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DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

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Figure 45.6 Modulated<br />

ramp waveform<br />

A system or subsystem with linear amplitude response<br />

has a transfer function whose output plots as a straightline<br />

function of its input; departure from this condition<br />

is differential gain error. Amplitude nonlinearity could<br />

be characterized directly by the departure of the<br />

transfer function from a straight line; ADC and DAC<br />

components and subsystems are specified this way.<br />

However, in video, amplitude nonlinearity is usually<br />

characterized by a parameter differential gain (DG) that<br />

is easier to measure.<br />

To measure DG, you present to a system the sum of<br />

a high-frequency signal and a low-frequency signal –<br />

consider these as x and y in Equation 45.1 above. It is<br />

standard to use a sine wave at subcarrier frequency as<br />

the high-frequency signal. The low-frequency signal is<br />

either a ramp or a staircase. Figure 45.6 shows the<br />

preferred modulated ramp signal. The alternative<br />

modulated stair is shown in Figure 45.7 opposite.<br />

Ideally, when measuring DG, the high-frequency sine<br />

wave component should emerge at the output of the<br />

system having its amplitude independent of the lowfrequency<br />

component upon which it is superimposed.<br />

The DG measuring instrument has a filter that rejects<br />

(discards) the low-frequency component; DG is then<br />

determined from the amplitude of the remaining highfrequency<br />

(subcarrier-based) component. Nonlinear<br />

540 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES

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