Multimedia architecture -- Introduction to display

Preface ：

This article blog from LCD principle 、Display Basic knowledge of 、DSI Introduce 、HDMI relevant 、display Principles are related to concepts 、 An Zhuozhong Display Graphics frame 、Qcom display Frame these points to introduce Display.

One 、Display Overview

 1. Show application areas （mobile,PC）

2. Other related fields （ Projector 、VR glasses ）

Two 、LCD Display principle

1. Backlight IC Give off a bright light , Through the lower polarizer , Liquid crystal array , Through the upper polarizer , The human eye receives the light source , That is, you will see the displayed image .

2. Pixel point (RGB)+ Backlight = Display module

3.RGB :

   RGB Color mode is the most widely used color standard at present , It's through red (R)、 green (G)、 blue (B) The changes of the three color channels and their superposition to get a variety of colors

   RGB565=  Every pixels By 2 Byte composition ,R->5bit,G->6bit,B->5bit;

  

4.RGB Color space is through red (R)、 green (G)、 blue (B) The change of the three color channels and their mutual superposition result in pixels .

The image is composed of points , This point is called a pixel . One LCD How many pixels are there , Is the resolution ; Such as 720P,1080P

Many pixels make up the image content we see ;

3、 ... and 、Display Basic knowledge of

1. Pixels （pixel）

The image consists of points , This point is called a pixel . One LCD How many pixels are there , It's resolution （resolution）. for example ：1080P、720P etc. .

2. A deep （bit-depth）

position ”( bit ) It is the smallest unit in the system memory , It is used to record the value of each pixel color . The richer the color of the image ,“ position ” The more . The number of bits used by each pixel in the system is “ Bit depth ”.

Nowadays, most mobile phones use 24bit Of panel, in other words , Can be displayed 2^(8x3) Color ( As shown in the previous figure ).

3. The interface specification

–DBI: That's what they usually say MCU excuse , Be commonly called I 80 system Interface , Transfer control commands and data through parallel interface ,LCM Module comes with GRAM Update the data to refresh the screen ( It is mainly used in low-end instrument display ).   

– DPI : That is to say RGB Parallel interface , Use common synchronization 、 The clock 、 Signal line to transmit RGB data , use SPI Wait for the control line to complete the command control .

 

–DSI: It is also the most commonly used interface on mobile phones , It is a serial transmission mode , Including data 、 Instructions 、 Other information ( At present, the mainstream manual interface ).

Four 、DSI Introduce  

1.MIPI-DSI Interface profile ：

  •MIPI：Mobile Industry Processor Interface

  •DSI： Display Serial Interface

  •CSI： Camer Serial Interface

  •PHY：Physical Layer

  •HSHigh ：Speed

  Fps：Frames per second

2.DSI There are four layers , Corresponding D-PHY、DSI、DCS standard 、 The hierarchical structure is as follows ：

  • PHY Defines the transmission medium , Input / Output circuit and clock and signal mechanism .

  • LaneManagement layer ： Send and collect data streams to each lane.

  • LowLevel Protocol layer ： It defines how to frame, analyze and detect errors .

  • Application layer ： Describe high-level coding and parsing data streams .

3.DSI（display serial interface）

5.Command and Video Pattern

• DSI Compatible peripherals （ Display device ） Support Command or Video Operation mode , Which mode to use depends on the architecture of the peripheral

• Command Mode refers to sending commands and data to the controller with display cache . The host indirectly controls peripherals through commands .Command The mode adopts two-way interface （data lane0）.

• Video Mode refers to the flow of real pixels when transmitted from the host to the peripherals . This mode can only be transmitted at high speed . To reduce complexity and save costs , Use only Video A schema system may have only one one-way data path .

5、 ... and 、HDMI relevant

1.High Definition Multimedia Interface-- High definition multimedia interface is a kind of digital video interface / Audio Interface Technology , It is a special digital interface suitable for image transmission , It can transmit audio and video signals at the same time , The data transmission speed is 5Gbps;

2. application ： Mobile HD video , TV ,DVD, HD camera , Digital camera ;

3. characteristic ：HDMI Not only can you satisfy 1080P The resolution of the , Can also support DVD Audio And other digital audio formats , Support eight channels 96kHz Or stereo 192kHz Digital audio transmission ;

4. Same as DVI comparison ： The interface is simple , High data transmission rate ; high resolution , Hot plug support ; Differential transmission , Support file encoding HDCP Data copyright protection ;

5.HDMI Transmission principle ：

     

6、 ... and 、Display principle Conceptual relevance

1. Frame buffer driver

 Linux Working in protected mode , So user mode processes can't be like DOS Using a graphics card like that BIOS The interrupt call provided in is used to write the screen directly ,Linux Abstract out FrameBuffer This device is used for user mode process to write screen directly .

Framebuffer The mechanism imitates the function of the graphics card , Abstract the hardware structure of graphics card , Can pass Framebuffer Read and write directly to the video memory operation .

Framebuffer It does not have any ability to calculate data , It is like a pool for temporary storage and drainage .CPU Put the result of the operation into the pool , The pool then streams the results to the display . The data will not be processed in the middle . 

2.DMA

APP Put the plotted display data into DDR Internal video memory area （framebuffer）,LCD The controller maps the video memory data to DMA Data area , start-up LCD Inside DMA function ,DMA Will automatically and continuously transfer the data in the video memory to LCD Peripheral interface display to complete data stream transmission .

7、 ... and 、Android in Display  Graphics frame

1.Display principle ：Software Architecture：

2.SurfaceFlinger The service is responsible for drawing the composition Android Of UI

•APP be responsible for draw layer,SF be responsible for buffer Apply and manage

        • Draw composite

•

•OpenGLES Applied to embedded OpenGL, Rendering API

•HWComposer Yes Layer To synthesize

8、 ... and 、Qcom display frame

1.Android frameworks – Surface texture, SurfaceFlinger


2.QTI HALs – Overlay, graphics allocation, and hardware composer


3.Primary panel interfaces – MIPI DSI


4.External display – WFD


5.Snapdragon Display Manager (SDM) core – SDE drivers, pipe, configuration, and clocks/power/performance


6.Postprocessing and color management – CABL, sharpening, color, conversion, and so on.

7.Qcom Display frame

•SourceSurface Processor (ViG, RGB, DMA - SSPP)  – Formatconversion and quality improvement for source surfaces (video, graphics,etc.)

•LayerMixer (LM)  – Blendand mix source surface together

•DestinationSurface Processor (DSPP)  – Conversion,correction, and adjustment based on panel characteristics

•Write-Back/Rotation(WB)  – Writeback to memory –Performrotation if required

•Displayinterface –  Timing generator and interfaceconnecting the display peripheral