Translate the website into your language :)

Latest News

Powered by Blogger.

TV Engineering lab: CRT Operation (Part 1)

Study the sample operation of a CRT monitor for extracting video (Red (R), Green (G), Blue (B)) and synchronous signal by using video graphic array (VGA) port.


To observe the waveform of RGB signal in Oscilloscope.
To examine the horizontal and vertical synchronous waveform of composite video signal.

The purpose of this lab is for you to understand the working principles of cathode ray tube. In this experiment, you will have the opportunity to extract the video and synchronous signal for picture tube operation. For this purpose, we have used VGA port of Altera DE1 board. The DE1 board includes a 16-pin D-SUB connector for VGA output. The VGA synchronization signals are provided directly from the Cyclone II FPGA, and a 4-bit DAC using resistor network is used to produce the analog data signals (red, green, and blue). This can support standard VGA resolution (640x480 pixels, at 25 MHz).

VGA (video graphics array) is a video display standard introduced in the late 1980s in IBM PCs and is widely supported by PC graphics hardware and monitors. In this experiment, we discuss the design of a 640-by-480 resolution interface for CRT (cathode ray tube) monitors. The intensity of the electron beam and the brightness of the dot are determined by the voltage level of the external video input signal, labeled mono in the following figure. The mono signal is an analog signal whose voltage level is between 0 and 0.7 V.

A vertical deflection coil and a horizontal deflection coil outside the tube produce magnetic fields to control how the electron beam travels and to determine where on the screen the electrons hit. The following figure shows the CRT scanning pattern.

The monitor’s internal oscillators and amplifiers generate sawtooth waveforms to control the two deflection coils. For example, the electron beam moves from the left edge to the right edge as the voltage applied to the horizontal deflection coil gradually increases. After reaching the right edge, the beam returns rapidly to the left edge (i.e., retraces) when the voltage changes to 0.
Video controller
A video controller generates the synchronization signals and outputs data pixels  serially. A simplified block diagram of a VGA controller is shown in following figure. It contains a synchronization circuit, labeled vga-sync,  and a pixel generation circuit.
The vga-sync circuit generates timing and synchronization signals.  The hsync and vsync signals are connected to the VGA port to control the horizontal and vertical scans of the monitor.  The two signals are decoded from the internal counters, whose outputs are the pixel_x and pixel_y signals.  The pixel_x and pixel_y signals indicate the relative positions of the scans and essentially specify the location of the current pixel. The vga_sync circuit also generates the video-on signal to indicate whether to enable or disable the display.
VGA Synchronization
The video synchronization circuit generates the hsync signal, which specifies the required time to traverse (scan) a row, and the vsync signal, which specifies the required time to traverse (scan) the entire screen.  This principle is based on a 640-by-480 VGA screen with a 25-MHzpixel rate, which means that 25M pixels are processed in a second. Note that this resolution is also known as the VGA mode. The screen of a CRT monitor usually includes a small black border, as shown at the top of following figure. The middle rectangle is the visible portion. Note that the coordinate of the vertical axis increases downward. The coordinates of the top-left and bottom-right comers are (0,0) and (639,479), respectively.

A period of the hsync signal contains 800 pixels and can be divided into four regions:


TV Engineering lab: CRT Operation (Part 1)
  • Title : TV Engineering lab: CRT Operation (Part 1)
  • Posted by :
  • Date : 17:00:00
  • Labels :
  • Blogger Comments
  • Facebook Comments


Post a Comment