回顾:Magewell USB Capture HDMI 4K Plus

Over the last ten years, I have owned a lot of video capture cards for my various webcast systems. I have built many mobile webcast workstations in computer cases and most recently in road-rack cases. These builds all have had PCIe capture cards or external hardware connected to a PCIe card. I had been very happy with these solutions, but they aren’t as portable as a laptop-based solution.

三年前，我买了我的第一个USB采集卡，一个Magewell USB 3.0 HDMI capture dongle, and paired with my old laptop, I had a truly portable webcast solution. Over the next two months I bought an SDI version and then a second HDMI version. These USB capture devices were fantastic for when I produced a one- or two-camera webcast without a video switcher or when I wanted to record or stream the program output from a video switcher. They also saw a lot of action on my mobile workstation solutions to add additional inputs or when I needed a device that was able to convert the video resolution and frame rate to something that was compatible with where the video was going. This was especially important when I used to produce webinars on Flash-based solutions that were limited to the video standard of 480p20, 这不是现代任何东西都支持的视频标准.

The beauty of a USB capture card is that the capture card hardware does all the heavy processing and it outputs over USB 3.0是通用视频编解码器兼容的视频信号. 换句话说, you don’t need to install any drivers with this protocol and your computer sees the signal as a webcam, 这意味着它也适用于网络研讨会等服务, 视频会议, 视频聊天, 不接受传统的PCIe视频采集卡.

捕捉4 k

我最近的USB捕获卡是Magewell USB捕获HDMI 4K Plus (下面的图1). It is a perfect companion to my Sony FS5 and X70 4K video cameras because these cameras output UHD 4K only over HDMI, 而3G-SDI输出仅限于高清分辨率.

图1. Magewell USB Capture HDMI 4K Plus

如果你使用的是支持6G-HD-SDI的4K摄像机, 那么你会想要考虑Magewell USB Capture SDI 4K Plus. In addition to taking a 2160p30 HDMI or SDI signal and converting it to a UVC signal over USB cable, 这两种型号都有一个方便的HDMI或SDI环路直通(下面的图2).

图2. HDMI and audio I/O USB Capture HDMI 4K Plus (left) and USB Capture SDI 4K Plus (right)

I use 4K workflows both when I want to record and webcast in 4K and when I want to create several HD virtual inputs from a single 4K video feed. Having a 4K camera set to wide allows me to have multiple virtual HD cameras with up to 2-3x lossless zoom each. This works because UHD video has 2x the vertical and horizontal resolution of 1080 HD video and 3x the resolution of 720 HD video. I can even use my recording software to zoom in or out and pan on the 4K image in pre-programmed movements.

注意这一点很重要, 开箱即用, 这些设备设置为支持YUY2和NV12捕获格式. 其他格式也可用, 但你必须通过Magewell USB捕获实用程序启用它们. 我一直在这个地方工作, but I have to admit that these terms were new to me and I had to do a bit of research before I could understand the differences between the two formats and confidently know which one to use when.

当你使用4K信号时, you will want to be especially careful that your device is outputting the NV12 format. YUY2使用4:2:2的色彩空间, 我用的摄像机中常见的高清格式是什么, while NV12 works with a 4:2:0 color space and is the UHD 4K format that my video cameras use. I found out the hard way not to leave my webcast and video recording software set to the “default” video format. 最初, I was very frustrated when I kept dropping frames in my 4K tests but thankfully Magewell support pointed me in the right direction. While I could force my capture card to work with only the NV12 format by using the USB Capture Utility, I decided it best to manually select NV12 within vMix so that when I used the same card with HD video, 我会在合适的色彩空间(下面的图3和图4).

图3. 在vMix中选择NV12视频格式

图4. 没有更多的丢失帧，在vMix统计屏幕报告