Concerning "legal" and
"valid" video signals

Copyright © 1998-03-24
Charles Poynton

There is confusion in the video industry concerning the terms legal, illegal, valid and invalid.

In 1990, Tektronix published a white paper Solving the Component Puzzle. That document states,

The various CAV interconnect standards specify a voltage range called the "gamut," for each component channel. If a component signal stays within the limits specified for its channel, it is said to be "legal" (or to have a legal gamut).

If a color difference signal produces legal components when translated to RGB [sic] format, it is said to be "valid." But if a signal will not translate properly to RGB [sic], the signal is invalid - even if it falls within the legal range in its own standard. ...

... Valid signals can be translated, encoded, or input to any part of a video system without causing amplitude-related problems.

I emphasize Tektronix's use of unadorned symbols R, G, and B. These are gamma-corrected signals, and are properly written with prime symbols, R', G', and B'.

In other words, according to Tektronix,

Tektronix uses the word "gamut" to describe the coding range of each of the components individually. I consider that to be a terrible choice of words, since "gamut" is defined in the domain of color science to refer to a representable color, not a representable signal or a representable component of a color. Gamut ought to refer to the gamut of video, that is to the unit R'G'B' cube: Gamut ought to be related to Tektronix's "legality," not to validity.

An NTSC transmitter has zero carrier at 120 IRE. For NTSC transmission, the composite signal must not excurse beyond 120 IRE. So the third Tektronix paragraph seems to be in error: 100% yellow or 100% cyan present a serious amplitude-related problem when clipped to 120 IRE prior to NTSC VHF or UHF transmission. Most people would call 100% yellow and 100% cyan "illegal" for NTSC, but the Tektronix definition clearly implies that they are both legal and valid, because the unit R'G'B' cube contains both.

A hard clip at or near 120 IRE will introduce undesirable effects; it is preferable to limit the chroma excursion by desaturating colors appropriately. Most VHF/UHF standards for PAL transmission accommodate composite PAL from synctip through peak saturated chroma (or related back to baseband, from -300 mV to +1000 mV). In other words, zero carrier is at the equivalent of +133 1/3 IRE or higher. So, chroma limiting is not necessary for PAL.

Snell & Wilcox have published a Glossary of Video Terms, by Steinberg and Warren. (I have an undated draft from early 1997; the book has now appeared in hard cover form.) That document uses "legal" for what Tektronix calls "valid":

Legal Colour. A colour with R, G, B [sic] values each of which must lay within the range Reference Black to Reference White. Non-legal values may commonly be seen, for example, after the processing of 4:2:2 signals.

Snell & Wilcox should also know to write R, G, and B with prime symbols.

There is no entry for Valid on its own, but the document has this entry:

Valid Ramps. A component test signal containing three ramps, covering correspondingly the total level range of Y, Pb, Pr channels, each ramp is accompanied by smaller ramps in the other two channels to keep within the valid [sic] range of R, G, B [sic] levels from Reference Black to Reference White.

This seems in conformance with Tektronix "validity," and seems to call into question Snell & Wilcox's own definition of "legal." (These two definitions may have changed in the final version of the book.)

Charles - Video engineering
Copyright © 1998-03-24
Edited 1998-03-25