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Choosing a Capture Card for Multicamera Live Production

To choose the best capture card for mixing live events with PC-based software, you should pursue multi-part analysis. In this article, I'll get you up to speed on what features to look for, but you're going to have to carry the load on the critical subjective items.

Scaling and Deinterlacing and Other Adjustments

If you need to save ISO feeds from your incoming sources, you’ll want to capture at full resolution. Otherwise, with both internal and external capture devices, you may want to scale the incoming video to the output resolution of your capture device to improve scaling quality and save CPU cycles on the host computer. Not all products can perform this scaling, making on-board scaling and deinterlacing features worth checking for.

Also check if the capture device provides video adjustment features such as a ProcAmp with scopes, like those available on Osprey video cards. One aspect of streaming live is that you can’t fix it in post, so the ability to adjust video characteristics such as contrast and saturation can be a valuable adjunct to the typical exposure and color adjustments found on most camcorders.

Does it Provide the Required Outputs?

For many streaming producers, the sole output will be a streaming signal to one or more services. However, if you’re broadcasting in an arena or similar venue, you may also need HD-SDI/or HDMI output of the program stream to feed a monitor or IMAG system. Many capture cards are input only, so if you need HD-SDI or HDMI out, make sure it’s supported, either directly, or via a separate add-in card. To be perfectly clear, note that you need the full output of the program stream, not simply a passthrough of one of the input channels, which many cards provide.

Another common requirement is ISO recording, which stores the individual inputs or the program feed to hard disk where it can easily be edited or uploaded to YouTube or other sites. Note that Wirecast and other programs can store a lower-resolution encoded copy of the program stream, but this is different from true ISO recording, which uses hardware processing on a card such as the Matrox VS4 (Figure 3, below) to store broadcast-quality output ready for further processing.

Figure 3. The Matrox VS4 is used in many Wirecast-based turnkey systems because of its performance and ISO recording capabilities.

How Do You Achieve the Required Density?

Keep in mind that density, or the number of supported inputs, has two discrete components: enough ports to physically connect the required inputs, and sufficient CPU horsepower to mix and compress the incoming streams without dropping frames. In this regard, while you may be able to input three 1080p signals into a MacBook Air via its two USB 3.0 ports and single Thunderbolt port, that doesn’t mean that the 1.6 GHz dual-core Intel Core i5 CPU can mix the feeds and produce the required streaming output without dropping frames.

On the other hand, a 15" MacBook Pro with a 2.6GHz quad-core Intel Core i7 CPU just might do the trick, as Imry Halevi, Harvard’s Director of Multiple Production for the Department of Athletics, explains in his blog post High Definition, Three Camera Inputs, One Laptop, $3,500. In his setup, Halevi plugged two Blackmagic Design Mini Recorders ($149) into the available Thunderbolt ports, and a Blackmagic Intensity Shuttle USB 3.0 ( $199) into one of the MacBook Pro’s USB ports, for input.

At the risk of sounding politically incorrect, as Halevi’s post suggests, there are many ways to skin a cat. Though Blackmagic and AJA seem to be the only vendors with Thunderbolt capture dongles, there are multiple suppliers of inexpensive single- or dual-port USB capture, including Magewell, Epiphan, and AverMedia. Those seeking higher-end capabilities are a higher price point could substitute a single AJA Io 4K (Figure 4, below $1,995) to enable four HD-SDI inputs, HDMI input, with superior monitoring and output capabilities as well as 4K support in and out.

Figure 4. Connections available on the AJA Io 4K, a four-input external Thunderbolt capture device

Internal cards are subject to the same CPU caveats. Just because you can install an 8-port board into a computer doesn’t mean the computer will be able to successfully mix the streams. If your system is powerful enough, you can find a variety of single-, dual-, quad-, and eight-input cards, and if you have enough slots, you can install multiple capture boards into a single computer to achieve the required density. Producers who dislike the mini HD-SDI connectors used on most 8-port cards can install two 4-input cards to reach the same number of inputs.

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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.
This article explains what features to look for when choosing a capture card to incorporate into your live switching and streaming workflow.