Using Thunderbolt Capture Cards for Multi-cam Live Productions

过去，外部捕捉设备用于单摄像头或双摄像头事件, and internal boards for serious productions. That changed when Apple introduced Thunderbolt on its notebooks, 推出了没有捕捉设备插槽的新Mac Pro. Suddenly, if you wanted to produce live events on a relatively new Mac, Thunderbolt was your only option.

This isn’t necessarily a bad thing. Technically, Thunderbolt 2结合了一个4倍PCI Express插槽和一个迷你DisplayPort适配器，因此Thunderbolt可以驱动监视器并从捕获设备输入. Copper Thunderbolt cables can be up to three meters long, while optical cables can be up to 60 meters long, 两者都突破了高达20gbps(每秒千兆比特)的理论极限。. 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.

Interestingly, this gives you two alternatives. First, you can choose an external Thunderbolt capture device, or multiple Thunderbolt capture devices, to support your required inputs. Or, 您可以购买外部Thunderbolt扩展机箱，如Sonnet Technologies Echo Express III-D (Figure 1, below), and install internal PCIe cards in the chassis.

Figure 1. Sonnet Technologies Echo Express III-D

Why go the expansion route? 因为它开辟了一个全新的捕获板的范围，你可以考虑. This is important, 因为虽然有多个单通道Thunderbolt捕获设备可用, there’s only one four-channel external Thunderbolt device, the AJA Io 4K. Though it looks like a fabulous product, an expansion chassis increases your available options, 尽管它增加了显著的成本(899美元)和系统的体积.

最重要的是，如果你是为了在当前型号的Mac上混音而购买, you have two choices: an external capture device or devices (USB 3.0 or Thunderbolt), or a Thunderbolt expansion chassis. 在Windows台式机上，你可以使用内部或外部产品. 一些Windows笔记本电脑有Thunderbolt连接器，支持与mac电脑相同的选项. 其他使用ExpressCard插槽，可以支持扩展机箱，如Echo Express, 虽然这是一个带宽有限的选项，可能只能支持单个高清源.

在考虑Thunderbolt捕获设备时，请牢记几个与产品相关的细节. 首先，有多个标准，Thunderbolt 1和Thunderbolt 2，以及即将到来的Thunderbolt 3. Thunderbolt 1 and 2 are very similar; the key difference is that Thunderbolt 1 has two independent bidirectional 10Gbps channels for a maximum throughput of 10 Gbps in either direction. Thunderbolt 2结合了这些通道，所以你可以在两个方向上传输高达20gbps的数据. Thunderbolt 2外设应该在Thunderbolt 1端口中工作，但最高速度为10gbps.

其次，这些传输速度是结合DisplayPort和PCIe吞吐量的理论最大值. The maximum reserved for PCIe on Thunderbolt 2 is about 11 Gbps, but if you’re displaying video at 4K 60 fps with DisplayPort monitors, on the same Thunderbolt channel, the PCIe allocation drops to about 8 Gbps (see this Intel slide on the AJA site). Even so, 它不太可能影响你输入四个高清流的能力, 在Wirecast和其他视频混频器使用的8位YUV格式中，每个应该大约1gbps.

Finally, 在规划基于thunderbolt的捕获和混合时，需要了解两个概念:控制器和端口. 控制器是连接CPU或PCH (Platform Control Hub)到Thunderbolt端口的芯片. Each controller has the full bandwidth described above, which it divides among the ports attached to the controller. So if you have one controller with two ports, 当两者都被充分利用时，它们将达到该控制器总带宽的一半.

这些概念对于理解系统的功能以及如何配置系统上的Thunderbolt和其他设备至关重要. For example, most MacBook Pros with Thunderbolt have two ports, but only one controller, so the ports share the total bandwidth. In contrast, the Mac Pro has six Thunderbolt ports connected to three controllers, with ports 5 and 6 on one controller, 1 and 3 on another, and 2 and 4 on a third.

如果您正在配置多个同时运行的高带宽设备, 例如本文中讨论的ajio 4K捕获设备和4K DisplayPort监视器, make sure they don’t share a common controller.