TV Engineering lab: CRT Operation (Part 2)

Display: region where the pixels are actually displayed on the screen. The length of this region is 640 pixels.

Retrace: region in which the electron beams return to the left edge. The video signal should be disabled (i.e., black), and the length of this region is 96 pixels.

 

Right border: region that forms the right border of the display region. It is also known as the front porch (i.e., porch before retrace). The video signal should be disabled, and the length of this region is  16 pixels.

Left border: region that forms the left border of the display region.  It is also known as the backporch (i.e., porch after retrace). The video signal should be disabled, and the length of this region is 48 pixels.

During the vertical scan, the electron beams move gradually from top to bottom and then return to the top.  This corresponds to the time required to refresh the entire screen. The format of the vsync signal is similar to that of the hsync signal, as shown in following figure. The time unit of the movement is represented in terms of horizontal scan lines.  A period of the vsync signal is 525 lines and can be divided into four regions: Display, Retrace, Bottom border, and Top border.

As in the horizontal scan, the lengths of the top and bottom borders may vary for different brands of monitors. The vsync signal can be obtained by a special mod-525 counter and a decoding circuit. Again, we intentionally start counting from the beginning of the display region. This allows us to use the counter output as the vertical (y-axis) coordinate.  This output constitutes the pixel-y signal. The vsync signal goes low when the line count is 490 or 491. As in the horizontal scan, we use the v-video-on signal to indicate whether the current vertical coordinate is in the displayable region. It is asserted only when the line count is smaller than 480.

Timing calculation of VGA synchronization signals

The s parameter specifies how fast the screen should be refreshed.  For a human eye, the refresh rate must be at least 30 screens per second to make the motion appear to be continuous. To reduce flickering, the monitor usually has a much higher rate, such as the 60 screens per second specification above. The pixel rate can be calculated by the three parameters: 

As mentioned earlier, we assume that the pixel rate is 25 MHz.

Report

Your report should start with a general description of what you did in this experiment, followed by the results and observations you obtained. You should answer all the following questions using the corresponding figures of spectrum or images. 

In your report, include the horizontal and vertical synchronous logic diagram by using ModelSim logic simulator.

Record the wave shape of the RGB signals those are taken from the VGA port of Altera board and displayed in the Oscilloscope.

Explain why the B signal shows an extra dc level in the Oscilloscope ?