Narrow-bandwidth Television Association

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Chapter 14

Gamma for NBTV

The CCIR TV-system uses in its video signal a principle that is named "Gamma". In NBTV video signals this gamma should also be used.

Picture tubes
Gamma has everything to do with the reproduction of shades of grey. Television picture tubes are far from linear. When we make a video signal that is a linear ramp, then the brightness on the screen increases in a quadratic way. This is because of the way cathode ray tubes work. Quadratic will say: To the power of 2. This power is called the Gamma. A picture tube has a gamma of 2.

A picture tube has a gamma of 2.	This must be corrected as described below.

Gamma correction
The gamma of the picture tube must be corrected, otherwise we don't see the correct shades of grey on our screen. This gamma correction is the inverse operation of quadratic, i.e. the square root. Square root is the same as "to the power of ½", so the gamma corrector has a gamma of ½.
For historical reasons the gamma correction is done in the TV-camera. In the beginning there were much more TV's than camera's so this was the cheaper solution. This has never been changed, so still every studio camera, cam-corder and web-cam has a built in correction with gamma equals ½.

Added noise
Wouldn't a linear working TV-system be better than a system with gamma? The answer is no. Our eyes are more sensitive to changes in brightness in dark parts of a picture, than in bright parts. When a TV set receives a weak station there is noise added to the received video signal. In a linear system the same amount of added noise would be much more visible in dark areas than in bright parts. But now the gamma of the picture tube compresses the noise in dark areas and expands the noise in the bright parts of the picture. This compensates more or less the sensitivity curve of our eyes for added noise.

Digital video
Noise is not only added due to weak reception. Also if video signals are digitised this digitalisation adds noise: the so-called quantisation noise. This quantisation noise is stronger if less bits per sample are used. Now again if we digitise gamma?corrected signals we need less bits than if we digitised linear video signals.

NBTV practice
Simple Nipkow disc monitors have a gamma of almost 1. Neon lamps and LED's give light proportional (linear) to the applied current. This is the reason that when you look for the first time at test pictures coming from our NBTV CD's you will observe that the dark grey blocks can be distinguished very well, while the bright grey blocks all seem to have the same brightness. There is a big difference if your monitor is based on an oscilloscope. Its picture tube has a gamma of 2, and all grey blocks are clearly visible.
Also in the live video from the CD's, the Nipkow disc shows the brighter parts (faces) bleached out. If you lower the brightness setting, you will see that there are many more details to see! This happens because the live video of the CD was recorded from Pete Smith's camcorder, with gamma corrected video signals of course (CCIR gamma = 0.5), through his linearly working scan converter.

Gamma in NBTV
For NBTV the club should make the choice to use a gamma correction of 0,5 in our video signals. This makes noise added to the video signal equally visible in dark and bright parts and it minimises the amount of bits that we need for digitalisation of video signals. It also gives the best compatibility with CCIR-video. Scan converters from CCIR to NBTV and from NBTV to CCIR now have to do nothing more than the scan conversion. NBTV monitors based on a picture tube, like oscilloscopes, have already a built in gamma of 2. And yes, Nipkow disc cameras should also include gamma correction and also disc monitors now should have an artificial gamma of 2 built in.

Gamma in a disc monitor
It is simple to build a disc monitor so that it displays grey levels as if it had a gamma of 2. In chapter 3 an LED drive circuit is given. The two diodes in the source circuit of the driver FET are there to include this gamma. By adjusting the value of the resistors you can adjust the grey scale tracking. A method to get a good setting is to run the circle test pattern of the club CD, look at the left and right edge grey bars and set the resistors so that you see almost equal grey steps in the dark and bright areas.

Computers
Notice that computers have had the same dilemma. For years they used cathode ray picture tubes to display their colour pictures. Now LCD screens are used as well. The grey scale tracking and colour tracking of these screens must be matched to picture tube monitors to have the same colour reproduction, especially for photographs and moving video. In the settings menu of the display you might be able to set the gamma of the video output of the PC.

 

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