The standard "SVGA" 15-pin D-sub video connector is familiar. So too ought the signal it carries.
It is designed to work with normal CRT monitors, which are scanned by an electron beam moving left to right, top to bottom. Each can line is left to right, and the beam is blanked whilst the voltage on the deflection plates is changed to move the beam from the right back to the left for a new row, and similarly whilst it moves from the bottom to the top of the screen.
Humans worry about the vertical refresh rate, that is, how often the whole picture is rescanned. Below about 65 to 70Hz they perceive a flickering, and feel unhappy. It is alleged that for a small range above this they fail to perceive the flickering, but still feel unhappy, and they are generally content once the refresh rate reaches 75Hz. However, not all humans are identical. (The persistence time of the phosphor is typically <1ms.)
The signals for the red, green and blue guns are transmitted separately, each over a shielded wire. Considering in detail one encoding of a signal of resolution 1280x1024 and refresh rate 75Hz, the individual pixels are emitted at a rate of 135MHz. The part of the horizontal scan for which the electron beam is turned on is clearly 1280 pixels, but the full width is 1688, so the time taken to reset the electron beam to the start of the next row is almost a third of the time taken to complete a single row. In this blank peiod, a synchronisation pulse is transmitted to help keep the video card and monitor in step with each other.
The rate of production of horizontal scans is thus 80kHz, as 80kHz x 1688 = 135MHz. In a similar fashion, 1024 such rows, followed by a synchronisation pulse within a longer blank period, make up the vertical timing, with the picture repeating after enough time for 1066 rows. This gives the vertical refresh rate of 80kHz/1066=75Hz.
When configuring a monitor using XFree86, the precise length of the blank period, and the position and lengths of the synch pulse within it, can all be specified, along with the resolution and pixel clock. Most other X servers and graphical OSes are less flexible.
The "pixel clock" (135MHz) is limitted by how fast the digital to analogue converters on the video card can spit out the signal (and, indeed, how fast the video memory on the card can feed data to the DACs: if stored as 32 bits per pixel, even 135MHz is 540MB/s), and by how high a frequency the video cable and amplifiers in the monitor can cope with before distortion becomes significant. The horizontal and vertical refresh rates are further limitted by the ranges that the monitor is able to synchronise (or lock) to.
The synchronisation pulses may be transmitted on separate wires, or may be transmitted on the same wire that carries the green signal (sync on green). The former is generally preferred.
Modern monitors also have a low-speed serial data connection which runs over the video cable. It enables the computer to ask the monitor which refresh rates it supports (and who made it). This information is usually obtained only when the computer starts its graphical interface, so the order of booting and plugging in the monitor may matter.
Various power saving modes (DPMS) are signalled by dropping the horizontal and/or vertical synch. signals. A monitor which does not understand this will be very confused, and may attempt to display a picture in the absence of a synch. signal.
The analogue signal works well with CRTs. With LCD screens it works less well: LCDs are not scanned by an electron beam, and have distinct pixels which are somewhat like DRAM cells. They also don't need rapid refreshing, as any flickering is normally caused by their fluorescent back-lights. The response time of the display is usually >10ms, and anyway the cells can be kept actively on with no need for any refresh.
When presented with an analogue signal, they need to divide up the long horizontal scan of 1280 pixels into 1280 distinct pixels, and to do this precisely the same way each time the signal is retransmitted (i.e. 75 times a second). Any drift, even 0.1% or less, will show up as quite severe shimmering, particularly around sharp boundaries between black and white areas of the display.
The digital DVI signal is closely based on the above analogue scheme. The connector is a very different shape, with 24 pins for digital data, and a further five (R,G,B, Sync, ground) for analogue. Thus a DVI-I lead can carry an analogue signal (a DVI-D one lacks the analogue connections).
The lead has one pair of leads for each of red, green and blue, and one for a clock which runs at a maximum of 1.65GHz. The data on the RGB lines are sent as uncompressed bits, using an encoding of ten bits for each byte in a manner which minimises transitions. Hence the maximum pixel rate is 165MHz. The clock signal ensures perfect synchronisation between the video card and the monitor, and the digital nature of the signal ensures there is no degredation in the cable.
The timings for the signal are similar to those for the CRT monitor: there is still a blank period at the end of each scan. The maximum resolution for a DVI monitor is thus 1600x1200 or 1920x1080, both at 60Hz.
The DVI connector has one extra trick: the cable has enough wires for two video signal sharing a single clock signal. Equipment not wishing to use this "dual link" capability may omit the six pins that are used for the second link. Equipment using them can use up to 330M pixels/sec, or up to 2048x1536 at 60Hz.
DVI monitors always support a low-speed serial "Display Data Channel" connection to enable the monitor to tell the video card what capabilities it supports. It is not necessarily the case that a low-resolution monitor will support a 1.65GHz clock, for instance: a 1280x1024 (native) monitor here supports a 1.1GHz maximum, and thus does not support the VESA 1280x1024@75Hz standard, but merely 1280x1024@60Hz.