SDI signal range
From the old days of analog TV, we inherited the "YUV" image encoding – with the Y’CbCr format being the variant for digital images. From its analog history, Y’CbCr also inherited the concept of "headroom". The headroom of analog signals was designed to make these signals more robust to visible image artifacts.
The Y‘CbCr encoding of the SDI signal still implements that headroom in the form of not-to-be-used code values. For example, let‘s consider the 10-bit signal of an SDI signal. 10-bits of information can encode 1024 different states (2^10 = 1024), typically used as code values numbered 0 to 1023.
The "regular" values for the Y‘ channel‘s encoding (luma) are specified as just the values from 64 to 940. That is the range called "legal range" (other names are "video range", "SMPTE range", or "narrow range"). Outside of the legal range lies the headroom.
When sticking to the standards for "legal range", the code value 64 represents the blackest black that a display can reproduce, and the code value 940 the brightest white that a display can reproduce.
In practice
In HD-SDI broadcast workflows, we still work almost exclusively with 4:2:2 signals, and 4:2:2 is Y‘CbCr encoded and that is almost always expected to be a video/legal range signal. The exception being some of the HDR and log based formats.
In film production we typically use 4:4:4, which normally is RGB (although it can be YUV as well) and RGB usually is expected to be a full range signal. In Pixotope we we do not currently support 4:4:4 and/or RGB.
Special Cases for Full/Extended range video
Already in the early years of HD-SDI, the idea came up to use the unused and "wasted" headroom for transferring image information in digital Y’CbCr signals. The outlook of 15% more code values per channel has led to two main approaches for extending the use of code values beyond the "legal range":
Some systems with digital video (SDI) interfaces include settings for switching between "full range" and "legal range" video signals. These settings change all video processing of the device from a range of 64..940 to 4..1020 to extend the fidelity of the video. For example, these settings have been used for HD-CAM SR video tapes in some use cases.
This is not relevant for Pixotope, as we convert all incoming video into 32bit floating point processing space, which offer essentially "unlimited" fidelity for processing.
Some cameras use the headroom above code value 940 for sending out "super whites" / "super brights" beyond white. That way the increased dynamic range of camera sensors beyond the "legal" white level of SDR monitors can be transferred for further use, without changing the default appearance on a monitor. Typically the code values from 64 to 1019 are used in an SDI signal, and that range is called "extended range".
This is something to be mindful of when working with HDR range content, see table below.
Please consult your camera documentation or video engineer to learn how the signal is encoded, but as a reference:
ARRI LogC, Canon CanonLog2, RED Log3G10: These log signals are encoded in a legal range Y’CbCr signal.
Sony Slog2/Slog3, Panasonic VLog: These log signals are encoded in an extended range Y’CbCr signal.
The recommendation is to use narrow/legal range for HLG
Some PQ Signals require extended range
Camera output set to Rec.709: This is almost in every case a legal range Y’CbCr signal.
SDI inputs of professional monitors expect a legal range signal on default.