如何选择用于实时切换和流媒体的视频采集卡
本文解释了在选择捕获卡以将其合并到您的实时切换和流工作流程中时应该寻找哪些特性.
选择视频采集卡是一个更复杂的决定，你会做出当你把一个实时切换和流媒体工作流程. In this article, 我将解释当您浏览当今市场上广泛选择的捕获卡时应该寻找哪些功能.
Video Inputs
When choosing a capture card, 你需要考虑的两种最常见的视频输入是HDMI和HD-SDI. Whenever possible in my workflows, 我更喜欢使用HD-SDI电缆，因为它们提供安全的BNC卡口连接, introduce less delay in the signal workflow, and don’t interconnect with every other interconnected device, as HDMI does.
HD-SDI can be used to transport HD and 4K signals, 所以一定要选择一个可以接受你打算使用的分辨率和帧率的捕获卡(Figure 1, below). The 1.5G HD-SDI standard is limited to 1080/60i signals, 所以我建议考虑3G HD-SDI作为现代工作流程的最低标准，因为它支持1080/60P信号. 超高清(UHD)/4K需要6G的2160/30P和12G的2160/60P工作流. These two 4K signals are common on Blackmagic video cameras, 但没有得到其他依赖HDMI输出4K的制造商的广泛支持.
Figure 1. Blackmagic DeckLink 4K Extreme是一款可以通过HDMI捕获2160/60P的PCIe卡, HD-SDI, and even optical fiber.
This is the case in my own workflows with Sony FS5 (Figure 2, below) and X70 video cameras. 这些4K超高清摄像机可以在内部录制4K，但HD-SDI输出仅限于3G SDI信号. 要从这些摄像机中获得2160/30P，你必须使用HDMI输出.
Figure 2. 索尼PXW-FS5数字电影摄像机有一个能够输出1080/60P的3G HD-SDI输出和一个能够输出2160/30P的HDMI输出.
我提到我自己的摄像机的限制，提出的问题，如果你不能输出2160/60P信号, then you might not want to invest in a capture card that can, as these tend to be more expensive than 2160/30P-capable cards. 只是因为我用的索尼摄像机只能通过HDMI输出提供UHD/4K, 这可能并不意味着我应该限制自己的4K HDMI捕获卡，如果, somewhere along my workflow, I will be converting this HDMI signal to HD-SDI. 我想将HDMI转换为HD-SDI的原因之一是HDMI仅限于较短的电缆运行. Also, 如果我打算使用硬件视频切换器，如Blackmagic ATEM 4K系列中的一个, then the Program and Aux outputs are both HD-SDI signals, 这些是我用来连接外部录音机和网络广播编码器的输出.
For some workflows, you might still prefer sticking with an HDMI capture card, 但如果你不确定，或者你需要这两种投入来生产一个特定的产品, 那么您将需要考虑具有HDMI和HD-SDI输入的视频采集卡. Beyond HDMI and HD-SDI capture cards, 如果您需要支持传统的模拟格式(如VGA)，则可能需要考虑DVI捕获卡, component, 数码HDMI和DVI标准(Figure 3, below). The DVI-I standard supports both analog and digital signals, meaning that by using a simple passive adapter or breakout cable, you can add support for several additional formats.
Figure 3. The Magewell Capture Plus Family of USB capture cards offers HDMI, HD-SDI, and DVI input cards with internal hardware scaling.
Related Articles
本文将讨论如何组装具有尽可能多功能的4K实时交换网络广播工作流的基本组件，价格低于5美元,000.
确定系统产生和交付实时事件流所需的功能, we tested a range of computers (old and new), capture devices, and multiple live streaming software programs, streaming to one or more streaming services, recording archive/ISO files in various formats, and measuring CPU utilization. Read on for the results.
为混合现场事件和基于pc的软件选择最佳捕获卡, you should pursue multi-part analysis. In this article, I'll get you up to speed on what features to look for, 但是你必须在关键的主观问题上承担责任.
Don't think of Thunderbolt as an external connector like USB; think of it as another PCI Express expansion slot that you can access without opening your computer. You can choose external Thunderbolt capture devices, 也可以购买外接Thunderbolt扩展机箱，在机箱内安装内部PCIe卡.