How to Choose a Video Capture Card for Live Switching and Streaming
This article explains what features to look for when choosing a capture card to incorporate into your live switching and streaming workflow.
Selecting a video capture card is one of the more complicated decisions you will make when you put together a live switching and streaming workflow. In this article, I am going to explain what features to look for when you navigate the wide selection of capture cards on the market today.
When choosing a capture card, the two most common video inputs you need to consider are HDMI and HD-SDI. Whenever possible in my workflows, I prefer to use HD-SDI cables because they offer a secure BNC bayonet connection, 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, so be sure to select a capture card that can accept the resolution and frame rates that you intend to work with (Figure 1, below). The 1.5G HD-SDI standard is limited to 1080/60i signals, so I recommend considering 3G HD-SDI as the minimum for modern workflows as it supports 1080/60P signals. Ultra HD (UHD)/4K requires 6G for 2160/30P and 12G for 2160/60P workflows. These two 4K signals are common on Blackmagic video cameras, but have not gained widespread support among other manufacturers who rely on HDMI for 4K output.
Figure 1. The Blackmagic DeckLink 4K Extreme is a PCIe card that can capture 2160/60P over HDMI, HD-SDI, and even optical fiber.
This is the case in my own workflows with Sony FS5 (Figure 2, below) and X70 video cameras. These 4K UHD video cameras can record internally in 4K but have HD-SDI outputs that are limited to 3G SDI signals. To get 2160/30P out of these video cameras, you must use the HDMI output.
Figure 2. The Sony PXW-FS5 Digital Motion Picture Camera has a 3G HD-SDI output capable of outputting 1080/60P and an HDMI output capable of outputting 2160/30P.
I mention my own video camera limitations to raise the issue that if yours cannot output a 2160/60P signal, 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. Just because I am filming with Sony video cameras that offer UHD/4K only from the HDMI output, this may not mean that I should limit myself to a 4K HDMI capture card if, somewhere along my workflow, I will be converting this HDMI signal to HD-SDI. One of the reasons that I would want to convert HDMI to HD-SDI is that HDMI is limited to shorter cable runs. Also, if I intend to use a hardware video switcher such as one in the Blackmagic ATEM 4K line, then the Program and Aux outputs are both HD-SDI signals, and these are the outputs I would use to connect to external recorders and webcast encoders.
For some workflows, you might still prefer sticking with an HDMI capture card, but if you’re unsure or if you require both inputs for a particular production, then you will want to consider a video capture card that has both HDMI and HD-SDI inputs. Beyond HDMI and HD-SDI capture cards, you may want to consider DVI capture cards if you need to support legacy analog formats such as VGA, component, and composite video in additional to the digital HDMI and DVI standards (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.
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, or you can purchase an external Thunderbolt expansion chassis and install internal PCIe cards in the chassis.
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