Using Thunderbolt Capture Cards for Multi-cam Live Productions

It used to be that external capture devices were for single- or perhaps dual-camera events, and internal boards for serious productions. That changed when Apple introduced Thunderbolt on its notebooks, and introduced the new Mac Pro without accessible slots for capture devices. 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 combines a 4x PCI Express Slot with a mini DisplayPort adapter so Thunderbolt can drive a monitor and input from a capture device. Copper Thunderbolt cables can be up to three meters long, while optical cables can be up to 60 meters long, both pushing a theoretical limit of up to 20 Gbps (gigabits per second). 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, you can purchase an external Thunderbolt expansion chassis like 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? Because it opens up a whole new range of capture boards that you can consider. This is important, because while there are multiple single-channel Thunderbolt capture devices available, 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, though it adds significant cost ($899 direct) and some bulk to the system.

The bottom line is that if you’re buying for mixing on a current-model Mac, you have two choices: an external capture device or devices (USB 3.0 or Thunderbolt), or a Thunderbolt expansion chassis. On Windows desktops, you can use either an internal or external product. Some Windows notebooks have Thunderbolt connectors, enabling the same options as Macs. Others use an ExpressCard slot that can support an expansion chassis such as the Echo Express, though this is a limited-bandwidth option that likely could only support a single HD source.

When considering a Thunderbolt capture device, keep several product-related details in mind. First, there are multiple standards, Thunderbolt 1 and Thunderbolt 2, with Thunderbolt 3 coming. 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 combines those channels, so you can transfer up to 20 Gbps in either direction. A Thunderbolt 2 peripheral should work in a Thunderbolt 1 port, but at a maximum speeds of 10 Gbps.

Second, these transfer speeds are theoretical maximums for combined DisplayPort and PCIe throughput. 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, it’s extremely unlikely that this will impact your ability to input four HD streams, which should be about 1 Gbps each in the 8-bit YUV format used by Wirecast and other video mixers.

Finally, there are two concepts to understand when planning for Thunderbolt-based capture and mixing: controllers and ports. The controller is the chip that connects the CPU or Platform Control Hub (PCH) to the Thunderbolt ports. 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, when both are fully utilized they will max out at half the total bandwidth for that controller.

These concepts are critical for understanding both your system’s capabilities and how to configure Thunderbolt and other devices on that system. 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.

If you’re configuring multiple high-bandwidth devices that operate simultaneously, such as the AJA io 4K capture device and a 4K DisplayPort monitor discussed in this article, make sure they don’t share a common controller.