has strict requirements on the synchronization time of receiving end and transmitting end of equipment, which is technically more difficult to realize than coaxial transmission. The advantages of optical fiber technology in long-distance transmission are very obvious, and there will be no problem of excessive attenuation of coaxial cable in long distance, so it has also been favored by many users with distance restrictions and new decoration.

In addition, whether optical fiber transmission or coaxial cable transmission is used, it is necessary to purchase a separate connecting wire, which means an increase in the use cost and complexity for users.

HDMI technology combines the advantages of the above two: in the physical layer, mature cable connection is used instead of optical fiber connection with strict synchronization timing. Secondly, in theory, HDMI can realize lossless digital audio signal transmission of up to 20 meters, which is also acceptable for users with distance requirements. Finally, the combination of video cable and audio cable effectively reduces the purchase cost of users, and can also make the equipment side slim, while reducing the production cost of manufacturers.

HDMI interface has become a necessary interface for a new generation of AV power amplifier.

Judging from the types of audio formats supported by HDMI, its main positioning is still home theater applications. In PC field, using HDMI to output audio signals needs more cooperation from video card and sound card manufacturers, so the most mainstream digital audio format in AV field will be the goal of HDMI. In the version of HDMI 1.0, support for Dolby Digital 5. 1 and DTS, the two most widely used digital multi-channel audio streams, has been added. At the same time, with the popularity of digital TV, digital sound function will also be widely used in HDMI. This means that all users of DVD, HD video and digital TV in the world will be potential users of HDMI standard.