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Monday, January 17, 2000

Luminance Ranges in Video

Different video systems have differing internal definitions of “black” and “white.”

IRE Issues: You’ve Been Set Up

It is common to confuse the digital luminance value issues discussed in this article with the analog measurement of brightness, known as IRE (Institute of Radio Engineers) units. They are indeed both definitions of how to represent black and white, but in different worlds - they are not interchangeable nor a replacement for one another.

In analog video, the electrical reference for white is generally defined as 100 IRE, using near-identical electrical levels regardless of format. The electrical reference for black depends on what video format you are using, and even what country you are in: PAL and the Japanese version of NTSC always use 0 IRE to define black. Composite NTSC video in North America uses an electrical value of 7.5 IRE, which is sometimes referred to as set-up. Component video in North America can use either 0 or 7.5 IRE for black; 7.5 seems to be more common.

A comparisons between different IRE references for black and white. The one on the left is for NTSC composite video in North America; the center is used in Japan; the one on the right is for PAL, and is also occasionally used for NTSC component video.

The analog-to-digital and digital-to-analog conversion portions of your video card or deck decide how to translate luminance values between these two worlds. For example, if set to 0 IRE and digitizing to a 601-specification data stream, a video signal at 0 IRE would be translated to a value of 16, and video at 100 IRE would be translated to 235. Hot spots brighter than 100 IRE would be captured as values above 235.

If this same data stream was output through hardware with 7.5 IRE setup enabled, a value of 16 would create a video signal at 7.5 IRE, and a value of 235 would create a 100 IRE signal. Most video editing systems have a 0/7.5 IRE switch, but note that some - such as the Media 100 - actually only change this reference on the output, always keeping the input at 7.5 IRE for NTSC systems.

Some mistakenly assume that a luminance range of 16-235 means 7.5 IRE setup was used, but this is not true. For example, a value of 16 still defines black in any 0 IRE system, including PAL and Japanese NTSC. If a videotape was recorded with 7.5 IRE setup, but digitized through hardware that uses 0 IRE for its black reference, and then translated through a codec that stretched video black down to a numeric value of 0, “black” on the 7.5 IRE tape would indeed decode at a value around 16 - but only because someone made a mistake.

You can imagine the number of permutations that can pop up if someone does not align both their digital and their analog luminance ranges throughout the production chain. Again, it’s just a matter of tracking which definition each part of the chain is using, and translating where necessary.

If you are working with a codec that stretches the common internal values of 16-235 out to 0-255 for software, take particular care when digitizing tapes to make sure your source material doesn’t regularly measure over 100 IRE - otherwise, these areas will just get clipped off later by the codec. Tame these by adjusting your input processing amplifier (either external, or built into your editing system).

How can you tell if a videotape was recorded at a reference of 0 or 7.5 IRE? Look at a waveform monitor on input while looking at some color bars, and see where the lowest level of the picture information is landing. If your system lacks a waveform monitor, one unscientific method is to eyeball the color bars at its head - specifically, the three skinny “PLUGE” (Picture Line-Up Generator) bars in the lower right corner. The middle of these three skinny bars is supposed to be reference black; the ones on either side are 4 IRE lower and higher than reference black.

In theory, you should adjust your monitor so that the middle reference bar and the darker one beside it blend together. This makes sure black in the video signal truly is black on your monitor. In reality, most monitors I’ve seen default to a brighter setting. Because of this, if you can see all three bars, chances are good that the tape you are viewing was recorded at 7.5 IRE (the darkest bar is at 3.5 IRE); if you see one double-wide bar and one lighter bar, chances are it was recorded at 0 IRE (and the “minus” bar is blending into the middle reference bar).

In a color bar pattern, the black square that’s second from the lower right actually contains three skinny bars, known as the PLUGE (picture line-up generation equipment). The middle skinny bar - as well as the two squares on either side - are true black; the skinny bars on either side of the center bar are lighter and darker than black.

This is not a dependable system - after all, your monitor might actually be calibrated correctly - but it’s often a clue if you have no other idea. To remove some of the guesswork, we now label all our tapes either 0 or 7.5 IRE; it is a good practice to get into, and to encourage others you work with to do as well.

Editing • Motion Graphics • Post Production • Visual Effects •

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Chris Meyer | 02/15- 07:45 PM

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