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[ The PC Guide | Systems and Components Reference Guide | Monitors | CRT Characteristics ]

Analog and Digital CRTs

All monitors today are called analog monitors, while older monitors are often called digital. This is kind of confusing--don't computers deal with digital information? How can the monitor be analog? Well, it isn't totally accurate to suggest that the monitor itself is really analog. The circuitry that controls it is still digital. What is analog is the color signals that are received from the video card (which of course is part of the main PC box and is also digital except for the circuit that generates the video signal, called the RAMDAC).

Originally, monitors used digital color signals, meaning each color had only a certain pre-set number of color levels that were supported. This was the case for CGA and EGA video cards and the monitors that work with them. Digital video signals are also sometimes called "TTL" for "transistor-to-transistor logic".

Starting with IBM's VGA standard, the switch was made to analog color to allow for more possible shades of each of the three primary colors. An analog signal can have any of a continuous range of values, so the number of different color levels is in theory unlimited. In practice, standard analog color normally uses a range of 256 different color values for each color, yielding a total of 16.7 million different colors.

All modern monitors and video cards use analog signaling. During the transition phase of the late 80s, when there were a lot of VGA cards in use but also many older adapters, some companies put out monitors that can respond to both analog and digital signals. These usually use a toggle switch on the back of the case to select which mode they are working in. Most of these are quite old and obsolete at this point.

Note: Calling a monitor "analog" or "digital" refers to the type of color signals it uses. Since analog monitors can also have analog or digital controls, some companies confusingly call their monitors "analog" or "digital" based on this. However, this is inaccurate; in almost every case the monitor itself is analog either way.

Next: Dot Pitch


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