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specs/freevga/vtiming.htm
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specs/freevga/vtiming.htm
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<HTML>
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<HEAD>
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<META NAME="Author" CONTENT="Joshua Neal">
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<META NAME="Description" CONTENT="Pure VGA/SVGA hardware programming (registers, identification, and otherlow-level stuff.)">
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<META NAME="KeyWords" CONTENT="VGA SVGA hardware video programming">
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<TITLE>FreeVGA - Video Timing Information</TITLE>
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</HEAD>
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<BODY>
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<CENTER><A HREF="home.htm">Home</A> <A HREF="#intro">Intro</A> <A HREF="#basics">Basics</A>
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<A HREF="#measure">Measurements</A> <A HREF="#horiz">Horizontal</A> <A HREF="#vert">Vertical</A>
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<A HREF="#considerations">Considerations</A> <A HREF="home.htm#background">Back</A>
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<HR><B>Hardware Level VGA and SVGA Video Programming Information Page</B></CENTER>
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<CENTER>Video Timing Information
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<HR></CENTER>
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<A NAME="intro"></A><B>Introduction</B>
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<BR> This page is written to give the
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necessary background on video timing that is useful for video programming.
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This is not a comprehensive reference on the subject, rather it just gives
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the minimum information needed to know to perform mode setting and the
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creation of custom video modes. It includes a small bit of information
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about the messy side of video adapters, the electrical output and how that
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is interpreted by the monitor. Much of this information pertains
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both to monitors and other CRT devices such as television displays, and
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is less applicable to LCD displays as they have different timing requirements.
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<P><A NAME="basics"></A><B>Basic Description</B>
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<BR> The video hardware produces
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a continuous signal on its output connector, except when it is in reset
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mode, where the video outputs are held in a single state. The continuous
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signal is required because the pixel information is only displayed for
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a short period of time, and relies on the persistence of the phosphor glow
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on the monitor as well as the ability of eyesight to perform automatic
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averaging to appear to be a steady image. That signal is usually
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output on multiple pins of the monitor connector, although it could also
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be a TV compatible output. LCD displays use a similar technique,
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although the timing is more advanced and depends on the specific type of
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panel and its driver circuitry. The signal includes both the pixel
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data that the monitor displays, as well as timing and "framing" information
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that the video display uses to drive its internal circuitry.
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<BR> The image's pixels are "scanned"
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on to the screen from left to right, top to bottom, and is broken up into
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"dot periods" or pixels, which is controlled by the "dot clock" for the
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mode, which is the basis for all the other video timings. Each horizontal
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"scan line" of dot periods is called a horizontal refresh as it "refreshes"
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the information on the display in a horizontal line. Many of these
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scan lines (the amount depending on the video mode), scanning from top
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to bottom, make up a vertical refresh, also known as a "frame". There
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are many vertical refreshes per second, where a higher refresh rate produces
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an image with less flicker.
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<P><A NAME="measure"></A><B>Timing Measurements</B>
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<BR> One of the important pieces
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of terminology to understand is how timing is measured. These include
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terms such as megahertz, kilohertz, and hertz. The first three are
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a measure of frequency which is based on the term hertz (abbreviated hz),
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which can be replaced by the term "Cycles per second." In video timing,
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hertz is used to describe the frequencies of the timing signals, such as
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when saying that the vertical refresh frequency is 60 hertz (or 60hz).
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This means that there are 60 cycles per second, which means that there
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are 60 vertical refreshes per second. Another case where hertz is
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used is when saying the horizontal refresh rate, such as when saying 31500
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hz, which means that there are 31,500 horizontal refresh cycles per second.
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One abbreviation frequently found is the term kilohertz (abbreviated Khz)
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which means 1,000 cycles/per second. For example, 31.5 kilohertz
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means 31.5 x 1000 hertz, or 31500 hz. This is used to save writing
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a few zeros and is a bit more concise. Similarly the term megahertz
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(abbreviated Mhz) is used, which means 1,000,000 cycles/per second.
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For example, instead of saying that a certain mode uses a 25,000,000 hz
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dot clock, or saying that it uses a 25,000 Khz clock, it can be concisely
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be stated by saying that it uses a 25 Mhz dot clock.
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<BR> Similarly, the periods of
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time involved in video timing are very short as they are typically small
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fractions of a second. The terms millisecond, microsecond, and nanosecond
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are useful for expressing these small periods of time. The term millisecond
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(abbreviated ms) means one thousandth of a second, or 0.001 seconds.
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In one second, there are 1,000 milliseconds. This is used to describe things,
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such as the length of time a vertical refresh takes, for example a 16.6
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millisecond vertical refresh means 16.6 thousands of a second, or 0.0166
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seconds. In one second, there are 1,000,000 microseconds. The
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term microsecond (abbreviated us) is used to describe something in terms
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of millionths of a second, or 0.000001 second. For example the length
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of a horizontal refresh could be 31.7 microseconds, or 31.7 millionths
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of a second, 0.0000317 second, or 0.0317 ms. The term nanosecond
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(abbreviated ns) is used to describe one billionth of a second, or 0.000000001
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seconds. There are 1,000,000,000 nanoseconds in one second.
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One circumstance where this is used, is to describe the period of time
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one dot period takes. For example, one dot period could be stated
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as 40 nanoseconds, 0.04 us, 0.00004 ms, or 0.00000004 seconds. In
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each case, the most concise term is used, to provide a shorter, more concise
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description.
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<BR> Because the unit hertz is
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defined using a unit of time (second), the period of one cycle can be determined
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by division. The simplest example is 1 hz, where the length of the
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cycle, by definition would be 1 second. For other values, it can be calculated
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according to the following formula:
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<UL>
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<LI>
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Period (in seconds) = 1 / frequency (in hertz)</LI>
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</UL>
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For example, a 60 hertz vertical
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refresh would last 1 / 60 second, which is approximately 0.0166 seconds,
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or 16.6 ms. Similarly, a 31.5 Khz horizontal refresh would be 1 /
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31500 second, which is approximately 0.000031 seconds, or 31.7 us.
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A 25 Mhz dot clock would produce a dot period of 1 / 25000000 second, which
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is 0.00000004 seconds, or 40 ns. If the period of a cycle is known,
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then the frequency can be calculated as:
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<UL>
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<LI>
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Frequency (in hertz) = 1 / Period (in seconds).</LI>
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</UL>
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For example, a 16.6 ms period
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would equate to 1 / 0.0166, which produces a frequency of approximately
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60 hz. Similarly a 31.7 us period would produce approximately a 31.5
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Khz frequency, and a 40 ns period would produce a 25 Mhz frequency.
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<P><A NAME="horiz"></A><B>Horizontal Timing</B>
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<BR> From a monitor's standpoint,
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the timing is fairly simple. It detects the horizontal sync pulses
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on the hsync line, then based on the polarity, frequency, and/or duration
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of those pulses sets up its horizontal scan circuitry to scan across the
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screen at the desired rate. During this period it continuously displays
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the signal input on the analog RGB pins/connectors. It is important
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to to know the horizontal sync frequency ranges of the monitor, as well
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as the acceptable width of the sync pulse for those sync ranges.
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If the width of the sync pulse is incorrect, it can make the displayed
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information too large or too small, as well as possibly preventing the
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monitor from synchronizing to the generated signal. The acceptable
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range of sync pulse width and polarity for a given frequency should be
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given in the specifications for the monitor; however, this is frequently
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overlooked by the manufacturer. It is recommended to contact the
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manufacturer, otherwise the result has to be determined by trial and error
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which can be damaging to the monitor's circuitry.
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<BR> In addition to those that
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control horizontal sync frequency and width, there are other horizontal
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timing registers which tell the display generation hardware when to output
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the active display, when to output the overscan, and when to perform blanking.
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The active display is when pixel data from the frame buffer are being output
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and displayed. This could also be overlaid by data from another source,
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such as a TV or MPEG decoder, or a hardware cursor. The overscan
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is the border to the left and right of the screen. This was more
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important on older video hardware such as those monitors lacking horizontal
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and vertical picture controls, and is provided for compatibility reasons
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although current hardware typically reduces the need for this portion completely.
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The blanking period is used during the retrace portion of the horizontal
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display cycle which is the period in which the horizontal sweeps from the
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right of the screen back to the left. Outputting non-zero intensities
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during this period would end up being stretched, in reverse across the
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end of the current scan line to the beginning of the next scan line which,
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while interesting and possibly useful in a small number of circumstances.
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would add a bit of blurring to the image. Blanking is signaled to
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the monitor by sending zero intensities of the red, green, and blue components
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on the analog lines.
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<BR> In the display generator,
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horizontal timings are specified by the number of dot periods they take.
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The dot period is controlled by selecting the desired dot clock frequency
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by programming certain registers.
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<P><A NAME="vert"></A><B>Vertical Timing</B>
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<BR> Vertical timing is nearly
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the same as the horizontal timing, except that it controls the vertical
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movement of the display scan, spacing the scan lines the right width apart
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so that they seem to form a rectangular image. The monitor detects
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the vertical sync pulses on the vsync line, then based on the polarity,
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frequency, and/or duration of those pulses sets up its vertical circuitry
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to scan down the screen at the desired rate. It is necessary to know
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the vertical sync frequency ranges for a given monitor, and the range of
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acceptable vertical sync widths and polarities for those ranges.
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The rage of vertical sync frequencies supported by the monitor are nearly
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always given my the monitor's specifications, but like the horizontal sync
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widths, the vertical sync widths are not commonly specified. Contact
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the manufacturer, as attempting to guess the correct vertical sync width
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can possibly cause the monitor to fail.
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<BR> As well as being programmed
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with the vertical sync frequency and pulse width, the display generation
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hardware has other registers which control when to output the active display,
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when to output the overscan, and when to perform blanking. In vertical
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terms, the active display is the scan lines which contain horizontal active
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display periods. The overscan is the border on top and bottom of
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the screen and, if present, consists of one or more entire scan lines in
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which the active display period is replaced with the overscan color.
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The blanking is used during the vertical retrace, and consists of one or
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more (usually more) scan lines in which the active display and overscan
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periods are replaced with blanking period, making the entire line effectively
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blanking. This prevents intensity from overlaying the screen during
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the vertical retrace where the monitor sweeps the vertical back to the
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top of the screen. Non-zero intensities output during this period
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would be written in a zig-zag pattern from the bottom to the top of the
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screen. In the display generator, the vertical timings are specified
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in terms of how many horizontal sync periods they take.
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<BR>
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<BR><A NAME="considerations"></A><B>Programming Considerations</B>
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<BR> For maximum flexibility,
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video timings should be configurable by the end users to allow for the
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specifications of their monitor. However, it is probably a wise idea
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to maintain a table of monitors and their rated specifications, to allow
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the users to select thieir monitor and determine whether or not the configured
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video timings are within the rated specifications of their monitor and
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warn the user about this. There is a distinct need for a comprehensive
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and accurate software-usable database of monitor specifications in a platform
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and video hardware independent form with sufficient information for a program
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to both select timings for a particular video mode, as well as verify that
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a given set of timings will function properly on the end-user's hardware.
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This database should contain a human readable description of the monitor
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make and model, as well as software parsable fields giving corresponding
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ranges of horizontal and vertical frequencies and sync polarities for those
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ranges if applicable, as well as a method of determining the acceptable
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widths of horizontal and vertical sync pulses for a given frequency in
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the corresponding rages. Framing information could be included in
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this table in a frequency independent fashion, although this is something
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that can be safely adjusted by the end user without risk of damage to the
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monitor, thus it is preferrable to provide a method or interface for the
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end-user to adjust these parameters to their preference.
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<BR>
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<P>Notice: All trademarks used or referred to on this page are the property
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of their respective owners.
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<BR>All pages are Copyright © 1997, 1998, J. D. Neal, except where
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noted. Permission for utilization and distribution is subject to the terms
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of the <A HREF="license.htm">FreeVGA Project Copyright License</A>.
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<BR>
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<BR>
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</BODY>
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</HTML>
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