local
Aug 1996
fbset
Tools for managing a frame buffer's video mode properties
NAME
fb.modes - frame buffer modes file
DESCRIPTION
/etc/fb.modes contains an unlimited number of video mode descriptions. The general format of a video mode is:
mode
geometry <xres> <yres> <vxres> <vyres> <depth>
timings <pixclock> <left> <right> <upper> <lower> <hslen> <vslen>
options <value>
endmodetimings <pixclock> <left> <right> <upper> <lower> <hslen> <vslen>
options <value>
OPTIONS
geometry options:
| xres | visible horizontal resolution (in pixels) |
| yres | visible vertical resolution (in pixels) |
| vxres | virtual horizontal resolution (in pixels) |
| vyres | virtual vertical resolution (in pixels) |
| depth | display depth (in bits per pixel) |
timing options:
| pixclock | |
| length of one pixel (in picoseconds) | |
| left | left margin (in pixels) |
| right | right margin (in pixels) |
| upper | upper margin (in pixel lines) |
| lower | lower margin (in pixel lines) |
| hslen | horizontal sync length (in pixels) |
| vslen | vertical sync length (in pixel lines) |
other options:
the first value of this options is the default
| hsync {low|high} | |
| the horizontal sync polarity | |
| vsync {low|high} | |
| the vertical sync polarity | |
| csync {low|high} | |
| the composite sync polarity | |
| extsync {false|true} | |
| enable or disable external resync. If enabled the sync timings are not generated by the frame buffer device and must be provided externally instead. Note that this option may not be supported by every frame buffer device | |
| laced {false|true} | |
| enable or disable interlace. If enabled the display will be split in two frames, each frame contains only even and odd lines respectively. These two frames will be displayed alternating, this way twice the lines can be displayed and the vertical frequency for monitor stays the same, but the visible vertical frequency gets halved | |
| double {false|true} | |
| enable or disable doublescan. If enabled every line will be displayed twice and this way the horizontal frequency can easily be doubled, so that the same resolution can be displayed on different monitors, even if the horizontal frequency specification differs. Note that this option may not be supported by every frame buffer device | |
INTERNALS
Generally a frame buffer display is organized as follows:
The length of one line can now be calculated with pixclock
and we have the horizontal frequency
To get the vertical frequency vtotal must eventually adjusted. If the display is laced, vtotal must be halved or if the display is a doublescan one, vtotal must be doubled. Now we can calculate the length of one frame
and we get also the vertical frequency
+---+---------------+---+---+
| | ^ | | |
| | | 5 | | |
| | v | | |
+---#################---+---+
| # ^ # | |
| # | # | |
| # | # | |
| 1 # | 2 # 3 | 4 |
|<->#<------+------>#<->|<->|
| # | # | |
| # | 6 # | |
| # | # | |
| # v # | |
+---#################---+---+
| | ^ | | |
| | | 7 | | |
| | v | | |
+---+---------------+---+---+
| | ^ | | |
| | | 8 | | |
| | v | | |
+---+---------------+---+---+
| | ^ | | |
| | | 5 | | |
| | v | | |
+---#################---+---+
| # ^ # | |
| # | # | |
| # | # | |
| 1 # | 2 # 3 | 4 |
|<->#<------+------>#<->|<->|
| # | # | |
| # | 6 # | |
| # | # | |
| # v # | |
+---#################---+---+
| | ^ | | |
| | | 7 | | |
| | v | | |
+---+---------------+---+---+
| | ^ | | |
| | | 8 | | |
| | v | | |
+---+---------------+---+---+
1 left margin
2 xres
3 right margin
4 horizontal sync len
5 upper margin
6 yres
7 lower margin
8 vertical sync len
2 xres
3 right margin
4 horizontal sync len
5 upper margin
6 yres
7 lower margin
8 vertical sync len
The area bordered with ‘#’ is the visible display area. Horizontal and vertical frequencies can now easily be calculated, for this the sum of horizontal or vertical values are important
htotal = left + xres + right + hslen
vtotal = upper + yres + lower + vslen
vtotal = upper + yres + lower + vslen
line = pixclock * htotal
hfreq = 1E12 / line = 1E12 / (pixclock * htotal)
if (lace) vtotal /= 2
if (double) vtotal *= 2
if (double) vtotal *= 2
frame = vtotal * line
vfreq = 1E12 / frame = hfreq / vtotal
