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Chapter 23

Gamma for NBTV

Broadcast standard TV systems use a principle called 'Gamma'. This principle can also be usefully applied to NBTV video signals.

Picture tubes

Gamma relates to the reproduction of shades of grey. Television picture tubes are far from linear. When a linear ramp video signal is applied, then the brightness on the screen increases in a quadratic way. This is because of the way cathode ray tubes work. Quadratic means 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.

Gamma correction

Correct grey scale reproduction requires correction for picture tube gamma. This gamma correction is the inverse operation of quadratic, i.e. the square root. Square root is the same as 'to the power ½', so the gamma corrector has a gamma of ½ or 0.5.

To correct the picture tube a gamma of 1/2 is needed.

Historically the TV signal as transmitted has embodied this correction. TV cameras, both professional and domestic, still incorporate this correction with gamma equals ½.

Added noise

A TV system that uses gamma can be shown to have certain advantages over a linear system. 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 more visible in dark areas than in bright parts. The gamma of the picture tube compresses the noise in the dark areas and expands the noise in the bright parts of the picture. This compensates, more or less, our eye's noise sensitivity curve.

Digital video

Noise is added not only due to weak reception. If video signals are digitised this digitisation also adds noise: the so-called quantisation noise. This quantisation noise is stronger if fewer bits per sample are used. Gamma corrected video signals enable fewer bits to be used for digitisation than linear video signals.

NBTV practice

A straight forward Nipkow disc televisor has a gamma of almost 1. Neon lamps and LED's give light proportional (linear) to the applied current. If this televisor is used to look at test patterns 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 higher gamma, and all grey blocks are clearly distinguishable.
Also in live video from the CD's the linear disc televisor shows the brighter parts (e.g. faces) bleached out. If you lower the brightness setting you will see that there is much more detail to see! This happens because the live video of the CD was recorded from Peter Smith's camcorder, with gamma corrected signals of course (gamma = ½), through his linear working scan converter.

camera gamma=1 . camera gamma=½ . camera gamma=½
monitor gamma=2 . monitor gamma=2 . monitor gamma=1.
darks almost black . . . correct greys . . . brights bleached out

Gamma in NBTV

For the satisfactory display of scan-converted NBTV video originating from broadcast type sources with a gamma of ½ it is recommended that a gamma correction of around 2 should be applied to the video signals in your monitor. NBTV monitors employing cathode ray tubes, like oscilloscopes, have already a built-in gamma of around 2 and therefore no gamma correction is needed.
Where monitors have a fixed gamma of around 2 it follows that all input sources, including mechanical cameras, will theoretically require the inverse gamma correction to be applied. Otherwise, the gamma correction would need to be made switchable to cater for alternative input signals with varying amounts of gamma.

Gamma in a disc televisor

It is simple to build a disc televisor so that it displays grey levels as if it had a gamma of 2. In previous chapters LED driver circuits are given. The three resistors and two diodes in the circuit of the driver transistor are there to introduce this non linearity. Correct selection of the resistor values ensures a satisfactory display of grey scale gradations.

Computers

The same considerations apply in the world of computers. As LCD screens replace cathode ray tube types their characteristics must be matched to pre-serve grey scale and colour fidelity. It is sometimes possible to set the gamma of a computer's video screen output in the display setting menu.
A typical NBTV viewer program will include a gamma adjustment alongside contrast and brightness settings, allowing its effects to be clearly observed.

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