fb (5) Linux Manual Page

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>

endmode

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:

+—+—————+—+—+
|   |       ^       |   |   |
|   |       | 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

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

The length of one line can now be calculated with pixclock

line = pixclock * htotal

and we have the horizontal frequency

hfreq = 1E12 / line = 1E12 / (pixclock * htotal)

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

if (lace)   vtotal /= 2
if (double) vtotal *= 2

frame = vtotal * line

and we get also the vertical frequency

vfreq = 1E12 / frame = hfreq / vtotal

See Also

fbset(8), fbdev(4)