Anybody who has ever tried to connect a CRT-based display to a computer or stimulus generator knows how difficult it can be to a find an acceptable combination of line-rates, frame-rates back- porches and other arcane parameters to provide the required resolution. So here is a resume of some the terminology used in the video world.

Raster scanning is the technique whereby an electron beam or spot of laser light is swept slowly from one side of the display area to the other and then returns quickly to the first side at a point slightly below where it started on the sweep above. The whole area is swept like this in a zig-zag or sawtooth fashion until the spot reaches the bottom right-hand corner from where is rapidly taken back to the top left-hand corner and the process begins again. As the beam is scanned, it is turned on and off, or modulated, to cause the presence or absence of a dot on the image plane. The grid of lines so-produced is called the raster, the short sweeps from left to right form the lines of the display and the slower sweep from top to bottom forms the frame. The quick return of the beam at the end of a line is called the line-flyback while the return from bottom to top of the screen is called the frame-flyback. In order not to disrupt the display during the flyback the beam is always turned off, or blanked, at this time. Well-designed systems keep the flyback time as short as possible so as not to waste valuable display time. In a laser printer the laser is scanned by rotating a mirror and only one complete scan is needed to draw the image onto the photosensitive drum while in a CRT the electron beam is scanned by a magnetic field and repeated scans are need to keep the display refreshed; each time the electrons impinge on the phosphor a small packet of light is produced which decays to nothing in about 1 or 2ms. From a technological point of view, the scanning in a laser printer is easier to achieve than that in the CRT as it doesn’t need to be performed in real time. That is to say that it is not important (within reason) how long it takes to scan the image as that will only affect the delay period between the computer sending the data and the first page being printed. In a CRT however, the scan time is dictated by the requirement to perform at least 60 or 70 complete scans per second in order to give the impression of a continuous display and this is much more difficult.

For historical reasons the evolution of CRT displays and laser printers have taken different paths; a typical laser printer comes complete with its own computer and framebuffer and needs only be sent the data (very easy) while CRT monitors are very simple and need a fully featured video signal to modulate their electron beams and synchronise to the raster. This is normally generated by the host computer or graphics card. The requirement to ensure that the raster on the display is locked to the video signal being supplied necessitates some extra synchronisation signals. It is conventional that the video generator, in this case the host computer system, should supply two additional bits of information to the display, a pulse to indicate the start of a scan line and a pulse to indicate the start of a frame. These are the line and frame synchronisation signals and they happen during the line flyback and the frame flyback respectively. It is then up to the display electronics to ensure that it scans in synchrony with the generator even though there is no method of feeding back if this is really the case. In an effort to keep to a minimum the wires between the display and the computer these are often combined together to form a combined (or H/V) sync signal or combined and added to the green video signal (composite sync). This should not be confused with composite video used in television where all the video and sync information is modulated onto one channel. One further term is needed in relation to the signals and that is porch; the time between the end of the active video and the start of the synchronisation pulse at the end of a scan line is called the horizontal front porch while the time at the beginning of each line during which the display is blanked between the end of the synchronisation pulse and the start of the active video signal is called the back porch. The equivalent times at the end and start of each frame are called the vertical front and back porches respectively.

Figure 13 A typical video signal

5.6.1    Monitor timing - a rough calculation

What are the important parameters for a display running at 120Hz with a resolution of 800 pixels by 600 lines?

Assume that the line and frame flyback times are 10% of the line and frame display times. Then:

Frame scan rate = 120Hz

Line scan rate = 120Hz . 600lines . 110% = 79.2kHz

Pixel rate = 79200Hz . 800pixels . 110% = 69.7MHz

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