REC. 709

In addition to the above video signal timings, there are two color space formats that are typically encountered, called 4:2:2 and 4:4:4. (http://en.wikipedia.org/wiki/Chroma_subsampling#4:2:2)

A 4:2:2 signal fits into a single HD-SDI cable link, and is the most common signal for monitoring and standard video acquisition. A 4:4:4 signal requires 2 HD-SDI cable links (called, appropriately enough, “dual-link” HDSDI), and is more commonly used in high end feature film production. The signal bandwidth requires a recording deck that can handle this; the most common recording deck found on these productions is a Sony SRW-1. (http://pro.sony.com/bbsc/ssr/product-SRW1/)

The color space used on most 4:2:2 video signals is called “Rec. 709” (http://en.wikipedia.org/wiki/Rec._709). This corresponds to a standard, video-type color space, but it’s pretty common on episodic shoots because everyone understands it.


Log Color

The color space used on most 4:4:4 shoots is some form of logarithmic encoding. This is done to more closely simulate the way a film negative records light, since the modern digital cameras have enough range (11 + stops) to simulate film quite closely. The problem with shooting “log” is that the uncorrected images look kind of flat in a monitor, so a temporary “LUT” (look up table) is applied to them for on-set viewing. This doesn’t touch the underlying image, but makes it so people can get an idea of what the final shot will look like.

There are several different flavors of log recording — Sony’s S-Log, Panavision’s Panalog, etc. They all basically do a input/output transfer curve that causes brighter highlights to “roll off” smoothly, instead of clipping harshly, which is a prime giveaway of a cheap video camera. They all need to have reverse transfer functions used when importing video recorded in log format into whatever system you’ll use for editing/FX work.

Previzion presently uses a standard 4:2:2 single link HDSDI video input for its live keyer, and assumes Rec. 709 encoding, as it’s the on stage standard for monitoring. We have a second 4:2:2 live single link input that works, and at some point we’ll tackle reading in 4:4:4 log encoded video, but that’s a ways off.