Pointing out the right colours


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In the old days of PAL and NTSC analogue television, a vectorscope was an essential tool for examining chroma at every part for the programme production and transmission chain. This was because the colour information was carried as a phase and amplitude modulation which could be sensitive to a variety of transmission or recording non-linearities and problems. These could cause colour degradation or be completely wrong. In the digital age, colour information is carried in digits which do not usually get corrupted at each regeneration or link. So why would you want to look the chroma (colour information), portrayed on the same analogue style instrument?

In analogue television systems, there is always the potential for hue and saturation problems as well as timing and 8 field-sequence issues in PAL. In an ‘all digital’ world, the colour values are carried as a stream of numbers rather than as complex analogue waveforms. Images from cameras are essentially analogue so there is always every chance for the colour to be wrong, particularly when matching cameras to look the same. There could also be differences in lenses, optics and sensors. The human factor also comes into play, like having left the tungsten setting on when the camera should be set for daylight.

Transmission problems also exist in the digital world. Most digital transmission and recording formats use component YCbCr (or YUV), having started with RGB in the camera. Whenever format or standards conversion takes place, de-matrixing and re-matrixing are going on because there are different ‘mixes’ for the colour components between HD and SD, PAL and NTSC (actually matrix coefficients but let us not get bogged down with the maths). If these processes are not performed accurately, colour purity is going to suffer. An even more important role for the modern vectorscope is in the colour grading suite. Getting the right artistic look from shot to shot and avoiding gamut error.

Hence vectorscope displays are still very much in use but decreasingly in separate analogue instruments. Rasteriser boxes have been around for about 25 years. These can digitise both waveform and vectorscope data in such a way that a normal video display can be used in combination. Nowadays, these features can be integrated into the monitor itself. These displays are in most cases only an indication of the chroma vectors with a crude graticule.

Fully featured vectorscopes with calibrated displays can provide much more detail and the ability to switch graticules and scaling as well as homing in on a region of interest in the image. These can be still found in a stand-alone box with or without and built-in screen. More often the vectorscope can be brought up on the computer monitor as a tool or plug-in to an editing or grading package. These too can have limitations. Sometimes their speed is well below real-time or the size quite small due to the limitations of running within the editing software. Often, the graticules looks crude or have a non-standard aspect ratio.

For real-time conventional 2D images, the Cel-Soft Reel-Check Solo uses PC software image processing that is fast enough even for 4K as well as 2K/HD vectorscope windows in combination with waveforms, audio or other displays. And there is no special hardware. This type of software product can replace the rasteriser or stand-alone box.

In the 3D world, the Cel-Scope3D software creates vectorscopes which may be combined so that left and right eye image chroma can be superimposed for comparison and matching at any scale in the same sort of way.

A vectorscope is very similar in concept to semaphore signalling with flags. It points at where the right colours should be. Other chroma display methods are also around but the familiar vectorscope styles are easy to understand and can provide a comforting consistency in the suite or on-set.


Robin Palmer is Managing Director of Cel-Soft and is currently involved with solutions for 3D & TV technology.

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