Calculating Video Bandwidth for VGA Systems

In 1987 IBM introduced the VGA video standard using RGB analog signals, which specifies a 640x480-pixel format with 60 Hz refresh rate and the capability of producing sixteen colors. The analog RGB standard quickly became known as the VGA standard, and most computers today still retain the 15-pin, D-Sub blue connector for VGA monitors. The VGA standard is now maintained by the Video Electronics Standards Association (VESA), which defines the signals and how to measure and test them.

Today the VGA family includes an alphabet soup of names, ranging from the original VGA (Video Graphics Array) up through UXGA (Ultra Extended Graphics Array). Industry experts
expect that this standard will remain a part of the PC world until 2015 or later.

Some systems must route the VGA signal through an amplifier or switch, yet it is not obvious how a designer calculates the bandwidth required for such a switch or amplifier. Making that calculation requires some mathematics and an understanding of CRT monitors. Since the CRT was once the most common display device by far, a basic video bandwidth calculation still depends on the physics of the old-style CRT monitors.

Even though CRT monitors are rapidly being displaced by LCD monitors, they are part of the standard, and one must therefore understand them to do the bandwidth calculations. Within a CRT, the electron beam is magnetically deflected across the screen from top to bottom, thereby tracing a "raster" pattern (see Figure 1). Information is displayed during the active part of the display (the trace). At the end of a trace the beam must return to the left side to begin another trace, and during that re-trace it must be invisible, which is accomplished by "blanking" it to the black level. The time taken for re-trace must be taken into account. Similarly, a vertical re-trace to the start position is made when the beam reaches the bottom of the CRT, and that time must also be accounted for in the bandwidth calculation.

Example: 1280x1024 pixels; 60Hz refresh

For a 60Hz refresh rate, the time required to display one frame is 16.6 ms, and we assume the vertical retrace time is 1.6 ms. That leaves 15 ms for the active scan time. If a frame includes 1024 active lines, then each line is completed in 14.6 ms. We assume the horizontal-retrace time is 1.8 ms, leaving 12.8 ms for the active trace.

We need to display 1280 pixels in 12.8 ms, so the duration of each pixel is 10 ns. To calculate the worst-case bandwidth needed, we assume an alternate on, off, on, off display of any color vs. white. If, for example, we apply a square wave to the green signal, with 10 ns per pixel, the result is a square-wave with a 20 ns period, which is 50 MHz (see Figure 2).

This waveform can display alternate G, BK, G, BK (512 alternating green/black pixels) across the screen. What happens, however, if the waveform is a sine wave instead of a square wave?

Figure 3 shows a sine wave superimposed on a square wave. If the sine wave alone is displayed on a CRT monitor, it is clear that the average value of the pixel would be less, and that it would not become as intense as quickly. The pixel would be a little "fuzzy," achieving full brightness only in the middle of its 10ns interval. In place of a nice crisp dot, the sine wave signal would produce edges somewhat less defined, or "smeared."

A perfect signal passing though a 50 MHz filter might produce a usable display, but the image would not appear as "crisp" as that produced by a device with a wider-bandwidth. Because most of today's monitors are based on the LCD instead of the CRT, it is no longer obvious how much bandwidth is necessary. We know, however, that the system must be able to pass the fundamental frequency as shown in the Table.

The Table below lists the combination of horizontal and vertical pixels, refresh rate, and fundamental video frequency corresponding to each standard of resolution. It is assumed that 66% of the signal is active, and the video bandwidth equals the fundamental frequency. Note that increasing the refresh rate (from 60 Hz to 80 Hz, for example) has the same effect on bandwidth as does an increase in the pixel rate. For today's LCD monitors, the refresh rate for optimum viewing is rarely higher than 60 Hz. Thus, a reasonable rule of thumb for determining the video bandwidth of any amplifier or switch in the system is 2x to 3x the fundamental frequency.

This table is useful when selecting components to meet one or more resolutions. For example, if the target resolution is a VGA switch (1280 X 1024 at 60Hz), we see that the bandwidth (fundamental) is 60 MHz. As an example, the MAX4885 or MAX4887 VGA switch chips offer a bandwidth of greater than 400 MHz. The fundamental and 3rd harmonic(180 MHz) pass almost unattenuated. Either of these switches would work quite well, inserting less than 0.5 dB loss into the system, with no need for video buffers. If a video buffer were to be used, then the designer should select a buffer with a bandwidth of greater than 200 MHz to match the switch, such as the MAX4219.