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How to Choose Video Converters and Scalers for Live 4K and HD Production

Signal conversion is required in most live HD video productions, and this article will discuss how to convert modern and legacy video signals to the most common outputs required in modern HD and 4K broadcasts.

As 4K video workflows move closer to the production mainstream, the progression of video standards from SD to 4K is important to understanding overall signal compatibility, which is critical for live video productions. Any workflow is only as strong as its weakest link. If you have to mix multiple video and computer inputs, and both record an archive and send a live stream video signal, you’re likely to find yourself working with a variety of video and computer resolutions, scan types, frame rates, and connections. When working with different signals, you might struggle to find a common signal. That is where converters and scalers come into play.

I recently started to move my productions to the new 4K UHD video resolution standard. Now I own two cameras that are capable of 4K HDMI outputs via their HDMI ports and two more that can even record 4K internally. When I pair these with my 4K video switcher, I have the start of a professional 4K UHD workflow.

At this writing, none of my clients have asked me to deliver their projects in 4K, but that hasn’t stopped me from filming in 4K. A similar situation arose a few years back when I moved to HD capture at a time when SD delivery was still the norm. My long-term clients benefit from my early HD adoption when I am able to offer them an HD version of a product they’d thought was only in SD, and the additional revenue from these revisited past projects is helping to fund my 4K upgrade investment. Although many viewers don’t have 4K monitors, streaming platforms such as YouTube and Vimeo support 4K today, so I don’t have to wait to start delivering 4K today for those clients for whom I’m creating on-demand content.

Managing Multiple Standards

Of course, my productions didn’t just jump from one SD standard to one HD standard overnight, and there is likewise more than one 4K standard. From 4:3 SD we moved to 16:9 SD (which had the same 720x480 resolution but changed the pixel aspect ratio from 0.9 to 1.2), then to 1440x1080 HDV, 1920x1080 60i HD, 1920x1080 25/30p HD, and finally to 1920x1080 50/60p HD. Sure, higher frame rates are available in HD, but they’re reserved mainly for high-speed acquisition.

UHD and 4K have a similar story, although I’m thankful that we don’t have to work with funny pixel aspect ratios or legacy interlaced video signals in the 4K world. On the acquisition side, DCI 4K is 4096x2160 and UHD is 3840x2160, but outside of productions destined for the big screen, UHD will be the widely adopted delivery resolution, and frame rates will be the standard 24p, 30/60p (NTSC countries), and 25/50p (PAL countries).

Matching Your Gear to Your Deliverables

I prefer cameras for a live production to be capable of putting out a 1080/30p signal for live switching and a 720/30p signal for webcasting. This preference is designed to match my current deliverables, but if I’m also sending a signal to the projector for IMAG, I often need to work with 1080/60p. Right off the bat you can see that having a professional video camera that allows you to select an output resolution and frame rate is critical to starting your workflow with a progressive video signal.

Prosumer and consumer video camera models often lack the ability to select different output signals, and older HD cameras are limited to 1080/60i signals. If I jump down the workflow path, I have to keep in mind that if I’m filming in 60p and want to webcast in 30p, I need to use hardware or software that supports translating video from one frame rate to another. Recently, I changed my webcast software because it lacked this very ability, and 720/60p was a higher frame rate than is practical for a webcast.

Depending on the capabilities of your video switcher, you might not be able to mix and match video signals. This means you’ll need to either use a common video signal or add a converter or scaler into your workflow to make sure your video switcher can see all your video inputs.

HDMI vs. HD-SDI

Let’s talk about video connections for a moment. HDMI and HD-SDI are the two most common HD connectors (Figure 1, below). HDMI is a consumer connection and can easily be pulled out, so as a rule I no longer buy video cameras that don’t have HD-SDI outputs that I can lock in. HDMI also behaves differently from HD-SDI, and so the connection and reconnection aren’t instantaneous. Every signal along the way—especially if you’re using an HDMI distribution amplifier—will cause a signal interruption and reconnect if one of the signals is unplugged.

Figure 1. HDMI and HD-SDI output connectors on the Sony PXW-X70

This problem arises more often that you might suspect, as it’s common to split an HDMI signal to a video switcher with one feed and a monitor and/or an external recorder with addition feeds from a distribution amplifier. There is nothing worse than having to change the battery on your video monitor and knowing that if you do that, because it’s connected to the HDMI chain, it will interrupt your live video signal.

The two most relevant HDMI standards that you need to know about are HDMI 1.4 and 2.0. HDMI 1.4 supports 1080/60p and 2160/24p while HDMI 2.0 is required if you need 2160/30p or 2160/60p. In addition to the HDMI interconnect and non-locking connection, HDMI is also limited to shorter cable runs of 50' as compared to HD-SDI. The 1.5G HD-SDI standard allows cable runs of 370', while 3G HD-SDI supports 250' cable runs. Ultimately, many producers choose to convert their HDMI signals to HD-SDI (Figure 2, below) so they can achieve longer cable runs, and so they can use the loop-through or second output on a signal converter to power their monitors or external recorder with, avoiding the HDMI interconnect.

Figure 2. An HDMI-to-HD-SDI converter from Atomos

HD-SDI is the professional standard that I prefer to work with. 1.5G HD-SDI is limited to 1080/60i video, Dual-Link HD-SDI and 3G HD-SDI support 1080/30p and 1080/60p video, and the soon-to-be ratified 6G and 12G HD-SDI standards are designed for 4K/30p and 4K/60p video signals. Newer HDMI and HD-SDI converters are rated for 4K UHD use and are reverse-compatible with HD signals, so keep this in mind when buying new or replacing older equipment. You might want to buy equipment that is ahead of your current needs so you don’t have to replace your converters when you move from interlaced HD to progressive HD and then again to 4K UHD.

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