Serial Digital What?

At AbelCine, we’ve embraced the trend toward external video recorders. Convergent Design’s nanoFlash, launched in 2009, was one of the first on-camera HD recorders that actually offered improved recording quality over what was found in the camera. Since then, companies like AJA, Atomos, Cinedeck, Codex, Blackmagic Design and Sound Devices have released their own models. These mini recorders all take advantage of the fact that many cameras can output uncompressed, or even raw, data over SDI or HDMI connections. They record the signals coming out of the camera and offer a variety of compression options that may be preferred for quality reasons or ease of post workflow. These new camera/recorder combinations often raise questions about camera outputs and signal types, which can be surprisingly complicated because there are so many different variations.

Recently, I was contacted by a production company shooting a commercial for a local TV news spot. They wanted to combine the Sony PMW-F3 camera with an external recorder that could capture a 1920×1080 resolution 60p 4:4:4 video signal. Basically, they wanted to record at 60 frames per second with no color reduction (subsampling) from the camera. The Sony F3 can output a 1080 60p signal, and it can output a 4:4:4 (RGB) signal, but it can’t do both at the same time. On top of which, not many recorders can even record that type of signal in the first place. In the process of helping the production company find a camera/recorder combination that would fit their needs and budget, even more questions arose about 4:4:4, RGB, SDI and all those other odd acronyms that keep our heads spinning.

3G? LIKE MY IPHONE?

When combining an external recorder with a camera, it’s important to know your camera’s output. The standard digital output of a professional camera is HD-SDI, or High-Definition Serial Digital Interface; now, HDMI is starting to show up, as well. These are both "uncompressed" outputs (read my article on "Raw, Log and Uncompressed" in the June 2012 issue to see why I put that in quotes), and they both carry video and audio over a single connection. Most cameras offer one or the other, and they both have their advantages.

HD-SDI is very popular for external recording because of its locking connectors (BNC). It’s standardized to include timecode and some limited metadata, as well. A single HD-SDI connection can support a 1920×1080 resolution signal at up to 60 interlaced fields per second in YCbCr 4:2:2 color sampling. I’ll explain more about 4:2:2 in a bit, but note that it can only take a 60i (interlaced) signal or less. 60i has the same bandwidth as 30p (progressive), and 24p has less information, so it will also work over a single cable. A single HD-SDI connection has a data rate of 1.485 Gb/s, which is used up with a 1080 60i 4:2:2 single cable. If you want to send out higher frame rates, say 60, or more color information, say 4:4:4, then a single cable just won’t work. You can begin to see the problem with the F3 question that I received. To get 1080 60p, you need more bandwidth than a single HD-SDI can offer, meaning that any recorder with only one HD-SDI input won’t do the trick. So what about more than one connection?

Dual link and 3G-SDI were created to handle the greater data needs of the latest digital cinema cameras. Dual link is just as it sounds: a combination of two HD-SDI signals to double your available bandwidth. 3G-SDI combines two single links into one connection. Remember that a single cable transfers roughly 1.5 Gb/s; add two of those together, and you get 3 Gb/s or 3G. With the extra bandwidth, they can carry up to 60p (double the frame rate of HD-SDI) or 4:4:4 (double the color resolution of 4:2:2). This is a great option, and cameras like Sony’s F3 can output either dual link or 3G signals—you just need a recorder that can handle the input.

You probably noticed that I said 3G/dual link can carry 4:4:4 or 60p, not both. To get the 1080 60p 4:4:4 signal that our client wanted, they’d actually need four times the regular HD-SDI information. The frame rate doubles the data rates needed, and so does the color resolution; so you’d need either four regular HD-SDI signals or dual 3G connections for a total of 6 Gb/s. Now that’s a whole lot of data to handle, and by now, our recording options were getting slimmer and slimmer. I explained this to our client, and then asked whether they were sure they needed 4:4:4. This was an important question, because 4:2:2 is significantly less data, but for many applications, it’s sufficient. To understand why, let me explain the color models.

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