JVC 250 service manual Video Processor PCB Operation

Chapter 5---Electronics

RGB_G2 - red, green, or blue G2 voltage adjust control signal to the Video Amplifier PCB.

G1_BIAS - G1 adjust control voltage (global adjustment) to the Video Amplifier PCB.

IIC_INT - IIC interrupt output to System Controller PCB.

Video Processor PCB - Operation

The Video Processor PCB accepts image and synchronizing inputs in either composite or separate RGBHV component format from the selected VIC. When the projector receives the power ON command, the System Controller PCB polls the VICs to determine what VICs are available. It then activates the VIC selected in the Channel Menu (see section 4-6 in the User’s Guide). The selected VIC sends a signal (/SEL_VIC) to the Video Processor PCB telling it which VIC is active.

The Video Processor separates the sync pulses and passes them (H_SYNC and V_SYNC) on to the Raster Timing Generator PCB. If the source sync type is Sync-on-Green, the Video Processor strips the horizontal and vertical syncs from the green input and sends that sync signal (G_SYNC) to the Raster Timing Generator PCB.

The Video Processor PCB adjusts the image input information with contrast, brightness, DC restore clamp, and blanking. Contrast changes the peak to peak amplitude of the External image while keeping the offset (black level) the same. Brightness shifts the DC level of the External image up or down while keeping the peak to peak amplitude the same. The DC restore circuit (or clamp pulse) restores the DC offset (black level) after each horizontal line. Contrast and Brightness commands come from the IIC data bus. The DC Restore and Blanking signals come directly from the Raster Timing Generator PCB.

The Video Processor PCB switches in the menu overlays and internal test patterns. It performs gamma correction adjustments, and applies the shading (sensitivity and threshold) adjustments. The menu overlay signals create the displayed menus and test patterns. Menu overlays are generated on the System Controller PCB. The gamma circuit corrects non-linearities, primarily due to the ILA® . Gamma correction consists of degrees of black and white boost. The gamma tracking controls adjust the gain of the image information near the 50% level.

Sensitivity and Threshold are shading adjustments that correct for non-uniformity in the CRT and ILA® . Sensitivity adjusts the gain (same as Contrast only local instead of global) and threshold adjusts the DC offset (black level) (same as Brightness, only local instead of global) to adjust the dark areas of the red, green, or blue channel. The sensitivity and threshold commands come from the System Controller PCB through the IIC bus and adjust both the internal and external image information. Sensitivity and Threshold are applied to the image information after the contrast, brightness and gamma correction.

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