XFree86 Video Timings HOWTO : Interpreting the Basic Specifications
Previous: What these control:
Next: About Bandwidth:

## 5. Interpreting the Basic Specifications

This section explains what the specifications above mean, and some other things you'll need to know. First, some definitions. Next to each in parens is the variable name we'll use for it when doing calculations

horizontal sync frequency (HSF)

  Horizontal scans per second (see above).

vertical sync frequency (VSF)

  Vertical scans per second (see above). Mainly important as the upper limit on your refresh rate.

dot clock (DCF)

  More formally, `driving clock frequency'; The frequency of the crystal or VCO on your adaptor --- the maximum dots-per-second it can emit.

video bandwidth (VB)

  The highest frequency you can feed into your monitor's video input and still expect to see anything discernible. If your adaptor produces an alternating on/off pattern, its lowest frequency is half the DCF, so in theory bandwidth starts making sense at DCF/2. For tolerably crisp display of fine details in the video image, however, you don't want it much below your highest DCF, and preferably higher.

frame length (HFL, VFL)

  Horizontal frame length (HFL) is the number of dot-clock ticks needed for your monitor's electron gun to scan one horizontal line, including the inactive left and right borders. Vertical frame length (VFL) is the number of scan lines in the entire image, including the inactive top and bottom borders.

screen refresh rate (RR)

  The number of times per second your screen is repainted (this is also called "frame rate"). Higher frequencies are better, as they reduce flicker. 60Hz is good, VESA-standard 72Hz is better. Compute it as

  > `

  >     	RR = DCF / (HFL * VFL)

  > `

  Note that the product in the denominator is not the same as the monitor's visible resolution, but typically somewhat larger. We'll get to the details of this below.

  The rates for which interlaced modes are usually specified (like 87Hz interlaced) are actually the half-frame rates: an entire screen seems to have about that flicker frequency for typical displays, but every single line is refreshed only half as often.

  For calculation purposes we reckon an interlaced display at its full-frame (refresh) rate, i.e. 43.5Hz. The quality of an interlaced mode is better than that of a non-interlaced mode with the same full-frame rate, but definitely worse then the non-interlaced one corresponding to the half-frame rate.

### 5.1. About Bandwidth:

### 5.2. Sync Frequencies and the Refresh Rate:

XFree86 Video Timings HOWTO : Interpreting the Basic Specifications
Previous: What these control:
Next: About Bandwidth: