在本文中, 我们将讨论CMOS传感器最新发展的优势-“全局快门”-以及全局快门成像仪如何大大减少或消除LED显示伪影.
我们大多数人在用大LED显示屏拍摄视频时都看到过缓慢移动的暗带或细线. The LED screen could be the focus of the shot or just in the background, 但不管怎样, these unwanted artifacts detract from the visual experience and vary in size, 数量, 强度, and movement depending on multiple factors. Most of these artifacts are caused by the type of imagers used in the cameras. Modern video and still cameras use CMOS sensors to capture images, and most have been “rolling shutter” CMOS imagers. 在本文中, 我们将讨论CMOS传感器最新发展的优势-“全局快门”-以及全局快门成像仪如何大大减少或消除LED显示伪影.
The Evolution of the Solid State Imager
The original solid state imager was the Charge Coupled Device (CCD), which was developed back in the early 1970s. CMOS sensors are digital devices and were developed in the late 1990s. Today, all solid state imagers use electronic shutters to control their exposure time. CCD和CMOS成像仪的主要区别在于CCD是模拟的而不是数字的，并且是(有效的)全局快门设备. All CMOS imagers were rolling shutter devices until recently. Rolling shutter CMOS sensors provide good performance with low noise, and they also cost less to manufacture, which helps keep camera costs down.
Manufacturing costs for CMOS global shutter imagers are higher, 因为在每个像素处有更多的电路来存储和处理图像的读出. 大多数全球快门高清制作或广播摄像机也是全逐行扫描(1080p60)。. But if you’re shooting in an LED environment, the benefits of global shutter are generally worth the extra cost.
Why are Global Shutter Cameras Needed Now?
The use of LED displays and LED lighting can create temporal artifacts, which are commonly perceived as flickering. This is particularly the case with the increasing use of lower-cost LED displays, screen processors and lighting. LEDs are controlled by image refresh frequency, along with Pulse Width Modulation (PWM) frequency for brightness. LED屏幕刷新率和PWM的相对频率与相机的快门频率相比会导致可见的伪影，如闪烁, rolling dark bands or thin lines. 它们也会引起“歪斜”——图像弯曲或弯曲，这是由快速移动的物体或在滚动快门成像仪的采集时间内快速移动的相机引起的.
人类的视觉系统有一种强大而独特的能力，叫做“视觉持久性”,这使得所有这些人工制品在我们直接看到时都显得正常. 然而, 卷帘式相机在获取图像的过程中会“看到”这些LED闪烁伪影.
Rolling Shutter Image Acquisition
滚动快门成像仪依次获取视频帧的每一行中的像素. This usually occurs over the frame rate of 59.94 Hz. 在此期间, LED显示屏刷新频率和PWM改变了图像的亮度，如下图中绿色脉冲所示. Some lines are displaced in time and will have different brightness from other lines. The image’s line readout takes place at the end of each line. 当所有顺序采集的线路作为单个视频帧传送到电视或监视器显示器时, this shows as bands of varying brightness. This “banding” may occur when shooting with LED video displays, LED lighting, or both.
Global Shutter – Image Acquisition
在这种情况下, the term “global” means “all-inclusive,，指的是全局快门CMOS成像仪在瞬间获得整个视频帧的事实. The entire frame is stored in the imager’s memory buffer for sequential readout, and the individual line readouts then proceed over the frame time, 通常59.94 Hz. 成像仪的快门速度和LED刷新率(绿色脉冲)之间的任何时间差都可以通过调整相机的快门来补偿，使其成为LED刷新率的偶数倍. This eliminates moving horizontal banding or lines in the picture.
帮助您进一步了解卷帘式快门和全局快门成像之间的区别, please see the animated video at www.hitachikokusai.com/ShutterAnimation.
波纹
畸变通常与成像仪的快门类型无关，在视频相机和静止相机中都会出现. When shooting any pixel-based display or LED video wall, moiré is when odd stripes and patterns appear in your images. 当拍摄对象上的精细图案与相机中成像传感器的像素图案相冲突时，就会发生这种情况, causing viewers to see a third, 单独的模式.
There are several ways to control moiré. 最简单的可能是通过稍微放大或缩小来改变相对模式来重新构图. 您还可以通过在背景中重新定位屏幕来降低捕获的LED显示屏的图像清晰度. Opening the iris has a similar effect by reducing the depth of field, putting the LED screen slightly out of focus behind the subject. 你甚至可以在镜头前使用扩散滤镜来略微降低相机分辨率.
LED Screens and Video Display Processors
When selecting LED displays, screen processors and cameras, 一定要彻底测试选定的LED显示屏和屏幕处理器与相机，正是你打算使用它们, before you purchase anything.
Inexpensive LED displays are like most everything else: you get what you pay for. High quality LED screens offer better DC stability, better matching of the individual LEDs, and the capability of higher refresh and PWM frequencies. Higher frequencies reduce flicker.
Like the LED screens themselves, 采用高品质LED视频处理器，具有genlock和可调刷新率，可以完全消除闪烁, banding and other artifacts when using a global shutter CMOS camera. 这是通过调整LED屏幕刷新率和相机成像仪快门速度来实现的，这样它们就可以以彼此的偶数倍来锁定.
CMOS Global Shutter Cameras from Hitachi Kokusai
日立Kokusai提供两款具有全高清逐行扫描(1080p60)的全球快门相机，在保持公司标志性的成本效益和性价比的同时，提供了这些优势:Z-HD5500和新的SK-HD1800. 两者都有可选择的HDR配置文件，而SK-HD1800也有一个远程电动滤光轮. 所有日立渐进扫描高清摄像机都可以通过SMPTE光纤和/或Triax连接到高度灵活的CU-HD1300摄像机控制单元(CCU)系列. 选择CU-HD1300型号可以从连接的高清摄像机输出上转换的4K/UHD图像，并同时进行HDR和SDR调整. SMPTE ST 2110 Media over IP output is also available.