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    How Monitors Work
    - Just the basics please

    Monitors have two timing components, 
    a: Horizontal timing 
    and 
    b: Vertical timing. 

    Monitors and televisions for that matter, utilize a special Electron Gun at the back of the CRT called the neck. This Gun emits three beams of electrons, one for Red, one for Blue and one for Green. On the neck, a Special Coil called the Yoke is mounted to deflect the electron beam as it travels towards the CRT's face, which ultimately displays the image.


    Horizontal Scan Time

    To display an image, the beam is first pointed at the upper left-hand corner of the screen. It is then moved horizontally across the screen from the left side of the screen to the right. The time it takes for the beam to be moved from the left side to the right side is referred to as the "Horizontal Active Time", as the beam is turned on. 

  • Horizontal Blank Time 

    Once the beam has reached the right side of the screen it is quickly moved back to the left side of the screen. As the beam is being directed back, it is always turned off and the time it takes to move back is called the "horizontal blank time", as the beam is now turned off. 

  • Horizontal Total Time 

    At this point, the beam has been directed to move one complete horizontal cycle, starting from the left edge of the screen to the right side and back again. The time it takes for this horizontal cycle is referred to as the "horizontal total time". Adding the Horizontal Active and Horizontal Blank times together equals the Horizontal Total Time. 

  • Horizontal Refresh Rate 

    Depending on how fast the monitor runs, 25,000 to 80,000 of these horizontal cycles may occur each second. The number of horizontal cycles per second is referred to as the Horizontal Refresh Rate and is usually in units of thousands of cycles, referred to as Kilohertz. For example, if a monitor horizontally cycles 70,000 times a second, its Horizontal Refresh Rate is 70.0 Kilohertz (written as 70.0 kHz). 

  • Horizontal Total Time 

    We generally speak of a monitor's horizontal speed by its Horizontal Refresh Rate (e.g. "the monitor runs at 70kHz horizontally"). When dealing with specific monitor timings though, we usually use the actual time of each horizontal cycle. That is, if a monitor runs at 50 kHz, each horizontal cycle takes 1/50,000 of second, or 20 microseconds (20 one-millionths of a second). When working with specific monitor timings, it is necessary to deal in actual time since the Horizontal Total Time is divided into the Horizontal Active and Horizontal Blank times. 

  • SCAN LINE 

    At this point, it is hopefully understood that the beam is moved back and forth horizontally. If the beam were not moved vertically also, the image on the monitor would be one horizontal line. To display a full image requires that each time the beam is moved back to the left side of the screen, it is moved down vertically. Each horizontal scan of the beam is called a "scanline". 

  • VERTICAL ACTIVE TIME 

    Depending on a monitor's vertical resolution, an image on the screen may be composed of 320 to 1600 horizontal scanlines. The total time it takes for all of the horizontal scanlines to be displayed is called the "vertical active time". 

  • Vertical Blank Time 

    Similar to the horizontal operation, once the beam reached the bottom of screen it is quickly returned to the top of the screen. The time it takes for the beam to be returned to the top is called the "vertical blank time". 

  • Vertical Total Time 

    Again, like the horizontal operation, the time it takes for the beam to start at the top of the screen, scan back and forth to the bottom of the screen, and return to the top, is called the "vertical total time". Adding the Vertical Active Time to the Vertical Blank Time will equal the Vertical Total Time. 

  • Vertical Refresh Rate 

    The Vertical Refresh Rate of a monitor is simply the number of vertical cycles completed per second, measured in Hertz. For example, if a monitor completes 72 vertical cycles per second, it's Vertical Refresh Rate is 72 Hertz (72 Hz).

    It should be noted that all the timings, both horizontal and vertical, are related, for example:
    To decrease the Horizontal Total Time, the Horizontal Active Time and/or the Horizontal Blank Time must decrease. 
    By decreasing the Horizontal Total Time, the beam will get to the bottom of the screen faster, thereby increasing the Vertical Refresh Rate. 
    If more Horizontal Blank Time is needed, either the Horizontal Total Time must be increased or the Horizontal Active Time must be decreased. 
    If the Horizontal Total Time is increased, it will take longer for the beam to reach the bottom of the screen, thereby decreasing the Vertical Refresh Rate. 

  • HORIZONTAL AND VERTICAL SYNCS 

    When the beam reaches the right hand side of the screen, the monitor must be instructed that it is time to return the beam to the left side. Similarly, when the last scanline on the bottom of the screen has been displayed, the monitor must be instructed to return the beam back to the top of the screen for the next vertical cycle. The Horizontal Sync Signal and the Vertical Sync Signal, respectively, perform these functions. 

  • Horizontal Sync Impulse 

    During the horizontal blank time, the monitor receives the horizontal sync pulse. The time between the end of the horizontal active time and the start of the horizontal sync pulse is called the Horizontal Front Porch. The amount of time that the horizontal sync pulse is active is called the Horizontal Sync Width. After receiving the horizontal sync pulse, the monitor returns the beam to the left side of the screen. The period of time between the end of the Horizontal Sync pulse and the start of the next horizontal active time is the Horizontal Back Porch. Adjusting the position of the horizontal sync pulse in relation to the horizontal blank time adjusts the location of the image on the screen to the right or left. 

  • Vertical Sync Impulse 

    Similarly, during the vertical blank time, the monitor receives the vertical sync pulse. The time between the end of the vertical active time and the start of the vertical sync pulse is called the Vertical Front Porch. The amount of time that the vertical sync pulse is active is called the Vertical Sync Width. After receiving the vertical sync pulse, the monitor returns the beam to the tope of the screen. The period of time between the end of the Vertical Sync pulse and the start of the next vertical active time is the Vertical Back Porch. Adjusting the position of the vertical sync pulse in relation to the vertical blank time adjust the location of the image on the screen up or down. 

  • Video Signals And Horizontal And Vertical Sync Interfaces 

    Monitors receive three different video signals, one for Red, one for Green, and one for Blue. In addition to the video signals, a monitor must also receive the horizontal and the vertical sync signals. 

    There are three methods of sending the Sync Signals, they are as follows: 

  • Separate horizontal and vertical syncs:
    The horizontal and vertical sync signals are sent as two separate signals. This sync interface is considered to be "external". In this configuration, monitors with BNC connections use five connections: red, green, blue, horizontal sync, and vertical sync. 

    Composite Sync:
    The horizontal and vertical sync signals are combined into one signal. This sync interface is also considered to be "external". In this configuration, monitors with BNC connections use four connections: red, green, blue, and sync. 

    Sync On Green:
    The horizontal and vertical sync signals are combined together with the green signal. This sync interface is considered to be "internal". In this configuration, monitors with BNC connections use three connections: red, green, and blue.