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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.

Audio Inputs

Most video cards accept embedded HDMI or HD-SDI audio. The easiest way to embed audio into a video workflow is by connecting an audio source (soundboard XLR or microphone) to your video camera, but some video capture cards also accept external mic or line-level audio capture. Similar to the USB video capture cards I will discuss below, there are also external audio capture cards that support RCA, 1/4", and XLR audio inputs.

Audio is important, so don’t forget to consider this part of your workflow.

Connection Type

After determining which input(s) you require, you can determine what connection types your workflow will support. PCIe is the standard for internal capture cards on desktop systems, and is the most common type of capture card.

The biggest limitation you will have is the number of PCIe lanes your motherboard has available, so if you’re building a new system, you will want to reserve your PCIe lanes for your graphics card and video capture card(s), instead of features that you could otherwise get on-board if you were to select a different motherboard. Wasting a PCIe lane on additional Ethernet and USB 3.x connections could be a costly mistake if it means you don’t have room for an additional video capture card.

If you’re using a laptop or a Mac that doesn’t have PCIe connections, then you have two additional connection options: USB and Thunderbolt. USB 3.0 is the faster version of USB 2.0 and is denoted by a blue connector. USB 3.1 is even faster and is normally connected using a USB-C connector. Thunderbolt 1 and 2 have the same DisplayPort connector (Figure 4, below), while Thunderbolt 3 can also support Thunderbolt 3 with a USB-C connector (Figure 5, below Figure 4). Just make sure, when you look at external capture cards in both USB and Thunderbolt, that your system supports that version or above and that you have the right cables and adapters as required.

Figure 4. The AJA Io 4K for Thunderbolt 2 supports 2160/30P over HDMI or Quad-link SDI.

Figure 5. Connecting a Thunderbolt 3 computer with a USB-C connection requires a USB-C-to-Thunderbolt 2 adapter.

Number of Cards and Inputs

If you are planning on using a video switcher and webcasting the program feed, you need only a single video capture card with a single input that matches the output from your video switcher. Otherwise, you will need a capture card input for each video camera you need to connect to your software video switcher, such as Telestream Wirecast or vMix.

There are PCIe and USB video capture cards that support from one to four HD inputs, and PCIe cards that support up to eight HD inputs, but I have not seen any 4K capture cards that accept more than one input. You can add multiple capture cards to your system, but you have to make sure you have enough PCIe channels and lanes available and a fast enough computer and graphics card to process your multiple video inputs.

If you run out of PCIe lanes (or don’t have any to begin with), then you can add additional inputs via USB and Thunderbolt. In general, if you have multiple USB capture devices, then you will want to connect them to different USB busses so that you don’t run into a bottleneck. If you run out of available connections on a laptop system, one workaround is to connect a second laptop to your video switching software using the NDI protocol. This way, you can install additional video capture cards in the second computer and pass the video signal over Ethernet.

Capture Card Hardware

Processing several uncompressed video inputs can be very taxing on your CPU and GPU. Some video capture cards have on-board hardware that can pre-process your video signals so that this burden is removed from your CPU and GPU. This is especially important when you need to scale, deinterlace, or change the color space of your video signal.

Some peer-to-peer and webinar services require a webcam signal and cannot accept an uncompressed video signal. You can use USB capture cards to connect professional HDMI and HD-SDI signals and convert them to the Universal Video Codec (UVC) that webcams use. The devices can both scale and adjust frame rate automatically depending on available computing power and internet speed.

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