HDMI interface

1. The source of the HDMI interface can be any device that supports HDMI output, and the access end can also be any device with an HDMI input interface. Whether they are audio equipment, video equipment or control equipment, HDMI interface can be used among them. The data signal in the HDMI interface uses the TMDS minimized transmission differential signal protocol. This kind of data transmission protocol has been widely used on DVI interface. The data signal on the HDMI interface also uses this protocol. This protocol converts standard 8-bit data into 10-bit signals, and uses differential transmission during the conversion process. The differential transmission technology has also been widely used in the data transmission of Gigabit Ethernet.

Three TMDS data channels can be used in the transmission of audio and video data in the HDMI interface. Video information is converted into continuous 24bit pixel data during transmission, and 10bit data can be transmitted per clock cycle. The pixel clock cycle transfer ratio is approximately between 25MHz and 165MHz. Generally speaking, the pixel clock transmission ratio of the standard NTSC 480i interlaced signal is about 13.5MHz. If the transmission signal rate is less than 25MHz, HDMI will use automatic loop technology to fill the code rate, increasing the signal rate to 25MHz. The HDMI interface can transmit up to 165M pixels of data per second, which is quite amazing.

2. HDMI interface connector

According to the difference in electrical structure and physical shape, HDMI interfaces can be divided into three types: Type A, Type B, and Type C. Each type of interface is composed of a socket for the device side and a plug on the wire side. It is driven by a low voltage of 5V, and the impedance is 100 ohms. All three plugs can provide reliable TMDS connection.

The A-type socket is a flat "D" shape with a wide top and a narrow bottom. A metal shielding layer with a thickness of 0.5 mm is provided on the outside of the interface to prevent various interference signals from the outside. Among them, the inner diameter of the socket for the device side is 14 mm at its widest point and 4.55 mm high. The 19 pins are arranged in two layers at the center. Compared with the A-type interface, the physical structure of the B-type interface has not changed much, and it is all "D"-type. But the maximum width of the socket end reached 21.3 mm, which is a full circle larger than the 14 mm of the A type. The C-type HDMI interface is designed for compact portable devices, so the size of the C-type socket is only 10.5×2.5 mm, and the plug is only 10.42×2.4 mm. Very small. The C-type interface can transmit 165MHz pixel information every clock cycle.

3. Data channel (TMDS channel), HDMI data channel and DVI data channel both use TMDS signals (minimize the transmission of differential signals). Transmit audio, video, and various auxiliary data through TMDS signals.

The CEC channel, the full text of CEC is Consumer Electronics Control, which is used to transmit industrial-standard AV Link protocol signals to support a single remote control to operate multiple AV machines. It is a single-core bidirectional serial bus.

DDC channel

The full text of DDC is Display Data Channel. The transmitter and receiver can use the DDC channel to learn each other's transmission and reception capabilities, but HDMI only needs to learn about the capabilities of the receiver (display) in one direction. Use I2C signal with 100kHz clock frequency.

SCL and SDA, the full text of SCL is Serial Clock, and the full text of SDA is Serial Data. These two pins are used to communicate between source (DVD) and display (TV). There is a memory inside the TV that stores the resolution supported by this TV, such as 720P or 1080P. If the source (DVD) ) If you don’t know what TV resolution is supported by the currently connected TV, source(DVD) doesn’t know what resolution signal to send. Therefore, at the beginning, the source (DVD) will read the resolution supported by the TV through these two pins, and only when the source (DVD) knows it, the source (DVD) will play the video screen that matches the TV resolution. Among them, SCL is the clock (CLOCK) pin, and SDA is the data pin.