汽车应用需要快速模拟视频切换
ADI公司 Fionn Hurley
尽管家庭娱乐已转向数字视频，但模拟视频在汽车行业等应用中仍有不小的市场。模拟视频是一种成熟、可靠的技术，经过拓展后可以满足汽车应用的要求。继续使用模拟视频的原因包括：很容易获得低成本电路设计、可以使用低廉的铜线缆，以及在嘈杂的环境中模拟视频性能的下降比较平缓。现在的轿车、越野车和卡车具备许多改进的特性，如环视摄像头、消费娱乐单元和导航系统等，所有这些都会产生模拟视频内容。一台包含这些特性的典型车辆如图1所示。
Despite the move to digital video for home entertainment, analog video still has a significant presence in applications such as the automotive industry. Analog video is a mature, proven technology which can be extended to meet the requirements of automotive applications. The reason for the continued use of analog video includes easy access to low cost circuit designs, use of inexpensive copper cabling, and analog video degrades “gracefully” in noisy environments. Cars, SUVs and trucks are now available with improved features such as surround view video cameras, consumer entertainment units, and navigation systems; all producing analog video content. A typical vehicle containing these features is shown in Figure 1.
由于对“车载视频”的需求不断增长，汽车信息娱乐应用中的模拟视频快速切换已成为一项关键标准。消费者希望能在眨眼之间（200 ms或更短）就无缝切换到不同的模拟视频源。当出于安全原因必须瞬间切换到后视摄像头等视频源时，快速切换特性尤其重要。在保持高视频质量的同时,进行快速切换的要求提升了车载显示单元所用视频解码器的性能标准。相比之下，多数视频解码器产品是针对电视市场而设计，其关键性能标准是高视频质量和对非标准、不良时基视频信号（如弱RF和VCR源）的鲁棒性。这些视频解码器必须维持恒定的输出时序，即使输入时基已被破坏。这类应用的锁定和同步要求正好与视频源之间快速切换的要求相反。本文探讨视频设计人员实现快速切换系统所面临的挑战，并提出切实可行的解决方案。
Due to the growing demand for this “video in car”, the fast switching of analog video in automotive infotainment applications becomes a key criteria. Consumers expect to switch seemlessly between analog video sources in the blink of an eye (200 msec or less). This is especially significant when switching to video sources such as rear view cameras, where it is critical to view an image instantaneously for safety reasons. The requirement for fast switching, while maintaining high quality video, “raises the bar” for the performance of video decoders used in a vehicle display unit. In comparison, most video decoder products were designed for the TV market where the key performance criteria were to provide high video quality and robustness to nonstandard and poor timebase video signals (such as weak RF and VCR sources). These video decoders were designed to maintain constant output timing, even when the input timebase was corrupted. The locking and synchronization requirements for these applications is the inverse to the requirement for fast switching between video sources. This article outlines the challenges facing video designers to implement systems with fast switching and proposes realistic solutions.
图1 典型车载视频系统
快速切换的挑战
Fast switching challenges
针对非标准和不良时基源颇具挑战性的要求，业界在视频解码器的开发方面进行了大量研究和设计工作。本文讨论的快速切换应用提出了新的挑战。然而，当我们设想一种能够同时适用于电视与汽车应用的解决方案时，我们发现这几乎是一项不可能完成的任务。这两种信号类型互相排斥，能够更好地处理一种信号类型的设计技术往往会削弱对另一种信号类型的处理，反之亦然。ADI公司视频解码器设计人员经过长时间的努力，终于在保持兼容性和图像质量的同时实现了快速切换性能。
Much research and design have been invested in developing video decoders to meet the challenging demands of non-standard and poor time base sources. The fast switching applications discussed here present new challenges. However when we try to envisage a solution that can be applicable to both TV and automotive applications, it becomes a near impossible task. The design techniques that will strengthen dealing with one signal type would weaken it with the other and vice versa. Analog Devices video decoder designers have worked a long time to achieve fast switching performance without compromising compatibility and image quality.
TV video decoders are designed to “flywheel” through interruptions on the input
当输入视频流出现中断时，电视视频解码器像飞轮一样依靠“惯性”度过中断期，然后在需要时重新产生标称同步信号。大部分解码器算法会忽略输入时序的中断，如图2所示。为了实现能够容忍视频流中断而不影响图像质量的视频解码器设计，需要克服为数众多的设计挑战。
video stream and, where necessary, to regenerate the nominal synchronization signals. Most decoder algorithms ignore interruptions on the input timing, as shown in Figure 2. The design challenges to implement video decoder designs that can tolerate interruptions of the video stream without impacting image quality are numerous.
图2 典型TV视频解码器应用
在汽车应用中，为了实现模拟视频的快速切换，必须确保视频源维持恒定且正确的时基。同步锁定视频源代价高昂，会增加线缆成本、电路板面积和控制处理要求，而所有这些都是汽车应用设计的关键考虑因素。高性能视频解码器原则上必须尽可能降低系统总成本。图3为需要快速切换的典型汽车应用的示意图。
The fast switching of analog video in an automotive application is based on the assurance that the video sources for these applications will maintain a constant and correct timebase. Genlocking (synchronizing) of the video sources is expensive in terms of the added cabling cost, board area and control processing requirements, which are all key considerations when designing for the automotive industry. High performance video decoders ideally have to minimize the overall system costs. Figure 3 provides a representation of a typical automotive application where fast switching is required.
图3 汽车视频解码器应用
选择正确的视频解码器
Selecting the correct Video decoder
针对快速切换应用选择视频解码器时，常常难以从数据手册的技术规格部分确定解码器的快速切换性能。自动检测切换速度、色彩锁定时间和垂直锁定时间等都是解码器快速切换性能的指标，但并不能反映全部情况。系统设计人员应确保所选的解码器能够针对快速切换应用做进一步优化。需要关注的一些关键产品特性包括：能够禁用Hsync（水平同步）和Vsync（垂直同步）处理模块；能够对强制应用标准进行控制，或者至少能够减少允许自动检测的输入标准数量；减少用于确定锁定和解锁状态的Hsync计数的控制选项；能够强制再获取模拟输入电路的时序；能够在解码器锁定或解锁时产生中断。
When choosing a video decoder for a fast switching appication it is often difficult to determine the decoders fast switching performance from the specifications in the datasheet. Autodetection switch speed, color lock-in time and vertical lock time are all indicators of the decoders fast switching performance but do not tell the full story. System designers should ensure that the decoder they choose can be further optimised for fast switching. Some of the key product features a system designer should look for are:
- Ability to disable Hsync and Vsync processing blocks
- Control to force the standard or, at least, reduce the number of input standards allowed to be auto detected
- Controls to reduce the number of Hsync counts for determining the lock and unlock status
- Ability to force the timing reacquire of analog input circuitry
- Ability to generate interrupt when decoder locked or unlocked
禁用Hsync和Vsync处理模块
Disabling Hsync and Vsync processing blocks
Hsync和Vsync处理模块支持高性能解码器锁定时基不良的信号。如果施加于解码器的输入源具有良好的时基，则所选的解码器应提供控制选项来禁用这些模块，以便缩短解码器的锁定时间。
The Hsync and Vsync processing blocks allow high performance decoders to lock to signals with poor time bases. If the input sources applied to the decoders have good time bases, then the chosen decoder should have the controls to disable these blocks, thus reducing the lock time of the decoder.
强制应用标准
Forcing standard
如果在设计期间已知道输入源标准，则应通过解码器设置锁定该标准，由于解码器不需要花时间执行自动检测算法来确定输入视频标准，这样便能缩短锁定时间。
If the input source standard is known during the time of design, the decoder should be programmed to lock to this standard, thus reducing the lock time as the decoder does not require time to use the autodetection algorithm to determine the input video standard.
减少Hsync计数
Reducing number of Hsync counts
针对快速切换应用选择解码器时，系统设计人员应确保解码器具有编程能力，允许用户减少视频解码器必须检测到的连续正确的Hsync脉冲数量（目的是确定它已锁定视频信号）。还应降低失锁计数(COL)限值，这样视频解码器只需通过更少的连续Hsync脉冲丢失数，就能判断它已失锁，进而搜索新的输入视频类型。
When choosing the decoder for a fast swtiching application the system designer should ensure that the decoder has programmabilty which allows the user to reduce the number of consecutive and correct Hsync pulses that must be detected by the video decoder to determine that it is locked to the video signal. The Count out of Lock (COL) limit should also be reduced, so the video decoder requires fewer consecutive Hsync pulses to be absent from the input video signal before the decoder determines that it is out of lock and goes searching for a new input video type.
强制再获取时序
Forcing timing reacquire
对于快速切换解码器的选择，强制复位视频解码器前端模拟电路的能力也很重要，其目的是重新初始化用于锁定视频信号的模块。通过强制再获取时序，前端模块复位并搜索视频信号，从而确保迅速锁定新的视频源。
The ability to force a reset of the front-end analog circuitry in the video decoder which reintialises the blocks used in acquiring a lock to the video signal is also key in a fast switching decoder choice. By forcing a timing reacquire, the front end block resets and “hunts” for a video signal, thus ensuring the speedy locking to a new video source.
在解码器锁定或解锁时产生中断
Generating Interrupt when decoder locked or unlocked
所选的视频解码器应具有一个专用中断引脚，当解码器锁定或失锁时，它能够发出提示，以便系统控制器响应这些事件，而不需要通过I2C持续监控解码器。
The chosen video decoder should have a dedicated Interrupt pin which can flag when the decoder becomes locked or unlocked. This enables the system controller to react to these events without the need to be constantly monitoring the decoder via I2C.
对输出视频的影响
Effects on output video
由于视频源并未同步锁定，因此无法从一路输入视频无缝切换到另一路视频。当解码器从一路输入的时序切换到另一路输入的视频时序时，输出视频流会受影响。如图4所示，切换期间的图像发生损坏。在异步视频源之间切换时，无法避免这一现象。
As the video sources are not genlocked, it is not possible to switch seamlessly from one input video to another. The output video stream is affected while the decoder switches from the timing on one input to the video timing on another input. As can be seen in Figure 4, the image becomes corrupted during the switching time. It is not possible to prevent this when switching between asynchronous video sources.
图4 输出视频切换时的截图
不同的系统采用不同的解决方案来防止屏幕上显示的视频图像发生撕裂。系统控制器应当能够对解码器产生的中断快速做出反应，控制切换期间屏幕上显示的图像，确保所显示的图像不含图4所示的撕裂现象。
Different systems implement different solutions to prevent the tearing of the video image being displayed on the screen. The system controller should have the ability to quickly react to the interrupt generated by the decoder and control the image displayed on the video screen during this switching time ensuring that the image displayed does not contain any of the tearing artifacts as shown in Figure 4.
切换期间的事件序列
Sequence of events during switching
本部分说明切换到新输入视频信号时发生的事件序列。这将有助于系统设计人员定量评估所选视频解码器的快速切换性能。
This section describes the sequence of events which occur when switching to a new input video signal. This will aid a system designer in quantifying the fast switching performance of their chosen video decoder.
图5 输入和输出视频信号波形
图5为获取视频信号之后发生的一系列事件标上了编号。下面按顺序说明这些事件：
The sequence of events followed when acquiring a video signal is numbered in Figure 5. These numbered events are described below.
1、对解码器执行I2C写操作。设置解码器，切换到新的视频输入，并告知视频标准。加快切换的写操作在此时执行。
1) I2C writes to decoder.
The decoder is programmed to switch into a new video input and is informed of the video standard. Writes to speed up the switching time are done at this point.
2、模拟箝位电路响应视频输入。在典型应用中，视频信号容性耦合到视频解码器中。然后，解码器必须确保对视频信号进行直流恢复，并箝位视频消隐电平，使得ADC输出一个特定的代码。有多种方法可以做到这一点，最常用的方法是在解码器的输入节点提供源电流和吸电流。图6为视频解码器箝位视频信号的典型示意图。
2) Analog clamps react to video input.
In a typical application, the video signal is capacitively coupled into the video decoder. The decoder must then ensure that the video signal is DC restored and the video blanking level is clamped so that the ADC outputs a specific code. There are several methods for achieving this; the sinking and sourcing of current onto the input node on the decoder is the most common. Figure 6 shows the typical clamping of a video signal by a video decoder.
图6 视频信号典型箝位
3、视频固定在正确的电平并且提取同步信号。高性能视频解码器产品同时包含粗调和精调箝位环路。粗调箝位电路将视频信号调整到接近正确电压之后，精调箝位电路对视频信号电平进行全面优化，并使视频输入保持在这一正确的直流电平。现在，输入视频固定在稳定的直流电平，消隐电平则设定为已知的ADC代码。解码器从视频信号中提取同步信号，并监控Hsync、Vsync和Field序列以便确定输入视频标准。如果使用自动检测功能，则确定视频标准的时间会延长。
3) Video fixed to correct voltage level and sync signal is extracted.
In high performance, video decoder products contain both coarse and fine clamp loops. Once the coarse clamps force the video signal to approximately the correct voltage, the fine clamps optimize fully the video signal level and maintain the video input at this correct DC level. The input video is now fixed at a stable DC level where the blank level is set to a known ADC code. The decoder extracts the sync signal from the video signal and monitors the Hsync, Vsync and Field sequence to determine the input video standard. If autodetection is used, the time to determine the video standard is increased.
4、解码器输出正确、稳定的视频时序。解码器锁定输入视频信号，针对该输入视频类型优化内部IP模块和滤波器，并开始输出稳定且正确的视频时序，从而在LCD面板或屏幕上显示正确的视频图像。
4) Decoder outputs correct stable video timing.
The decoder locks to the input video signal, optimizes internal IP blocks and filters for this input video type and starts ouputting stable and correct video timing. This results in the displaying of a correct video image on the LCD panel or screen.
上述结果全部基于利用ADI公司解码器（如ADV7180和ADV7181C）执行的测量，这些解码器符合快速切换要求。
All of the above results are based on measurements carried out using ADI decoders such as the ADV7180 and ADV7181C which have been designed to meet fast switching requirements.
ADI公司解码器广泛应用于汽车行业，满足许多汽车系统供应商的快速切换需求。ADV7180和ADV7181C具备本文所述的所有快速切换特性。
ADI decoders are used extensively throughout the automotive industry to meet the fast switching needs of many automotive system suppliers. The ADV7180 and ADV7181C contain all the fast switching features described in this article.
总结
Summary
本文说明了模拟视频在汽车应用中仍然受欢迎的原因，以及模拟视频相对于其它解决方案的优势。汽车制造商已经看到，功能丰富的信息娱乐系统有助于提高销售利润、增强安全性能。随着“车载视频”系统的快速发展，不久的将来很可能会出现一辆车配备5块液晶面板的情形。
This article describes the reasons why analog video is still popular for automotive applications and the advantages that analog video maintains over other solutions. Car makers have seen how full-featured infotainment systems help sell their cars at higher margins and improve safety ratings. The trend for expanded “video in car” systems is now moving to where 5 LCD panels per vehicle is a likely near-term scenario.
本文还讨论了模拟视频快速切换的需求以及相关的挑战。同时，还说明应当关注哪些重要特性，帮助系统设计人员正确选择视频解码器。ADV7180和ADV7181C等视频解码器产品已通过汽车应用认证，能够满足严苛的汽车信息娱乐应用快速切换要求。
The need for fast switching of analog video is outlined here along with the challenges associated with implementing fast switching in these applications. The key video decoder features that a system designer should look for are also outlined to aid them in their video decoder component selection process. Automotive qualified video decoders products such as the ADV7180 and ADV7181C are able to meet the demanding fast switching requirements found in automotive infotainment applications.
尽管家庭娱乐已转向数字视频，但模拟视频在汽车行业等应用中仍有不小的市场。模拟视频是一种成熟、可靠的技术，经过拓展后可以满足汽车应用的要求。继续使用模拟视频的原因包括：很容易获得低成本电路设计、可以使用低廉的铜线缆，以及在嘈杂的环境中模拟视频性能的下降比较平缓。现在的轿车、越野车和卡车具备许多改进的特性，如环视摄像头、消费娱乐单元和导航系统等，所有这些都会产生模拟视频内容。一台包含这些特性的典型车辆如图1所示。
Despite the move to digital video for home entertainment, analog video still has a significant presence in applications such as the automotive industry. Analog video is a mature, proven technology which can be extended to meet the requirements of automotive applications. The reason for the continued use of analog video includes easy access to low cost circuit designs, use of inexpensive copper cabling, and analog video degrades “gracefully” in noisy environments. Cars, SUVs and trucks are now available with improved features such as surround view video cameras, consumer entertainment units, and navigation systems; all producing analog video content. A typical vehicle containing these features is shown in Figure 1.
由于对“车载视频”的需求不断增长，汽车信息娱乐应用中的模拟视频快速切换已成为一项关键标准。消费者希望能在眨眼之间（200 ms或更短）就无缝切换到不同的模拟视频源。当出于安全原因必须瞬间切换到后视摄像头等视频源时，快速切换特性尤其重要。在保持高视频质量的同时,进行快速切换的要求提升了车载显示单元所用视频解码器的性能标准。相比之下，多数视频解码器产品是针对电视市场而设计，其关键性能标准是高视频质量和对非标准、不良时基视频信号（如弱RF和VCR源）的鲁棒性。这些视频解码器必须维持恒定的输出时序，即使输入时基已被破坏。这类应用的锁定和同步要求正好与视频源之间快速切换的要求相反。本文探讨视频设计人员实现快速切换系统所面临的挑战，并提出切实可行的解决方案。
Due to the growing demand for this “video in car”, the fast switching of analog video in automotive infotainment applications becomes a key criteria. Consumers expect to switch seemlessly between analog video sources in the blink of an eye (200 msec or less). This is especially significant when switching to video sources such as rear view cameras, where it is critical to view an image instantaneously for safety reasons. The requirement for fast switching, while maintaining high quality video, “raises the bar” for the performance of video decoders used in a vehicle display unit. In comparison, most video decoder products were designed for the TV market where the key performance criteria were to provide high video quality and robustness to nonstandard and poor timebase video signals (such as weak RF and VCR sources). These video decoders were designed to maintain constant output timing, even when the input timebase was corrupted. The locking and synchronization requirements for these applications is the inverse to the requirement for fast switching between video sources. This article outlines the challenges facing video designers to implement systems with fast switching and proposes realistic solutions.
图1 典型车载视频系统
快速切换的挑战
Fast switching challenges
针对非标准和不良时基源颇具挑战性的要求，业界在视频解码器的开发方面进行了大量研究和设计工作。本文讨论的快速切换应用提出了新的挑战。然而，当我们设想一种能够同时适用于电视与汽车应用的解决方案时，我们发现这几乎是一项不可能完成的任务。这两种信号类型互相排斥，能够更好地处理一种信号类型的设计技术往往会削弱对另一种信号类型的处理，反之亦然。ADI公司视频解码器设计人员经过长时间的努力，终于在保持兼容性和图像质量的同时实现了快速切换性能。
Much research and design have been invested in developing video decoders to meet the challenging demands of non-standard and poor time base sources. The fast switching applications discussed here present new challenges. However when we try to envisage a solution that can be applicable to both TV and automotive applications, it becomes a near impossible task. The design techniques that will strengthen dealing with one signal type would weaken it with the other and vice versa. Analog Devices video decoder designers have worked a long time to achieve fast switching performance without compromising compatibility and image quality.
TV video decoders are designed to “flywheel” through interruptions on the input
当输入视频流出现中断时，电视视频解码器像飞轮一样依靠“惯性”度过中断期，然后在需要时重新产生标称同步信号。大部分解码器算法会忽略输入时序的中断，如图2所示。为了实现能够容忍视频流中断而不影响图像质量的视频解码器设计，需要克服为数众多的设计挑战。
video stream and, where necessary, to regenerate the nominal synchronization signals. Most decoder algorithms ignore interruptions on the input timing, as shown in Figure 2. The design challenges to implement video decoder designs that can tolerate interruptions of the video stream without impacting image quality are numerous.
图2 典型TV视频解码器应用
在汽车应用中，为了实现模拟视频的快速切换，必须确保视频源维持恒定且正确的时基。同步锁定视频源代价高昂，会增加线缆成本、电路板面积和控制处理要求，而所有这些都是汽车应用设计的关键考虑因素。高性能视频解码器原则上必须尽可能降低系统总成本。图3为需要快速切换的典型汽车应用的示意图。
The fast switching of analog video in an automotive application is based on the assurance that the video sources for these applications will maintain a constant and correct timebase. Genlocking (synchronizing) of the video sources is expensive in terms of the added cabling cost, board area and control processing requirements, which are all key considerations when designing for the automotive industry. High performance video decoders ideally have to minimize the overall system costs. Figure 3 provides a representation of a typical automotive application where fast switching is required.
图3 汽车视频解码器应用
选择正确的视频解码器
Selecting the correct Video decoder
针对快速切换应用选择视频解码器时，常常难以从数据手册的技术规格部分确定解码器的快速切换性能。自动检测切换速度、色彩锁定时间和垂直锁定时间等都是解码器快速切换性能的指标，但并不能反映全部情况。系统设计人员应确保所选的解码器能够针对快速切换应用做进一步优化。需要关注的一些关键产品特性包括：能够禁用Hsync（水平同步）和Vsync（垂直同步）处理模块；能够对强制应用标准进行控制，或者至少能够减少允许自动检测的输入标准数量；减少用于确定锁定和解锁状态的Hsync计数的控制选项；能够强制再获取模拟输入电路的时序；能够在解码器锁定或解锁时产生中断。
When choosing a video decoder for a fast switching appication it is often difficult to determine the decoders fast switching performance from the specifications in the datasheet. Autodetection switch speed, color lock-in time and vertical lock time are all indicators of the decoders fast switching performance but do not tell the full story. System designers should ensure that the decoder they choose can be further optimised for fast switching. Some of the key product features a system designer should look for are:
- Ability to disable Hsync and Vsync processing blocks
- Control to force the standard or, at least, reduce the number of input standards allowed to be auto detected
- Controls to reduce the number of Hsync counts for determining the lock and unlock status
- Ability to force the timing reacquire of analog input circuitry
- Ability to generate interrupt when decoder locked or unlocked
禁用Hsync和Vsync处理模块
Disabling Hsync and Vsync processing blocks
Hsync和Vsync处理模块支持高性能解码器锁定时基不良的信号。如果施加于解码器的输入源具有良好的时基，则所选的解码器应提供控制选项来禁用这些模块，以便缩短解码器的锁定时间。
The Hsync and Vsync processing blocks allow high performance decoders to lock to signals with poor time bases. If the input sources applied to the decoders have good time bases, then the chosen decoder should have the controls to disable these blocks, thus reducing the lock time of the decoder.
强制应用标准
Forcing standard
如果在设计期间已知道输入源标准，则应通过解码器设置锁定该标准，由于解码器不需要花时间执行自动检测算法来确定输入视频标准，这样便能缩短锁定时间。
If the input source standard is known during the time of design, the decoder should be programmed to lock to this standard, thus reducing the lock time as the decoder does not require time to use the autodetection algorithm to determine the input video standard.
减少Hsync计数
Reducing number of Hsync counts
针对快速切换应用选择解码器时，系统设计人员应确保解码器具有编程能力，允许用户减少视频解码器必须检测到的连续正确的Hsync脉冲数量（目的是确定它已锁定视频信号）。还应降低失锁计数(COL)限值，这样视频解码器只需通过更少的连续Hsync脉冲丢失数，就能判断它已失锁，进而搜索新的输入视频类型。
When choosing the decoder for a fast swtiching application the system designer should ensure that the decoder has programmabilty which allows the user to reduce the number of consecutive and correct Hsync pulses that must be detected by the video decoder to determine that it is locked to the video signal. The Count out of Lock (COL) limit should also be reduced, so the video decoder requires fewer consecutive Hsync pulses to be absent from the input video signal before the decoder determines that it is out of lock and goes searching for a new input video type.
强制再获取时序
Forcing timing reacquire
对于快速切换解码器的选择，强制复位视频解码器前端模拟电路的能力也很重要，其目的是重新初始化用于锁定视频信号的模块。通过强制再获取时序，前端模块复位并搜索视频信号，从而确保迅速锁定新的视频源。
The ability to force a reset of the front-end analog circuitry in the video decoder which reintialises the blocks used in acquiring a lock to the video signal is also key in a fast switching decoder choice. By forcing a timing reacquire, the front end block resets and “hunts” for a video signal, thus ensuring the speedy locking to a new video source.
在解码器锁定或解锁时产生中断
Generating Interrupt when decoder locked or unlocked
所选的视频解码器应具有一个专用中断引脚，当解码器锁定或失锁时，它能够发出提示，以便系统控制器响应这些事件，而不需要通过I2C持续监控解码器。
The chosen video decoder should have a dedicated Interrupt pin which can flag when the decoder becomes locked or unlocked. This enables the system controller to react to these events without the need to be constantly monitoring the decoder via I2C.
对输出视频的影响
Effects on output video
由于视频源并未同步锁定，因此无法从一路输入视频无缝切换到另一路视频。当解码器从一路输入的时序切换到另一路输入的视频时序时，输出视频流会受影响。如图4所示，切换期间的图像发生损坏。在异步视频源之间切换时，无法避免这一现象。
As the video sources are not genlocked, it is not possible to switch seamlessly from one input video to another. The output video stream is affected while the decoder switches from the timing on one input to the video timing on another input. As can be seen in Figure 4, the image becomes corrupted during the switching time. It is not possible to prevent this when switching between asynchronous video sources.
图4 输出视频切换时的截图
不同的系统采用不同的解决方案来防止屏幕上显示的视频图像发生撕裂。系统控制器应当能够对解码器产生的中断快速做出反应，控制切换期间屏幕上显示的图像，确保所显示的图像不含图4所示的撕裂现象。
Different systems implement different solutions to prevent the tearing of the video image being displayed on the screen. The system controller should have the ability to quickly react to the interrupt generated by the decoder and control the image displayed on the video screen during this switching time ensuring that the image displayed does not contain any of the tearing artifacts as shown in Figure 4.
切换期间的事件序列
Sequence of events during switching
本部分说明切换到新输入视频信号时发生的事件序列。这将有助于系统设计人员定量评估所选视频解码器的快速切换性能。
This section describes the sequence of events which occur when switching to a new input video signal. This will aid a system designer in quantifying the fast switching performance of their chosen video decoder.
图5 输入和输出视频信号波形
图5为获取视频信号之后发生的一系列事件标上了编号。下面按顺序说明这些事件：
The sequence of events followed when acquiring a video signal is numbered in Figure 5. These numbered events are described below.
1、对解码器执行I2C写操作。设置解码器，切换到新的视频输入，并告知视频标准。加快切换的写操作在此时执行。
1) I2C writes to decoder.
The decoder is programmed to switch into a new video input and is informed of the video standard. Writes to speed up the switching time are done at this point.
2、模拟箝位电路响应视频输入。在典型应用中，视频信号容性耦合到视频解码器中。然后，解码器必须确保对视频信号进行直流恢复，并箝位视频消隐电平，使得ADC输出一个特定的代码。有多种方法可以做到这一点，最常用的方法是在解码器的输入节点提供源电流和吸电流。图6为视频解码器箝位视频信号的典型示意图。
2) Analog clamps react to video input.
In a typical application, the video signal is capacitively coupled into the video decoder. The decoder must then ensure that the video signal is DC restored and the video blanking level is clamped so that the ADC outputs a specific code. There are several methods for achieving this; the sinking and sourcing of current onto the input node on the decoder is the most common. Figure 6 shows the typical clamping of a video signal by a video decoder.
图6 视频信号典型箝位
3、视频固定在正确的电平并且提取同步信号。高性能视频解码器产品同时包含粗调和精调箝位环路。粗调箝位电路将视频信号调整到接近正确电压之后，精调箝位电路对视频信号电平进行全面优化，并使视频输入保持在这一正确的直流电平。现在，输入视频固定在稳定的直流电平，消隐电平则设定为已知的ADC代码。解码器从视频信号中提取同步信号，并监控Hsync、Vsync和Field序列以便确定输入视频标准。如果使用自动检测功能，则确定视频标准的时间会延长。
3) Video fixed to correct voltage level and sync signal is extracted.
In high performance, video decoder products contain both coarse and fine clamp loops. Once the coarse clamps force the video signal to approximately the correct voltage, the fine clamps optimize fully the video signal level and maintain the video input at this correct DC level. The input video is now fixed at a stable DC level where the blank level is set to a known ADC code. The decoder extracts the sync signal from the video signal and monitors the Hsync, Vsync and Field sequence to determine the input video standard. If autodetection is used, the time to determine the video standard is increased.
4、解码器输出正确、稳定的视频时序。解码器锁定输入视频信号，针对该输入视频类型优化内部IP模块和滤波器，并开始输出稳定且正确的视频时序，从而在LCD面板或屏幕上显示正确的视频图像。
4) Decoder outputs correct stable video timing.
The decoder locks to the input video signal, optimizes internal IP blocks and filters for this input video type and starts ouputting stable and correct video timing. This results in the displaying of a correct video image on the LCD panel or screen.
上述结果全部基于利用ADI公司解码器（如ADV7180和ADV7181C）执行的测量，这些解码器符合快速切换要求。
All of the above results are based on measurements carried out using ADI decoders such as the ADV7180 and ADV7181C which have been designed to meet fast switching requirements.
ADI公司解码器广泛应用于汽车行业，满足许多汽车系统供应商的快速切换需求。ADV7180和ADV7181C具备本文所述的所有快速切换特性。
ADI decoders are used extensively throughout the automotive industry to meet the fast switching needs of many automotive system suppliers. The ADV7180 and ADV7181C contain all the fast switching features described in this article.
总结
Summary
本文说明了模拟视频在汽车应用中仍然受欢迎的原因，以及模拟视频相对于其它解决方案的优势。汽车制造商已经看到，功能丰富的信息娱乐系统有助于提高销售利润、增强安全性能。随着“车载视频”系统的快速发展，不久的将来很可能会出现一辆车配备5块液晶面板的情形。
This article describes the reasons why analog video is still popular for automotive applications and the advantages that analog video maintains over other solutions. Car makers have seen how full-featured infotainment systems help sell their cars at higher margins and improve safety ratings. The trend for expanded “video in car” systems is now moving to where 5 LCD panels per vehicle is a likely near-term scenario.
本文还讨论了模拟视频快速切换的需求以及相关的挑战。同时，还说明应当关注哪些重要特性，帮助系统设计人员正确选择视频解码器。ADV7180和ADV7181C等视频解码器产品已通过汽车应用认证，能够满足严苛的汽车信息娱乐应用快速切换要求。
The need for fast switching of analog video is outlined here along with the challenges associated with implementing fast switching in these applications. The key video decoder features that a system designer should look for are also outlined to aid them in their video decoder component selection process. Automotive qualified video decoders products such as the ADV7180 and ADV7181C are able to meet the demanding fast switching requirements found in automotive infotainment applications.
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