Video Graphics Array - Technical Details - Signal Timings

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. The formula for the VGA horizontal frequency is thus (60 ÷ 1001) × 525 kHz = 4500 ÷ 143 kHz ≈ 31.4686 kHz. All other frequencies used by the VGA card are derived from this value by integer multiplication or division. Since the exactness of quartz oscillators is limited, real cards will have slightly higher or lower frequency.

All VGA timings can be varied widely by software that bypasses the VGA firmware interface and talks directly to the VGA hardware, as many MS-DOS based games did. However, only the standard modes, or modes that at least use exactly the same horizontal and vertical 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. Later "multisync" CRT monitors were usually much more flexible.

For most common VGA mode 640×480 "60 Hz" non-interlaced the horizontal timings are:

(Total horizontal sync time 6.60 µs)

The vertical timings are:

(Total vertical sync time 1.43 ms)

640 × 400 @ 70 Hz is video mode used for booting most x86 personal computers.

640 × 480 @ 60 Hz is the default MS-Windows graphics mode with 16 colors.

320 × 200 @ 70 Hz, the most common mode for VGA-era PC games, uses exactly the same timings as the 640 x 400 mode, just halving the dot clock and displaying each line of pixels twice.

The actual timings vary slightly. For example, for 640×480 @ 60 FPS, a 25.17 µs active video time with a pixel frequency of 25.174 MHz gives 633 pixels rather than the expected 640.