Gamut monitoring

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Mike Richardson of Harris sheds light on the importance of gamut monitoring in a multi-format environment.

Many articles have been written addressing colour space gamut differences between analogue composite, RGB and digital video.

Without repeating much of what has already been written, gamut "errors" are a product of the conversion of video signals originally produced for one video format into another. For instance, colour TV cameras produce RGB signals that are (still!) often encoded into composite video.

These two different ways of representing video - RGB and composite - also contain different colour representations.

Each one has an allowable range (or "gamut") that defines its colour space. In the broadcast world we are most often interested in discussing colour space conversions between the native colour space of SD and HD video (Y Cb Cr) and between RGB and composite.

The mathematics behind colour space conversion is relatively straightforward, but out of the scope of this article. It is enough to understand that converting between colour spaces is a simple mathematical process, involving nothing more than algebraic manipulation.

ITU and SMPTE standards contain all the appropriate detail, including the actual coefficients for conversion.

Beyond the mathematical curiosity, there are practical reasons for monitoring gamut excursions. Any video captured by a real-world device such as a camera will be captured as RGB since the camera sensors are RGB in devices.

Even if the camera has SD or HD outputs in Y Cb Cr colour space, these sensors are still in RGB space. At the opposite end of the chain, the video will be viewed in RGB colour space. All monitors and televisions, regardless of technology, are RGB devices.

Digital broadcast technology was designed to include more chrominance range (larger gamut), as well as more picture detail when compared to analogue.

An engineer working solely with hardware that conforms to the ITU BT.601 (SD-SDI) serial digital video specification will never encounter a need to compensate digital video for colour space limitations.

Equipment that is designed properly will not have any "gamut-related" problems.

However, as soon as the engineer needs to prepare SD-SDI video for conversion to high definition serial digital or for broadcast by an analogue transmission channel, gamut measurements and possible adjustments may be necessary.

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