Video amplifier, what does video amplifier mean

Video amplifier, what does video amplifier mean

Video Amplifier Overview

　　Video amplifier is used to amplify video signal to enhance the brightness, chroma and synchronization signal of video.
　　When the video transmission distance is long, it is better to use a video line with thicker wire diameter. At the same time, a video amplifier can be added in the line to enhance the signal strength to achieve the purpose of long-distance transmission. Video amplifier can enhance the brightness, chroma and synchronization signal of video, but the interference signal in the line will also be amplified. In addition, too many video amplifiers cannot be connected in series in the loop, otherwise saturation will occur, resulting in image distortion.
Important matching equipment
　　1. Video distributor
　　The video signal output by the video matrix switch may be sent to terminal devices such as monitors, recorders, transmission devices, hard copy imaging, etc. to complete the display and recording functions of imaging. Here, it is often encountered that the same video signal needs to be sent to several different places at the same time. When the number is two, it is completed by using a conversion plug or a two-way output device equipped on some terminal devices; but when the number is large, because the parallel video signal attenuation is large, after being sent to multiple output devices, due to impedance mismatch and other reasons, the image will be seriously distorted and the line will be unstable. A video distributor is needed to realize the function of one video input and multiple video outputs, so that the video output can be observed without distortion or loss of clarity. Usually, in addition to providing multiple independent video outputs, the video distributor also has the function of video signal amplification, so it is also called a video distribution amplifier. The
　　video distribution amplifier provides 4 to 6 independent 75Ω load capacity with independent and isolated complementary transistors or independent video amplifier integrated circuits, including color compatibility and a wide frequency response range (10 Hz ~ 7 MHz), and the video input and output are both BNC terminals.
　　2. Video monitor
　　The monitor is the standard output of the monitoring system. Only with a monitor can we watch the image sent from the front end. Monitors are divided into color and black and white, with sizes of 9, 10, 12, 14, 15, 17, 21, 29 inches, etc., and 14 inches is commonly used. Monitors also have resolution, which is expressed in lines like cameras. In actual use, it is generally required that the number of lines of the monitor should match that of the camera. In addition, some monitors also have audio input, S-video input, RGB component input, etc. Except for the audio input monitoring system, most of the other functions are used for image processing, which will not be introduced here. Definition: Color monitors are generally 300-500 lines, black and white monitors are generally 700-1000 lines. The difference between professional monitors and ordinary TVs is: first, the TV definition is higher; second, the anti-magnetic performance is good, so that there will be no mutual interference when installed side by side, while ordinary TVs do not have anti-magnetic function; third, the reliability is good, the monitor can accept long-term uninterrupted work, while ordinary TVs cannot. In the smallest system, there may be only a single monitor, while in the largest system, there may be a TV wall composed of dozens of monitors. The monitor can be black and white, but more often it is a color monitor. It can be a small screen monitor of 6 inches or 9 inches, or a large monitor of about 40 inches, a plasma flat panel display or a projection of hundreds of inches. In practical applications, either a professional-grade pure monitor can be used, or a cheap color TV can be used instead. In terms of image display quality, there are monitors with standard resolution, and there are also high-resolution monitors that pursue high image quality. From the perspective of use, practicality is the premise for making a choice, especially in the following points:
　　(1) The choice of monitor type should basically match the type of front-end camera. Black and white cameras generally have the characteristics of higher resolution and are relatively cheap. In a system composed mainly of black and white cameras, black and white monitors are preferably used.
　　(2) For occasions that require not only clear viewing but also color requirements, with the large-scale use of color CCD cameras, the video image must be displayed using a color monitor, but the resolution of the color monitor should be moderate at this time, and 350 to 400 lines is a more ideal standard.
　　(3) High-end CRT color monitors with a resolution of 600 to 800 lines generally have a refresh rate of 75 to 80 frames per second and are only suitable for use in situations where image quality requirements are extremely high.
　　(4) In addition to resolution indicators, the current fashion is that monitors have easy control and adjustment functions.
　　(5) Monitors have different scanning formats, and attention should be paid when selecting.
　　(6) For closed-circuit television monitoring systems, especially under conditions where funding is not abundant, the use of relatively cheap color TVs is one of the feasible compromises, but they must have video input terminals.
　　(7) The selection of monitor screen size should be based on the principle of matching the video image. Monitors used to display images output by multi-screen splitters should use large-screen monitors because there are multiple cameras outputting images on one screen.
　　III. Long-time video recorders
　　The so-called long-time video recorders refer to a 180-minute videotape that can record more than 8 hours of monitoring images. There are 24-hour and long-time types. Most of them record images intermittently in a time-division manner. The longest recording time can be up to 960 hours. This is called a time-lapse long-time video recorder. In addition, there are real-time long-duration video recorders that record 24-hour images continuously.
　　Long-duration video recorders are magnetic recording devices that record camera signals on magnetic tapes. Their characteristic is that they can record camera signals for up to 24 hours or even 960 hours on ordinary 180-minute tapes, which greatly saves tapes and facilitates management. The magnetic head of a long-duration video recorder is alternately stopped and started, which means that a certain amount of image time is lost in exchange for a long delay effect, so the image played back will have a significant effect.
　　Key parameters
　　TV horizontal definition: Generally, a VHS-mode video recorder can reach about 250 lines, and a SVHS-mode video recorder can reach about 400 lines.
　　Classification of long-time video recorders:
　　24-hour video recorders 480-hour video recorders 960-hour video recorders
　　According to the format, they are divided into VHS mode and S-VHS mode.
　　Unlike home video recorders, time-lapse video recorders can work for a long time and can record images for 24 hours (using ordinary VHS video tapes) or even hundreds of hours. They can be connected to alarm equipment and automatically start recording when receiving alarm signals. They can superimpose time and date, program the automatic recording program of the video recorder, select the recording speed, whether the video tape will automatically stop or rewind after reaching the beginning of the tape... Although the performance of time-lapse video recorders is outstanding, they are expensive, and the current resolution is not very high. Part of the image will be lost during time-lapse recording, and the playback image will jump from one stop to another.
　　4. Digital image recording device
　　As a full-function host that stores video with a digital hard disk, it has video compression, digital hard disk storage and video decompression functions, which can completely record the high-definition images of the camera, solve the low-definition loss in the back-end link of the closed-circuit television monitoring system, and make the video playback reach extremely high definition.
　　6. Video image printing output device
　　In addition to color inkjet printers and color laser printers, color video output devices also include color video printers. Color video printers are image output devices that mostly use thermal dye transfer imaging methods, with high resolution and good color reproduction. Color video printers are intelligent devices that can be operated by function menus as long as they are connected to a monitor.
How to couple a video amplifier (picture)
　　What is the difference between AC coupling and DC coupling in a video high-speed amplifier?
　　AC coupling simplifies circuit design by eliminating DC voltages on the transmission line and isolating the ground points of the transmit and receive systems through the use of input and output capacitors. However, these capacitors also compromise signal quality, which some systems can tolerate while others cannot.
　　To minimize this loss of signal quality, the output capacitance must be on the order of hundreds of microfarads. Since DC coupling can eliminate the output capacitor, it is particularly attractive in price-sensitive high-volume products. However, it is not attractive in situations where the input signal has positive and negative swings (think of the negative video sync pulse) because this complication requires an additional negative supply to ensure a common input voltage range that can accommodate the signal's positive and negative swings.
　　How to determine the output capacitor size?
　　Because the amplifier must drive a relatively low-impedance transmission line, the output capacitor COUT cannot be as small as CIN. Passing a 50 or 60Hz signal requires a large output capacitor greater than 100μF. 100μF is a balance between physical size and cost. In Figure 1, the transmission line is back-matched so that the effective resistance of the output capacitor at the input is 150Ω instead of 5kΩ. Although this is in parallel with the feedback network, the effect is negligible due to the relatively large feedback resistor, and a low-impedance output capacitor of 220μF (f0 = 9.6Hz) or higher is required to pass the 60Hz vertical sync signal. The
　　input/output capacitor values ​​must be calculated based on the requirement to pass the low-frequency components of the signal. (Figure 1)
　　Further frequency compensation for sag has the benefit of reducing the required output capacitor size at the expense of additional components. DC coupling can eliminate the output capacitor completely. (Figure 2)
　　What is sag? Does eliminating sag also reduce the size of the output capacitor?
　　Sag occurs when charge accumulates across the output coupling capacitor. In a field test signal or a bar graph test signal, sag appears as a slant in the video waveform during the scan, and on a TV monitor, sag appears as a brightness change from one side of the picture to the other or from top to bottom. For low-end products, this effect can be ignored, but it is definitely unacceptable in professional and broadcast equipment. To eliminate the sag, a smaller output capacitor can be used, but this comes at the expense of additional circuit components. Correction of the sag is based on low-frequency compensation of the high-pass filter formed by the back-end matched transmission line and output capacitor. The compensation circuit is shown in Figure 2. The product data sheet generally provides the best component values. In a typical case, RFB can be 10 kΩ, RIN and RSAG can be 1 kΩ, COUT can be 47μF, and CSAG can be 22μF.
　　If an amplifier that can handle the common input voltage is used, what effect will it have on DC coupling?
　　Many output drivers without or with internal gain adjustment resistors can control the signal for positive and negative swings. The disadvantage is that a ±5-V power supply is required. The additional complexity is acceptable in professional high-end equipment, but it is not attractive for designs facing cost and board space constraints. Fortunately, the latest amplifiers have an internal charge pump that can generate VEE up to -1.6 V, so that a 4.9-V common input voltage can be obtained using a single 3.3-V supply.
　　What DC-coupled video amplifier products are available on the market?
　　Until now, DC-coupled video amplifiers have been in a small range of applications. In mid-2005, two chip suppliers introduced DC-coupled video amplifiers based on integrated charge pumps. One of the products packaged one amplifier, while the other packaged three amplifiers. Both products have a gain of 6 dB per channel, and a 6-pole Butterworth filter is integrated on the chip to reconstruct the signal from the digital-to-analog converter at the video input. The two suppliers have slightly different ways of dealing with charge pump noise, but both have achieved excellent results. The single-amplifier chip uses a charge pump and linear regulator, while the three-amplifier chip uses a multi-stage charge pump. The
development and challenges of video amplifiers
　　Requirements for lower power consumption, smaller packages, and good matching
　　In recent years, with the continuous expansion of markets such as mobile phones and LCD TVs, users' requirements for video amplifiers have changed. Lower power consumption, smaller packages, and good matching performance have become very important. Companies such as Intersilicon, Linear Technology, Texas Instruments and Fairchild Semiconductor have introduced new amplifiers to meet the needs of drivers and buffers in video applications.
　　Linear Technology's LT6553 amplifier for high-performance video has a resolution of more than 1600x1200 pixels. The LT6553 is suitable for SXGA and UXGA LCD projectors and monitors, digital displays (presenters), scanners, automotive display systems such as car navigation and in-car video systems, digital cameras and CCD imaging systems. For simple multiplexing and signal routing, the LT6553 has an off/on function and can start within 50ns, suitable for spread spectrum and portable applications.
　　Intersilicon has introduced the EL536x series of new operational amplifiers to meet the requirements of high-resolution display interfaces. The EL536x series of three-electro-feedback amplifiers meet the requirements of high bandwidth and low noise, and most importantly, the pulse response is controlled. These new devices consume less power than combining three separate amplifiers together, and the monolithic architecture can also achieve better gain matching. Texas Instruments (TI)'s OPA693 video driver solution provides good pulse response for emerging high-resolution RGB applications. The OPA693 is the fastest +/-5V fixed gain amplifier on the market today, with significantly improved DC and AC accuracy. Its advantages are pulse fidelity under high pixel conditions provided by up to 700MHz bandwidth and 2500 V/us conversion harmony, and its price is lower than that of similar products.
　　Fairchild Semiconductor's FMS6418A is a highly integrated triple-output video driver that can be used for high-definition (HD) or standard resolution (SD) signal filtering to eliminate high-frequency noise before digitizing the signal (anti-aliasing processing), or to eliminate artifacts caused by the D/A conversion process in the encoder (reconstruction filtering). This product is aimed at leading video applications such as HDTV displays, cable and satellite set-top boxes, DVDs, PVRs, video on demand, audio/video receivers, etc., with higher performance and flexibility, and reduced component count. The FMS6418A has three 6-generation sequential filters, and the frequency can be 30 MHz (HD) or 8 MHz (SD). The FMS6418A combines a 2:1 multiplexer on each filter channel, with filtering and output amplification for all three channels. This offers cost and design advantages over discrete solutions.
　　Different Applications Have Different Needs
　　Despite the wide variety of products from manufacturers, different end products require different amplifier performance. The most demanding application is LCD projectors, where small, fragile, bright, high-resolution displays are required at a good price. This requires video amplifiers with higher bandwidth and slew rate, low power, small package size and good channel separation. Soule also pointed out that another challenge is getting video through long cables, such as old CAT5 cables. Linear Technology's new LT6552 is a differential video amplifier that solves this problem, allowing video to be transmitted over 1,000 feet of twisted pair.
　　When designing a video system for any application, the performance of the video amplifier is critical. One application is broadband video routers and switches, which have bandwidths of 100 to 500 MHz. To support such high bandwidths, the end product requires an internal op amp with extremely high bandwidth, even 10 times the required bandwidth. The bandwidth figures advertised by manufacturers can be faked, and the important bandwidth figure to focus on is the large signal bandwidth (LSBW) specification, which is usually stated as 1 volt peak-to-peak (Vpp) or higher. Many manufacturers advertise small signal bandwidth (SSBW), but generally speaking, video signals use large signals (2Vpp). Customers may be tempted by the small signal specification, but the higher LSBW number is a better value.
　　Recognizing the market demand for LSBW, National Semiconductor recently launched a three-bandwidth, 750 MHz operational amplifier LMH6738 and a three-bandwidth, 750 MHz programmable gain buffer (PGB) LMH6739, which have 400 MHz LSBW, 3300 V/us conversion rate and 200 MHz 0.1dB gain flatness, which outperforms similar products in driving high-resolution RGB video signals. An important feature of video amplifiers is 0.1dB gain flatness. Generally, the higher the gain flatness, the higher the supported video resolution. For example, HDTV systems require 30~50 MHz 0.1dB gain flatness. On the other hand, when facing high-resolution video signals (such as computer graphics), the required gain flatness is close to 100~200MHz 0.1dB. At
　　the same time, when using video amplifiers for design, you also need to pay attention to some issues, such as signal distortion, etc. This is very important to minimize the distortion brought to the video signal by the video amplifier. Most customers using professional-grade equipment will consider second and third harmonic distortion, and -80 to -90 dBc @ 5MHz is the minimum requirement for their systems. When faced with composite video signals, the differential gain/differential phase (DG/DP) figures are very important, and are generally required to be 0.02%/0.02, or lower. In composite video, color information is above brightness information, so the lowest DF/DP figures will produce the clearest and least distorted image quality.
　　For portable video devices such as digital cameras and cell phones that can play TV, the most important thing is high integration, low power shutdown, small size packaging and easy connection to the embedded video D/A converter on the DSP chip. Good video display is important, but the video requirements of consumer products are not as stringent as those in the broadcast and professional video fields. For this application, voltage feedback is an extremely important configuration. Important features of this type of amplifier include: integrated filters to help rebuild DAC signals, low power shutdown (preferably less than 5uA), low voltage (preferably running on a single power supply of 2.8-5V), SAG correction, ability to perform DC coupled output without affecting the sync pulse, and use of SC70, QFN or CSP packages.
　　Future market demand trends
　　There are many trends in the video field, one of the more important trends is higher and higher resolution. As the video resolution of portable products increases from VGA to SXGA and higher levels, the speed of operational amplifiers will also increase, and operational amplifiers must also improve their bandwidth and distortion performance to meet the requirements of high-resolution video. The overall demand trends in the market are generally the same: for example, the demand for lower voltage in portable products means that the voltage of video amplifiers is also lower, and more rail-to-rail amplifiers are needed; smaller packages are used, while more on-chip functions are integrated, such as gain resistors, and on-chip feedback paths are used to improve signal integrity and reduce crosstalk at higher bandwidths. Low power consumption; faster speeds to meet higher screen resolutions, such as SXGA, UXGA and HDTV formats; and finally, lower power consumption, such as the LT6210/1 current feedback amplifier from Linear Technology, which features an 'Rset' resistor that allows system designers to adjust the bandwidth, which is actually a speed-power knob. This means that system designers can set the amplifier so that it never consumes too much power at a given signal bandwidth.