fb.modes(8)

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>
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)