[Openchrome-users] X11 Modlines and Meaning for TVout on vt1625 for horizontal position

[Eric Gilbert]

I found the following link while trying to learn more about modlines
and what sort of tweaks will be needed to get the vt1625 tvout on the
composite working.

http://www.3dexpress.de/#FaqX11Modeline

I was able to follow the explanation/equation but I have some questions.

"1024x768 at 100" 126.64 1024 1056 1536 1568 768 781 794 807

    pixel frequency: 126.64 MHz
    horizontal: active      =             1024 pixels
                front porch = 1056-1024 =   32 pixels
                sync        = 1536-1056 =  480 pixels
                back porch  = 1568-1536 =   32 pixels
                total       =             1568 pixels
    vertical:   active      =              768 lines
                front porch = 781-768   =   13 lines
                sync        = 794-781   =   13 lines
                back porch  = 807-794   =   13 lines
                total       =              807 lines
    horizontal frequency: (pixel frequency) / (horizontal total)
                            = 126.64 / 1568 * 1000 = 80.76 KHz
    vertical frequency: (horizontal frequency) / (vertical total)
                            = 80.76 / 807 * 1000 = 100.08 Hz

I have the following output in my Xorg.0.log

(II) VIA(0): SharpTV: Using hsync value of 36.00 kHz
(II) VIA(0): SharpTV: Using vrefresh value of 60.00 Hz
(II) VIA(0): Clock range:  20.00 to 230.00 MHz
(--) VIA(0): Virtual size is 720x480 (pitch 720)
(**) VIA(0): *Default mode "720x480Under": 28.2 MHz (scaled from 0.0
MHz), 36.0 kHz, 60.0 Hz
(II) VIA(0): Modeline "720x480Under"   28.23  720 728 744 784  480 490
496 600 -hsync -vsync
(**) VIA(0): Display dimensions: (400, 300) mm
(**) VIA(0): DPI set to (45, 40)


So taking that modline from the log I plugged in the following to the
reference equation.

"720x480 at 60" 28.23 720 728 744 784 480 490 496 600

    pixel frequency: 28.23 MHz
    horizontal: active      =              720 pixels
                front porch =  728-720  =    8 pixels
                sync        =  744-728  =   16 pixels
                back porch  =  784-744  =   40 pixels
                total       =              784 pixels
    vertical:   active      =              480 lines
                front porch = 490-480   =   10 lines
                sync        = 496-490   =    6 lines
                back porch  = 600-496   =  104 lines
                total       =              807 lines
    horizontal frequency: (pixel frequency) / (horizontal total)
                            = 28.23 / 784 * 1000 = 36.00 KHz
    vertical frequency: (horizontal frequency) / (vertical total)
                            = 36.00 / 600 * 1000 = 60.00 Hz

Is this appropriate for TVout on an NTSC TV? Any thoughts on why the
front porch and back porch values are not equal in my equation, and
should they be? Also the sync from the second equation is much smaller
in relation to the sync in the second, does that mean the modline from
the log is being mis calculated and where does that come from.

At line 795 in via_vt162x.h I see the following being set for register
50-58 which I assume is in hex for for the 720x480Under is defined.

 0x0F, 0xCF,  0x23, 0x57, 0x22, 0x59, 0x83, 0x7F, 0x23

and translated to decimal I see

  15,  207,    35,   87,   34,   89,  131,  127,   35

>From the output from PrintTVRegs it looks like when I use
720x480Under at 24 with svn181 the HTP (Horizontal Total Pixels) is 89
with the HAP (Horizontal Active Pixels) 131 and the HSW (Horizontal
Sync Width) is 127. Why would the HTP be less than the HPA and why is
the HSW so large?

Thanks,
Eric