The structure and principle of HDMI

HDMI, which stands for High-Definition Multimedia Interface, is used to transmit high-definition audio and video signals. The transmission of HDMI is based on the TMDS encoding technology.

• HDMI contains three channels: TMDS channel, DDC (Display Data Channel), and CEC (Consumer Electronics Control) channel. When transmitting pixels, the TMDS channel allows each pixel to have a depth of 24-bit, 36-bit, or 48-bit, and supports pixel encoding formats such as RGB4:4:4, YCbCr 4:4:4, and YCbCr 4:2:2. DDC is the display data channel, used to transmit EDID information. EDID is a VESA standard data format that contains parameters about the display's performance, including image size, color depth, frequency range, etc. The sender and receiver can obtain each other's processing capabilities by obtaining the EDID information on the DDC channel. The CEC channel is a channel defined in the HDMI standard for control signal transmission between devices, and its function is to send AV Link signals.
• HDMI uses TMDS for data transmission. When transmitting video data, the received video data can be 24-bit, 36-bit, or 48-bit. The TMDS clock rate is considered the default transmission rate and is equal to the pixel clock rate. Each TMDS clock transmits 10-bit data, and the audio data and auxiliary data are packaged and processed before transmission. 
• In TMDS transmission, there are two parts: the sender and the receiver. The HDMI interface transmits 24-bit parallel data representing RGB pixel signals to the TMDS sender, encodes and performs parallel-to-serial conversion on these data using TMDS encoding technology, obtaining three data representing R, G, and B respectively, and sends R, G, and B to the receiver through the TMDS channel. After these serial data reach the TMDS receiver, the receiver decodes and performs serial-to-parallel conversion, transmits the obtained parallel data to the display control end, and receives the clock signal to achieve picture synchronization.

HDMI is classified according to link cycle mode into three types: video data cycle, data island cycle, and control cycle. Among them, audio signals and auxiliary information are transmitted during the data island cycle. In HDMI, the header (CTLx) at the end of the control cycle is used to indicate the next cycle mode.