[robski]

OK I've decided to put my engineers hat on again to investigate some quirks with CRT Monitor calibration. mainly to understand a number of points brought out by various websites giving advice on the subject. Initially to look into the pitfalls of using adjust by eye systems such adobe gamma. At some point I intend to get myself a calibration tool such as Spyder 2 and compare the results.

In a previous life I was involved in Television engineering. Gamma encoding was done at source ( TV Camera ) so we only had to worry about CRT cut-off (dark level ), gray scale, geometry and convergence adjustments.

I intend to try and keep it simple so that you can follow the gist of the subject. I have no experience of flat screen setup and I am sure they have their own set of problems.

The 2 areas I want to focus on are Gamma curves and the Brightness control.

To start with some background information as to why we need gamma correction. The CRT is an electronic device that goes back to the days of steam radio. In the UK we called them Valves in other parts of the world they are known as vacuum tubes. The little glass bottles that glowed in the dark in granny's radio. As a schoolboy I used to build valve radio projects given in the Practical Wireless magazine. They were the days.

Next a 30 second electronics course. One of the simplest Valves is a triode. As the name suggests it contains 3 basic components.

1, a heated cathode which emits electrons, connected to the negative voltage supply.
2, an anode connected to high a positive voltage supply to attract the electrons towards it.
3, a control grid that is positioned between the cathode and anode. By applying a small negative voltage with respect to the cathode the flow of electrons can be controlled.

Valves were typically used as amplifiers. The input signal applied to the control grid and an amplified signal taken from the anode.

In a CRT the electronic part is called an electron gun. The main difference being the device has a couple more anodes. In a colour CRT the anode that is used for focusing has typically 3000 Volts applied and the final Anode has 25,000 Volts applied. The idea is to accelerate the electrons so that they bypass the anode and hit the face of the CRT. The face of the CRT is covered with phosphor dots which illuminate when struck by the electrons. The screen brightness is dependent on the volume of electron flow. The volume is controlled by the video signal and brightness control circuit.

Unfortunately the electron flow is not linear over the whole range of operation with valve and transistor devices. Typically the first 40% of control is curved and becoming more linear ( approximating to a straight line ) for the second 60%. The second 60% is the region that audio and video amplifiers are interested in to avoid distortion.

The attached CRT Characteristic image illustrates a typically transfer curve. The input being the controlling signal and the output being the amount of light produced. 2 points to note. First we don't get into the linear section of the curve until we have pasted the 20% light output stage. Secondly the first 50% of input produces only 20% of the light and the last 50% of input produces 80% of the light.

If we were to adopt a linear approach as with audio designs our dark point would be at 20% of the CRT brightness. i.e. the black would be a dark gray. For our blacks to black on the CRT we need zero electron flow and we have to live with this non-linearity. This is where gamma encoding is used to compensate. The dark parts of the image are boosted in such a way to linearize the brightness range. The CRT itself is the gamma decoder.

The shape of the curve has a logarithmic function and the clever mathematicians can approximate these curves with a function of x^y i.e. x raised to the power of y. x being the input signal and y would equal 2.2 to give this shape curve.

2.2 being the gamma factor.

It would appear that everybody ( including Mac users ) have settled on using a gamma value of 2.2 as the standard.

I trust you have all manage to keep up with the technical stuff.

The next attachment (Video brightness levels) illustrates the effect of applying the gamma encoding by video card driver. On the left is a step wedge of equal pixel brightness steps (original image data) and on the right show the steps with a gamma 2.2 encoding applied. The darker levels are boosted and the brighter levels are reduced in amplitude. The effect seen on the screen should a step wedge of equal brightness steps.

In an ideal world the first level of pixel brightness should coincide with the brightness control black level cut off point. If you have a black desktop and the brightness control set to maximum you may see a faint glimmer from the phosphor dots. The black level cut off point is where the phosphor stop glowing as you reduce the brightness. In theory this is the darkest black the monitor can produce. Any brightness seen will be the result of ambient room light reflected back off the face of the screen. Some Manufacturers have developed screens to reduce the reflection factor to give richer blacks and improved contrast ratio.

Part 2 to follow