1. Preface: Establishing the concept of hierarchy
Speaking of timing, the first example we think of is the example of IIC, SPI, serial port and other interfaces. According to our previous understanding, timing is the order in which the level of the transmission line changes with time. But but but! In HDMI, we should establish a new concept: that is, timing does not necessarily correspond to the physical layer: that is, the transmission line. How is this going? Let's see:
Because in the HDMI protocol, the transmitted information is encoded, that is, TMDS encoding. The purpose is to make the level change of the transmission line small, so as to achieve the purpose of reducing interference and enhancing transmission reliability.
Speaking of this, we need to understand the overall structure of HDMI to help us establish a layered concept. Let me divide it into two: the physical layer and the information layer. The physical layer is the physical circuit connection and level fluctuation; the information layer is the data we need to transmit, corresponding to the familiar binary code. The physical structure of HDMI is shown below
For HDMI physical cables, we focus on TMDS-related cables. TMDS Channel0-2 are three data transmission channels, TMDS Clock is a clock channel, and each channel uses 2 cables for signal transmission ( minimize the transmission of differential signals, Transition Minimized Differential signal ), abbreviated as TMDS. The HDMI Transmitter in the picture encodes video, audio, and control signals, and then puts them on the transmission line for transmission.
Video and audio are what I call the information layer. Here we mainly analyze the timing of the video signal, and we can ignore the details of TMDS encoding. TMDS encoding has ready-made IP that can be used, if you are interested, you can learn more later.
Second, the timing of the video signal
First of all, we understand that the video is composed of one frame by one frame, and each frame of the picture is composed of pixels. When the video signal is output, it is scanned for each pixel, starting from the upper left corner, when A horizontal sync signal HSYNC (horizontal sync) is output every time one line is scanned, and then the next line is scanned. When a complete picture is scanned, that is, a vertical sync signal VSYNC (vertical sync) is output when one field is completed. Each pixel is composed of three colors, RGB, and each color has a depth of 8bit, that is, 3*8bit.
There may be a question again, how are so many signals distributed among the 4 TMDS channels? The answer is as follows: CTL0-3 is the control signal
We can ignore it again, just know that the signal we need to send to the TMDS encoder is:
1. Pixel information [7:0]R, [7:0]G, [7:0]B
2. Horizontal synchronization signal HSYNC, vertical synchronization signal VSYNC
3. Pixel Clock
The timing of the video signal can be represented by the following figure
The horizontal sync signal HSYNC of the horizontal axis changes with the scan x position. When HSYNC changes from low to high, it indicates the beginning of a new line (the end of the old line);
·The vertical sync signal VSYNC changes with the scan y position. When VSYNC changes from low to high, it indicates the beginning of a new picture (the end of the old field);
·In the transmission of a picture, not all the time is transmitting the image signal;
·The transmission of image signal is carried out in the time between Hor "Active" Video and Ver "Active" Video;
·The rest of the time when the image is not transmitted is called "blanking time", but the blanking time does not mean that other signals such as control signals and audio signals are not transmitted;
·The line blanking time is composed of Hor Sync Time + H Back Porch + H Front Porch, and the vertical blanking time is the same;
·Speaking hard, how to calculate or stipulate various parameters such as Hor Sync Time, H Back Porch, etc. under common resolutions. I didn’t understand it. I didn’t see it in the HDMI standard manual, but I can pass some Resources to obtain these parameters: http://tinyvga.com/vga-timing
The following are the timing parameters corresponding to the common video specifications given by the black gold materials
