Video display system is analyzed in detail

As consumers, we are all familiar with video systems in various forms.But from the perspective of an embedded developer, video looks like a tangled web of different resolutions, formats, standards, and displays.

Video display:

Simulate video display

Video encoder

Video encoders can convert digital video streams into analog video signals.The input of these video encoders is generally itu-rbt.656 or bt.601 format YcbCr or RGB video stream, and then the input signal is converted according to various output standards (such as NTSC, PAL, SECAM).A master processor can control the encoder through a serial interface, such as SPI or I2C. For example, parameters such as pixel timing, input/output format and brightness/chromaticity filtering can be set.As shown below, the common block diagram of encoder structure.Video encoders usually use one or more of the following analog output formats.

Video encoder block diagram for example

CVBS -- this abbreviation represents Composite Video Baseband Signal (or Composite Video Blanking and Syncs), which conforms to Video Baseband Signal (or Composite Video Blanking and synchronization).Compound video is connected by the ubiquitous yellow RCA jack shown in figure (a) below.He integrated information on brightness, chromaticity, synchronization and color pulses into a single cable.

Common analog video connectors

S Video -- connected using the connector shown in (b) above, can transmit information on brightness and chromaticity, respectively.Separating brightness information from chromatic aberration signals can greatly improve image quality, which is why S Video connection is popular in today's home theater systems.

ComponentVideo -- ComponentVideo, also known as YPbPr, this is the analog version of YcbCr digital video.In this video format, brightness and chromaticity channels are transmitted separately, and each channel has its own time sequence.This can ensure that the analog transmission image to achieve the best quality.Such component connections are common in high-end home theater systems, such as DVD players and A/V receivers, as shown in figure (c).

In analog RGB formats, red, green, and blue signals have separate channels.This can provide image quality similar to the component video, but it is usually used in the computer graphics neighborhood as shown in figure (d), while the component video is mainly used in the consumer electronics neighborhood.

Cathode ray tube (CRT)

In the display neighborhood, RGB is the most commonly used interface for computer displays and LCD displays.The oldest computer monitors produce images by connecting three separate pins to an analog video signal from a PC graphics card and adjusting three separate electron guns accordingly.Depending on which beam activates the dot on the screen, the dot will appear red, green, blue, or some combination of those colors.This is different from analog television, which USES a composite signal with all the color information superimposed on the same input and only one electron beam being modulated.Newer computer monitors use DVI (Digital Visual Interface) to accept both Digital and analog RGB signals.

The main advantage of CRT displays is that they are very low cost and can produce more colors than LCD displays of the same size.Also, unlike LCD displays, CRT displays can be viewed from any Angle.CRT monitors, however, have their downsides, such as being bulky, bulky, emitting large amounts of electromagnetic radiation and causing eye strain due to blinking.

Digital video display

Liquid crystal display panel (LCD)

There are two main categories of LCD technologies: passive matrix and active matrix.Passive arrays (common types include the STN, fully called Super Twisted NemaTIc, or Super Twisted NemaTIc, and its derived category), consist of a printed lead structure on a glass substrate, a printed lead structure on another glass substrate, and then a "liquid crystal sandwich" structure.These intersections of rows and columns are pixels.Therefore, in order to activate a pixel, the timing circuit needs to power the column in which the pixel is located and ground the row in which the pixel is located.Thus, a voltage difference at the pixel causes the liquid crystal at the corresponding location to change, making the point opaque and preventing light from passing through.

Passive array technology is simple, but it has some disadvantages.For example, the screen refresh time is relatively long (this can cause "dragging" of fast-moving images).In addition, the voltage at the intersection of the rows could leak to nearby pixels, making the liquid crystal in the nearby pixel area a little opaque and blocking the passage of light.For viewers, the images are blurred and the contrast is reduced.Also, the viewing Angle is relatively small.

Compared with passive array, active array LCD technology has made great progress in these aspects.In active array LCD technology, each pixel consists of a capacitor and a transistor switch.This gave it a more popular name, "thin film transistor liquid crystal displays" (tft-lcd).To position a pixel.First enable the row in which the pixel is located, and then apply a voltage to the column in which the pixel is located.This creates the effect of isolating only the pixels of interest, while leaving the surrounding pixels untouched.In addition, because the current required to control a particular pixel is reduced, the pixel switch is also faster, resulting in a higher refresh rate for tft-lcd than for passive arrays.More importantly, many discrete brightness levels can be produced by adjusting the voltage level applied to the pixel.Now, corresponding to the 8-bit intensity information, 256 brightness levels can be reached.

Connecting to a tft-lcd panel is not easy because there are many different components involved.First, there is the LCD panel itself, which contains an array of pixels that can be stroked with rows and columns to reference the clock frequency of the pixels.

The tft-lcd's backlight is often a CCFL (cold cathode fluorescent), which generates very little heat by exciting the gas to glow.The advantages of the CCFL are: durability, long life and very simple and direct drive requirements.LED is also a popular way to generate backlight, mainly for small and medium-sized LCD panels, its advantages include low cost, low voltage, long life, good brightness control characteristics.However, for a larger panel size, the LED backlight consumes more power than the CCFL.

The LCD controller contains most of the circuits needed to convert the input video signal into the format required for LCD display.Usually, this part of the circuit contains a timing generator, which is used to control the timing signal and clock signal of each individual pixel on the LCD panel.However, to meet LCD panel size and cost requirements, sometimes these timing circuits need to be externally supplied.In addition to the standard synchronization and data lines, timing signals are required to drive the individual rows and columns of the LCD panel.Sometimes, universal PWM timers in media processors can be used instead of these individual chips to reduce system costs.

The LCD control chip also has some other features, such as OSD (on-screen display) support, image overlap and blending, color lookup tables, color mixing, and image rotation.Some of the more sophisticated LCD control chips can also be very expensive, often exceeding the cost of the processors they are connected to.

An LCD driver chip is needed to provide the appropriate voltage for the LCD panel.The driver chip ACTS as a "converter" between the LCD controller output and the LCD panel.The rows and columns are generally driven independently and the timing signal is controlled by the timing generator.Liquid crystals must be driven by periodic polarity reversal signals, as the prevailing current puts pressure on the crystal structure and eventually degrades the crystal.Therefore, the polarity of the voltage applied to each pixel must vary on each frame, line, or pixel, depending on how it is implemented.

Media devices are becoming smaller and cheaper, a trend that has led to the integration of these LCD system components.Now, an integrated tft-lcd module with a timing generator and driver circuit is available, with only a data bus, clock/sync signal line, and power supply.In addition, some liquid crystal panels do not use parallel digital input, but the complex analog video input.

OLED (organic light-emitting diode) displays the "organic" in organic light-emitting diodes for materials sandwiched between two electrodes.When an electric charge passes through the organic material, the organic material glows.This display technology is still very new, it is expected to improve some LCD display problems.For example, it's a self-luminous technology that doesn't require backlighting.This means that the power, cost and weight of the display panel can be greatly reduced - OLED panels can be made very thin and light.In addition, OLED displays support more colors than LCD displays, and their moving images are better than LCD displays.What's more, oleds support wide viewing angles and have high contrast.OLED has an electronic signal and data interface similar to tft-lcd.

Despite these advantages, by far the biggest problem with oleds is that the organic material in them can break down after a few thousand hours of use, although in some displays that number has now risen to more than 10,000 hours -- perfect for many portable multimedia applications.In some areas, OLED has a promising market, such as mobile phones, digital cameras and similar products.But it's also very likely to see TV or computer displays based on OLED technology.