Video Processing Brings New Meaning To Motion

Analog video signals contain horizontal and vertical retrace intervals where the CRT's electron beam is redirected to the start of a new line or field. But while panels lack vertical or horizontal retrace, they do need a pixel clock.

Analog video signals don't provide a pixel clock, so a phase-locked loop (PLL) must generate it. HD resolution requires a PLL with low jitter, but the range of HD standards creates a design challenge for analog PLLs because it's hard to optimize the loop filter across horizontal frequencies from 10 to 150 kHz.

Offset is another intrinsic AFE challenge. The AFE video path typically consists of a dc-restore clamp, offset and gain correction, and analog-to-digital conversion. A good dc-restore function will eliminate the offset at the input to the AFE, but the active devices that follow reintroduce offsets. The offsets are random: They vary from device to device, and they usually have large temperature coefficients, causing them to drift as the display warms up.

Here's the problem with offset. In component video, the Y signal (luminance, the gray-scale information) and the Pb and Pr signals (chrominance, the color information) are sent over three discrete channels. The Y signal is unipolar, and Y offset on the Y channel will affect brightness. The Pb and Pr signals are bipolar, and they form the orthogonal color space.

Random offsets on Pb and Pr move the center of this space away from 0 V. This adds color to what should be gray images and shifts the overall color space, causing colors to be displayed incorrectly (Fig. 2). Historically, display manufacturers have done a one-time calibration during the display's production test or simply not addressed it at all, shipping devices with large black level and color variations. As a result, it's up to the user to manually adjust the settings.

See associated figure