VGA - Revision history

Jnc: Link 'resolution'

2024-02-27T10:41:30Z

Link 'resolution'

Jnc: rm nonexist template

2021-03-09T16:29:46Z

rm nonexist template

Jnc: Add Wikipedia credit, rm useless template calls, +cat

2019-09-04T18:00:15Z

Add Wikipedia credit, rm useless template calls, +cat

ForOldHack: Created page with "'''Video Graphics Array''' ('''VGA''') is a graphics standard for video display controller first introduced with the IBM PS/2 line of computers i..."

2019-03-13T15:23:12Z

Created page with "'''Video Graphics Array''' ('''VGA''') is a graphics standard for video display controller first introduced with the IBM PS/2 line of computers i..."

New page

'''Video Graphics Array''' ('''VGA''') is a graphics standard for [[video display controller]] first introduced with the [[IBM Personal System/2|IBM PS/2]] line of computers in 1987 <ref>{{cite web |title=Chronology of IBM Personal Computers |author=Ken Polsson |url=http://pctimeline.info/ibmpc/ibm1987.htm |accessdate=2015-01-28 |deadurl=no |archiveurl=https://web.archive.org/web/20150221071923/http://pctimeline.info/ibmpc/ibm1987.htm |archivedate=2015-02-21 |df= }}</ref><ref>{{Cite news|url=https://www.computerhope.com/jargon/v/vga.htm|title=What is VGA (Video Graphics Array)?|access-date=2018-08-13|language=en}}</ref>, following [[Color Graphics Adaptor|CGA]] and [[Enhanced Graphics Adaptor|EGA]] introduced in earlier IBM personal computers. Through widespread adoption, the term has also come to mean either an analog [[computer display standard]], the 15-pin [[D-subminiature]] [[VGA connector]], or the 640×480 [[Graphics display resolution|resolution]] characteristic of the VGA hardware.

VGA was the last IBM graphics standard to which the majority of [[IBM PC compatible|PC clone]] manufacturers conformed, making it the [[lowest common denominator]] that virtually all post-1990 [[Personal computer|PC]] graphics hardware can be expected to implement. It was officially followed by IBM's [[Extended Graphics Array]] (XGA) standard, but was effectively superseded by numerous slightly different extensions to VGA made by [[Clone (computing)|clone]] manufacturers, collectively known as [[Super video graphics array|Super VGA]].

Today, the VGA analog interface is used for high-definition video, including resolutions of [[1080p]] and higher. While the transmission bandwidth of VGA is high enough to support even higher resolution playback, there can be picture quality degradation depending on cable quality and length. How discernible this degradation is depends on the individual's eyesight and the display, though it is more noticeable when switching to and from digital inputs like [[HDMI]] or [[DVI]].

===Specifications===
The original VGA specifications are as follows (supporting e.g. no hardware sprites):

*256 [[kilobyte|KB]] Video [[Random-access memory|RAM]] (The very first cards could be ordered with 64 KB or 128 KB of RAM, at the cost of losing some or all high-resolution 16-color modes.)
*16-color and 256-[[List of 16-bit computer hardware palettes#MCGA and VGA|color paletted]] display modes.
*262,144-color [[List of monochrome and RGB palettes#18-bit RGB|global palette]] (6 bits, and therefore 64 possible levels, for each of the red, green, and blue channels via the RAMDAC)
*Selectable 25.175 MHz<ref>{{cite web|title=VGA Signal 640 x 480 @ 60 Hz Industry standard timing|url=http://www.tinyvga.com/vga-timing/640x480@60Hz}}</ref> or 28.322 MHz master pixel clock
*Usual line rate fixed at 31.46875 kHz
*Maximum of 800 horizontal [[pixel]]s<ref name="ps2_refman1992">PS/2 Video Subsystem Technical Reference Manual 1992</ref>
*Maximum of 600 lines<ref name="ps2_refman1992"/>
*Refresh rates at up to 70 [[Hertz|Hz]]<ref>{{cite web|title=VGA Signal timings|url=http://www.tinyvga.com/vga-timing|deadurl=no|archiveurl=https://web.archive.org/web/20160620145944/http://tinyvga.com/vga-timing|archivedate=2016-06-20|df=}}</ref>
*[[Vertical blank interrupt]] (Not all [[clone (computing)|clone]] cards support this.)
*[[Planar (computer graphics)|Planar]] mode: up to 16 colors (4 [[bit plane]]s)
*Packed-pixel mode: 256 colors ([[Mode 13h]])
*Hardware [[scrolling|smooth scrolling]] support
*No Blitter, but supports very fast data transfers via "VGA latch" registers
*Some "[[Raster graphics|Raster]] Ops" support
*[[Barrel shifter]]
*[[Split screen (computer graphics)|Split screen]] support
*0.7 [[Volt|V]] [[Amplitude|peak-to-peak]]<ref>{{cite web|title=VGA Electrical FAQ|url=http://www.microvga.com/faq/electrical|deadurl=no|archiveurl=https://web.archive.org/web/20160429175550/http://microvga.com/faq/electrical|archivedate=2016-04-29|df=}}</ref>
*75 [[ohm]] double-terminated [[Electrical impedance|impedance]] (18.7 mA, 13 mW)

As well as the standard modes, VGA can be configured to emulate many of the modes of its predecessors ([[Enhanced Graphics Adapter|EGA]], [[Color Graphics Adapter|CGA]], and [[IBM Monochrome Display Adapter|MDA]]), including their reduced global color palettes (with particular pre-set colors chosen from the VGA palette for text and 4- or 16-color, 200-line modes) and coarser text font grids. Compatibility is almost full at [[BIOS]] level, but even at register level, a very high value of compatibility is reached. VGA is not directly compatible with the special [[IBM PCjr]] or [[Hercules Graphics Card|HGC]] video modes, despite having sufficient resolution, color, refresh rate and memory capabilities; any emulation of these modes has to be performed in software instead.

===Signal timings===
The intended standard value for the horizontal frequency of VGA is exactly double the value used in the [[NTSC-M]] video system, as this made it much easier to offer optional [[TV-out]] solutions or external VGA-to-TV converter boxes at the time of VGA's development, a technique proposed by Zia Shlaimoun; it is also at least nominally twice that of CGA, which itself used broadcast-frequency monitors, essentially tunerless televisions with more direct signal inputs. The formula for the VGA horizontal frequency is thus (60 ÷ 1001) × 525 kHz = 4500 ÷ 143 kHz ≈ 31.4685 kHz, obtained in practice by the method common to all raster-based computer graphics of using a particular crystal oscillator or PLL frequency for the pixel clock and deriving all other horizontal and vertical frequencies by integer division. In this case, a 25.175MHz or 28.322MHz pixel clock, counting 800 or 900 pixels per line, arriving at 31.46875 or 31.46889kHz (and from there, 525 or 449 lines to arrive at 59.94Hz or 70.09Hz); the tiny mismatch (7 to 12[[Parts per million|ppm]]) is easily accounted for by, and is well within the limits of the retrace synchronisation system. In fact, CGA itself was further adrift (as it could only scan 262 lines per progressive frame, instead of 262.5 to match NTSC's interlaced nature), and the limited precision of quartz oscillators - and their tendency to drift slightly with temperature and supply voltage - means real cards will have slightly higher or lower frequencies whose variance vs the nominal figures can easily exceed the inherent mismatch.

All derived VGA timings (i.e. those which still use the master 25.175 and 28.322 MHz crystals and, to a lesser extent, the nominal 31.469 kHz line rate) can be varied widely by software that bypasses the VGA firmware interface and communicates directly with the VGA hardware, as many MS-DOS based games did. However, only the standard modes, or modes that at least use almost exactly the same H-sync and V-sync timings as one of the standard modes, can be expected to work with the original late-1980s and early-1990s VGA monitors. The use of other timings may in fact damage such monitors and thus was usually avoided by software publishers. Third-party "multisync" CRT monitors were usually much more flexible, and in combination with "super EGA", VGA, and later SVGA graphics cards using extended modes, could display a much wider range of resolutions and refresh rates at wholly arbitrary sync frequencies and pixel clock rates (within a particular lower/upper range, depending on model, typically encompassing a range spanning CGA's 15.7kHz to SVGA and XGA's 36kHz at a minimum, and including the 18.4, 21.8, 24.8 and 31.5kHz of MDA/Hercules, EGA, "25kHz medium resolution" (a standard popularised by several Japanese domestic-market computers) and VGA along the way), commonly reaching 640x400 thru 720x480 (at 56 thru 72Hz), 752x410, 800x560 and 800x600 as standard or near-standard settings.

For the most common VGA mode (640×480 "60 Hz" [[progressive scan|non-interlaced]]), the horizontal timings are:<ref name="Javier Valcarce timings list">{{cite web|title=Javier Valcarce VGA timings page|url=http://www.javiervalcarce.eu/wiki/VGA_Video_Signal_Format_and_Timing_Specifications|deadurl=no|archiveurl=https://web.archive.org/web/20150102175843/http://javiervalcarce.eu/wiki/VGA_Video_Signal_Format_and_Timing_Specifications|archivedate=2015-01-02|df=}}</ref><ref>HP D1194A Super VGA Display & HP D1195A Erognomic Super VGA Display Installation Guide, Hewlett Packard</ref>

{| class="wikitable"
|-
!Parameter !! Value !! Unit
|-
|Pixel clock frequency || 25.175 || [[Hertz|MHz]]<ref>Article [http://groups.google.com/group/sci.electronics.design/msg/b8ee15d937e50fa1 "Re: VGA specifications ,where ?"] posted 19 November 1997 to sci.electronics.design newsgroup by Jeroen Stessen</ref>
|-
|Horizontal frequency || 31.469 || [[Hertz|kHz]]
|-
|Horizontal pixels || 640 ||
|-
|Horizontal sync polarity || Negative ||
|-
|Total time for each line || 31.778 || [[1 E-6 s|µs]]
|-
|[[Front porch]] (A) || 0.636 || µs
|-
|Sync pulse length (B) || 3.813 || µs
|-
|[[Back porch]] (C) || 1.907 || µs
|-
|Active video (D) || 25.422 || µs
|}

(Total horizontal sync and blanking time = 6.356 µs; equivalent to pixel widths of A = 16, B = 96, C = 48, D = 640 and each complete line = 800)
{{Wide image|VGA_640×480_horizontal_timings.svg|1000px|VGA horizontal timings for 640×480}}
''NB. The same general layout applies, merely at a lower frequency, for the vertical timings.''

These timings are the same in the higher frequency mode, but all pixel counts are correspondingly multiplied by 9/8ths{{snd}}thus, 720 active pixels, 900 total per line, and a 54 pixel back porch.

The vertical timings are:

{| class="wikitable"
|-
!Parameter !! Value !! Unit
|-
|Vertical lines || 480 ||
|-
|Vertical sync polarity || Negative ||
|-
|Vertical frequency || 59.94 || Hz
|-
|Total time for each frame || 16.683 || ms
|-
|Front porch (A) || 0.318 || [[Millisecond|ms]]
|-
|Sync pulse length (B) || 0.064 || ms
|-
|Back porch (C) || 1.048 || ms
|-
|Active video (D) || 15.253 || ms
|}

(Total vertical sync and blanking time 1.43 ms; equivalent to line periods of A = 10, B = 2, C = 33, D = 480 and each complete frame = 525)

These timings are somewhat altered in "70Hz" mode, as although it uses the same line rate, its frame rate is not quite exactly 7/6ths that of "60 Hz", despite 525 dividing cleanly into 7{{snd}}and, of course, 480/400 is itself a larger 6:5 ratio. Instead, it compromises on a 449-line frame (instead of the expected 450), with the back porch extended to 34 lines, and the front porch to 13, with an unaltered 2-line sync pulse{{snd}}and the active image taking up 89% of the total scan period rather than 91%. The monitor is triggered into synchronising at the higher frame scan rate (and, with digital displays such as LCDs, the higher horizontal pixel density) by use of a positive-polarity VSync pulse, versus the negative pulse of 60 Hz mode.

Depending on manufacturer, the exact details of active period and front/back porch widths, particularly in the horizontal domain, may vary slightly. This does not usually cause a problem as the porches are merely intended to act as blanked-video buffers offering a little overscan space between the active area and the sync pulse (which triggers, in traditional CRT monitors, the phosphor beam deflection "flyback" to the upper or left hand side of the tube) and thus can be safely overrun into by a certain amount when everything else is operating correctly. The relationship between the front and back porches can also be altered within certain limits, which makes possible special features such as software-based image alignment with certain graphics cards (centering the image within the monitor frame by adjusting the location of the active screen area between the horizontal and vertical porches, rather than relying wholly upon the adjustment range offered by the monitor's own controls which can sometimes be less than satisfactory).

This buffer zone is typically what is exploited to achieve higher active resolutions in the various custom screen modes, by deliberately reducing porch widths and using the freed-up scan time for active pixels instead. This technique can achieve an absolute maximum of 704 pixels horizontally in 25 MHz mode and 792 at 28 MHz without altering the actual sync width (in real-world cases, e.g. with 800 pixel wide mode, the sync pulse would be shortened and a small porch area left in place to prevent obvious visual artefacting), and as much as 523 or 447 lines at the standard 60 and 70 Hz refresh rates (again, it is usually necessary to leave SOME porch lines intact, hence the usual maximum of 410 or 512 lines at these rates, and the 50 Hz maximum being 600 lines rather than 626). Conveniently, the practical limits of these techniques are not ''quite'' high enough to overflow the available memory capacity of typical 256 KB cards (800×600 consuming 235 KB, and even the theoretical 832×624 requiring "only" 254 KB), so the only concerns remain those of monitor compatibility.

← Older revision		Revision as of 10:41, 27 February 2024
Line 1:		Line 1:
−	'''Video Graphics Array''' ('''VGA''') is a graphics standard for [[video display]] [[device controller\|controller]] first introduced with the [[IBM PS/2]] line of computers in 1987, following [[Color Graphics Adaptor\|CGA]] and [[Enhanced Graphics Adaptor\|EGA]] introduced in earlier IBM personal computers. Through widespread adoption, the term has also come to mean either an analog [[computer display standard]], the 15-pin [[D-subminiature]] [[VGA connector]], or the 640×480 [[~~Graphics display resolution\|~~resolution]] characteristic of the VGA hardware.	+	'''Video Graphics Array''' ('''VGA''') is a graphics standard for [[video display]] [[device controller\|controller]] first introduced with the [[IBM PS/2]] line of computers in 1987, following [[Color Graphics Adaptor\|CGA]] and [[Enhanced Graphics Adaptor\|EGA]] introduced in earlier IBM personal computers. Through widespread adoption, the term has also come to mean either an analog [[computer display standard]], the 15-pin [[D-subminiature]] [[VGA connector]], or the 640×480 [[resolution]] characteristic of the VGA hardware.

	VGA was the last IBM graphics standard to which the majority of [[IBM PC compatible\|PC clone]] manufacturers conformed, making it the lowest common denominator that virtually all post-1990 [[personal computer\|PC]] graphics hardware can be expected to implement. It was officially followed by IBM's [[Extended Graphics Array]] (XGA) standard, but was effectively superseded by numerous slightly different extensions to VGA made by clone manufacturers, collectively known as [[Super video graphics array\|Super VGA]].		VGA was the last IBM graphics standard to which the majority of [[IBM PC compatible\|PC clone]] manufacturers conformed, making it the lowest common denominator that virtually all post-1990 [[personal computer\|PC]] graphics hardware can be expected to implement. It was officially followed by IBM's [[Extended Graphics Array]] (XGA) standard, but was effectively superseded by numerous slightly different extensions to VGA made by clone manufacturers, collectively known as [[Super video graphics array\|Super VGA]].

@@ Line 62: / Line 62: @@
 (Total horizontal sync and blanking time = 6.356&nbsp;µs; equivalent to pixel widths of A = 16, B = 96, C = 48, D = 640 and each complete line = 800)
-{{Wide image|VGA_640×480_horizontal_timings.svg|1000px|VGA horizontal timings for 640×480}}
 ''NB. The same general layout applies, merely at a lower frequency, for the vertical timings.''
-These timings are the same in the higher frequency mode, but all pixel counts are correspondingly multiplied by 9/8ths{{snd}}thus, 720 active pixels, 900 total per line, and a 54 pixel back porch.
+These timings are the same in the higher frequency mode, but all pixel counts are correspondingly multiplied by 9/8ths&nbsp;&ndash;&#32;thus, 720 active pixels, 900 total per line, and a 54 pixel back porch.
 The vertical timings are:
@@ Line 92: / Line 92: @@
 (Total vertical sync and blanking time 1.43&nbsp;ms; equivalent to line periods of A = 10, B = 2, C = 33, D = 480 and each complete frame = 525)
-These timings are somewhat altered in "70Hz" mode, as although it uses the same line rate, its frame rate is not quite exactly 7/6ths that of "60&nbsp;Hz", despite 525 dividing cleanly into 7{{snd}}and, of course, 480/400 is itself a larger 6:5 ratio. Instead, it compromises on a 449-line frame (instead of the expected 450), with the back porch extended to 34 lines, and the front porch to 13, with an unaltered 2-line sync pulse{{snd}}and the active image taking up 89% of the total scan period rather than 91%. The monitor is triggered into synchronising at the higher frame scan rate (and, with digital displays such as LCDs, the higher horizontal pixel density) by use of a positive-polarity VSync pulse, versus the negative pulse of 60&nbsp;Hz mode.
+These timings are somewhat altered in "70Hz" mode, as although it uses the same line rate, its frame rate is not quite exactly 7/6ths that of "60&nbsp;Hz", despite 525 dividing cleanly into 7&nbsp;&ndash;&#32;and, of course, 480/400 is itself a larger 6:5 ratio. Instead, it compromises on a 449-line frame (instead of the expected 450), with the back porch extended to 34 lines, and the front porch to 13, with an unaltered 2-line sync pulse&nbsp;&ndash;&#32;and the active image taking up 89% of the total scan period rather than 91%. The monitor is triggered into synchronising at the higher frame scan rate (and, with digital displays such as LCDs, the higher horizontal pixel density) by use of a positive-polarity VSync pulse, versus the negative pulse of 60&nbsp;Hz mode.
 Depending on manufacturer, the exact details of active period and front/back porch widths, particularly in the horizontal domain, may vary slightly. This does not usually cause a problem as the porches are merely intended to act as blanked-video buffers offering a little overscan space between the active area and the sync pulse (which triggers, in traditional CRT monitors, the phosphor beam deflection "flyback" to the upper or left hand side of the tube) and thus can be safely overrun into by a certain amount when everything else is operating correctly. The relationship between the front and back porches can also be altered within certain limits, which makes possible special features such as software-based image alignment with certain graphics cards (centering the image within the monitor frame by adjusting the location of the active screen area between the horizontal and vertical porches, rather than relying wholly upon the adjustment range offered by the monitor's own controls which can sometimes be less than satisfactory).

@@ Line 1: / Line 1: @@
-'''Video Graphics Array''' ('''VGA''') is a graphics standard for [[video display controller]] first introduced with the [[IBM Personal System/2|IBM PS/2]] line of computers in 1987 <ref>{{cite web |title=Chronology of IBM Personal Computers |author=Ken Polsson |url=http://pctimeline.info/ibmpc/ibm1987.htm |accessdate=2015-01-28 |deadurl=no |archiveurl=https://web.archive.org/web/20150221071923/http://pctimeline.info/ibmpc/ibm1987.htm |archivedate=2015-02-21 |df= }}</ref><ref>{{Cite news|url=https://www.computerhope.com/jargon/v/vga.htm|title=What is VGA (Video Graphics Array)?|access-date=2018-08-13|language=en}}</ref>, following [[Color Graphics Adaptor|CGA]] and [[Enhanced Graphics Adaptor|EGA]] introduced in earlier IBM personal computers. Through widespread adoption, the term has also come to mean either an analog [[computer display standard]], the 15-pin [[D-subminiature]] [[VGA connector]], or the 640×480 [[Graphics display resolution|resolution]] characteristic of the VGA hardware.
+'''Video Graphics Array''' ('''VGA''') is a graphics standard for [[video display]] [[device controller|controller]] first introduced with the [[IBM PS/2]] line of computers in 1987, following [[Color Graphics Adaptor|CGA]] and [[Enhanced Graphics Adaptor|EGA]] introduced in earlier IBM personal computers. Through widespread adoption, the term has also come to mean either an analog [[computer display standard]], the 15-pin [[D-subminiature]] [[VGA connector]], or the 640×480 [[Graphics display resolution|resolution]] characteristic of the VGA hardware.
-VGA was the last IBM graphics standard to which the majority of [[IBM PC compatible|PC clone]] manufacturers conformed, making it the [[lowest common denominator]] that virtually all post-1990 [[Personal computer|PC]] graphics hardware can be expected to implement. It was officially followed by IBM's [[Extended Graphics Array]] (XGA) standard, but was effectively superseded by numerous slightly different extensions to VGA made by [[Clone (computing)|clone]] manufacturers, collectively known as [[Super video graphics array|Super VGA]].
+VGA was the last IBM graphics standard to which the majority of [[IBM PC compatible|PC clone]] manufacturers conformed, making it the lowest common denominator that virtually all post-1990 [[personal computer|PC]] graphics hardware can be expected to implement. It was officially followed by IBM's [[Extended Graphics Array]] (XGA) standard, but was effectively superseded by numerous slightly different extensions to VGA made by clone manufacturers, collectively known as [[Super video graphics array|Super VGA]].
 Today, the VGA analog interface is used for high-definition video, including resolutions of [[1080p]] and higher. While the transmission bandwidth of VGA is high enough to support even higher resolution playback, there can be picture quality degradation depending on cable quality and length. How discernible this degradation is depends on the individual's eyesight and the display, though it is more noticeable when switching to and from digital inputs like [[HDMI]] or [[DVI]].
-===Specifications===
+==Specifications==
 The original VGA specifications are as follows (supporting e.g. no hardware sprites):
@@ Line 11: / Line 12: @@
 *16-color and 256-[[List of 16-bit computer hardware palettes#MCGA and VGA|color paletted]] display modes.
 *262,144-color [[List of monochrome and RGB palettes#18-bit RGB|global palette]] <small>(6&nbsp;bits, and therefore 64 possible levels, for each of the red, green, and blue channels via the RAMDAC)</small>
-*Selectable 25.175&nbsp;MHz<ref>{{cite web|title=VGA Signal 640 x 480 @ 60 Hz Industry standard timing|url=http://www.tinyvga.com/vga-timing/640x480@60Hz}}</ref> or 28.322&nbsp;MHz master pixel clock
+*Selectable 25.175&nbsp;MHz or 28.322&nbsp;MHz master pixel clock
 *Usual line rate fixed at 31.46875&nbsp;kHz
-*Maximum of 800&nbsp;horizontal [[pixel]]s<ref name="ps2_refman1992">PS/2 Video Subsystem Technical Reference Manual 1992</ref>
+*Maximum of 800&nbsp;horizontal [[pixel]]s
-*Maximum of 600&nbsp;lines<ref name="ps2_refman1992"/>
+*Maximum of 600&nbsp;lines
-*Refresh rates at up to 70&nbsp;[[Hertz|Hz]]<ref>{{cite web|title=VGA Signal timings|url=http://www.tinyvga.com/vga-timing|deadurl=no|archiveurl=https://web.archive.org/web/20160620145944/http://tinyvga.com/vga-timing|archivedate=2016-06-20|df=}}</ref>
+*Refresh rates at up to 70&nbsp;[[Hertz|Hz]]
-*[[Vertical blank interrupt]] <small>(Not all [[clone (computing)|clone]] cards support this.)</small>
+*[[Vertical blank interrupt]] <small>(Not all clone cards support this.)</small>
 *[[Planar (computer graphics)|Planar]] mode: up to 16&nbsp;colors <small>(4 [[bit plane]]s)</small>
 *Packed-pixel mode: 256&nbsp;colors <small>([[Mode 13h]])</small>
@@ Line 24: / Line 25: @@
 *[[Barrel shifter]]
 *[[Split screen (computer graphics)|Split screen]] support
-*0.7&nbsp;[[Volt|V]] [[Amplitude|peak-to-peak]]<ref>{{cite web|title=VGA Electrical FAQ|url=http://www.microvga.com/faq/electrical|deadurl=no|archiveurl=https://web.archive.org/web/20160429175550/http://microvga.com/faq/electrical|archivedate=2016-04-29|df=}}</ref>
+*0.7&nbsp;[[Volt|V]] [[Amplitude|peak-to-peak]]
-*75&nbsp;[[ohm]] double-terminated [[Electrical impedance|impedance]] (18.7&nbsp;mA, 13&nbsp;mW)
+*75&nbsp;[[ohm]] double-terminated [[impedance]] (18.7&nbsp;mA, 13&nbsp;mW)
 As well as the standard modes, VGA can be configured to emulate many of the modes of its predecessors ([[Enhanced Graphics Adapter|EGA]], [[Color Graphics Adapter|CGA]], and [[IBM Monochrome Display Adapter|MDA]]), including their reduced global color palettes (with particular pre-set colors chosen from the VGA palette for text and 4- or 16-color, 200-line modes) and coarser text font grids. Compatibility is almost full at [[BIOS]] level, but even at register level, a very high value of compatibility is reached. VGA is not directly compatible with the special [[IBM PCjr]] or [[Hercules Graphics Card|HGC]] video modes, despite having sufficient resolution, color, refresh rate and memory capabilities; any emulation of these modes has to be performed in software instead.
 ===Signal timings===
 The intended standard value for the horizontal frequency of VGA is exactly double the value used in the [[NTSC-M]] video system, as this made it much easier to offer optional [[TV-out]] solutions or external VGA-to-TV converter boxes at the time of VGA's development, a technique proposed by Zia Shlaimoun; it is also at least nominally twice that of CGA, which itself used broadcast-frequency monitors, essentially tunerless televisions with more direct signal inputs. The formula for the VGA horizontal frequency is thus (60 ÷ 1001) × 525&nbsp;kHz = 4500 ÷ 143&nbsp;kHz ≈ 31.4685&nbsp;kHz, obtained in practice by the method common to all raster-based computer graphics of using a particular crystal oscillator or PLL frequency for the pixel clock and deriving all other horizontal and vertical frequencies by integer division. In this case, a 25.175MHz or 28.322MHz pixel clock, counting 800 or 900 pixels per line, arriving at 31.46875 or 31.46889kHz (and from there, 525 or 449 lines to arrive at 59.94Hz or 70.09Hz); the tiny mismatch (7 to 12[[Parts per million|ppm]]) is easily accounted for by, and is well within the limits of the retrace synchronisation system. In fact, CGA itself was further adrift (as it could only scan 262 lines per progressive frame, instead of 262.5 to match NTSC's interlaced nature), and the limited precision of quartz oscillators - and their tendency to drift slightly with temperature and supply voltage - means real cards will have slightly higher or lower frequencies whose variance vs the nominal figures can easily exceed the inherent mismatch.
 All derived VGA timings (i.e. those which still use the master 25.175 and 28.322&nbsp;MHz crystals and, to a lesser extent, the nominal 31.469&nbsp;kHz line rate) can be varied widely by software that bypasses the VGA firmware interface and communicates directly with the VGA hardware, as many MS-DOS based games did. However, only the standard modes, or modes that at least use almost exactly the same H-sync and V-sync timings as one of the standard modes, can be expected to work with the original late-1980s and early-1990s VGA monitors. The use of other timings may in fact damage such monitors and thus was usually avoided by software publishers. Third-party "multisync" CRT monitors were usually much more flexible, and in combination with "super EGA", VGA, and later SVGA graphics cards using extended modes, could display a much wider range of resolutions and refresh rates at wholly arbitrary sync frequencies and pixel clock rates (within a particular lower/upper range, depending on model, typically encompassing a range spanning CGA's 15.7kHz to SVGA and XGA's 36kHz at a minimum, and including the 18.4, 21.8, 24.8 and 31.5kHz of MDA/Hercules, EGA, "25kHz medium resolution" (a standard popularised by several Japanese domestic-market computers) and VGA along the way), commonly reaching 640x400 thru 720x480 (at 56 thru 72Hz), 752x410, 800x560 and 800x600 as standard or near-standard settings.
-For the most common VGA mode (640×480 "60&nbsp;Hz" [[progressive scan|non-interlaced]]), the horizontal timings are:<ref name="Javier Valcarce timings list">{{cite web|title=Javier Valcarce VGA timings page|url=http://www.javiervalcarce.eu/wiki/VGA_Video_Signal_Format_and_Timing_Specifications|deadurl=no|archiveurl=https://web.archive.org/web/20150102175843/http://javiervalcarce.eu/wiki/VGA_Video_Signal_Format_and_Timing_Specifications|archivedate=2015-01-02|df=}}</ref><ref>HP D1194A Super VGA Display & HP D1195A Erognomic Super VGA Display Installation Guide, Hewlett Packard</ref>
+For the most common VGA mode (640×480 "60&nbsp;Hz" [[progressive scan|non-interlaced]]).
 {| class="wikitable"
@@ Line 40: / Line 42: @@
 !Parameter !! Value !! Unit
 |-
-|Pixel clock frequency || 25.175 || [[Hertz|MHz]]<ref>Article [http://groups.google.com/group/sci.electronics.design/msg/b8ee15d937e50fa1 "Re: VGA specifications ,where ?"] posted 19 November 1997 to sci.electronics.design newsgroup by Jeroen Stessen</ref>
+|Pixel clock frequency || 25.175 || [[Hertz|MHz]]
 |-
 |Horizontal frequency || 31.469 || [[Hertz|kHz]]<!--as per above-->
@@ Line 95: / Line 97: @@
 This buffer zone is typically what is exploited to achieve higher active resolutions in the various custom screen modes, by deliberately reducing porch widths and using the freed-up scan time for active pixels instead. This technique can achieve an absolute maximum of 704 pixels horizontally in 25&nbsp;MHz mode and 792 at 28&nbsp;MHz without altering the actual sync width (in real-world cases, e.g. with 800 pixel wide mode, the sync pulse would be shortened and a small porch area left in place to prevent obvious visual artefacting), and as much as 523 or 447 lines at the standard 60 and 70&nbsp;Hz refresh rates (again, it is usually necessary to leave SOME porch lines intact, hence the usual maximum of 410 or 512 lines at these rates, and the 50&nbsp;Hz maximum being 600 lines rather than 626). Conveniently, the practical limits of these techniques are not ''quite'' high enough to overflow the available memory capacity of typical 256&nbsp;KB cards (800×600 consuming 235&nbsp;KB, and even the theoretical 832×624 requiring "only" 254&nbsp;KB), so the only concerns remain those of monitor compatibility.