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Why do some of the test signals show a Gamut error directly from the generator+ and look really bad when converted to composite?

問題:

Why do some of the test signals show a Gamut error directly from the generator+ and look really bad when converted to composite?

答案:

Many of the signals generated in serial component format are designed for testing the response of a digital to analog conversion, in which the output of the system is analog component. Signals such as sweeps, multiburst, and 3 channel step signals fall into this category. These signals are not designed to be converted to RGB or composite, and doing so will result in gamut errors.

A multiburst or sweep signal is a monochrome signal in the composite environment. Figure 1 shows the composite multiburst.

Figure 1 Composite multiburst
Figure 1 Composite multiburst

The same signal would have information in only the Y channel if converted to the color difference format, as shown in figure 2.

Figure 2 Composite multiburst converted to component
Figure 2 Composite multiburst converted to component.

In the component environment, additional information is added to the color difference channels to provide a means of testing frequency response of all three channels. See figure 3

Figure 3 Component multiburst
Figure 3 Component multiburst

If converted to composite, the signal looks very jumbled, as shown in figure 4.

Figure 4 Component multiburst converted to composite
Figure 4 Component multiburst converted to composite.

You can clearly see here that the signal levels are much higher than 100 IRE, and significantly below 0 IRE, and that the Y+C gamut error is shown in red on the image. The multiburst is correct for color difference use only. Figure 5 shows the same component multiburst converted to RGB waveform.

Figure 5 Color Difference multiburst converted to RGB presentation
Figure 5 Color Difference multiburst converted to RGB presentation.

Note in figure 5 that the 0 and 700 mV maximum levels for RGB are violated in this conversion process. Also note that the RGB Gamut error is shown in red at the top of the image.

 

In summary, many test signals will not be usefull of valid when converted between formats. They are designed for testing within the format for which they were originally created.


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