Shop OBEX P1 Docs P2 Docs Learn Events
vga composite sync — Parallax Forums

vga composite sync

Lee MarshallLee Marshall Posts: 106
edited 2007-10-19 23:19 in Propeller 1
ive learnt most of the stuff i know about vga from this page:
web.mit.edu/6.111/www/s2004/NEWKIT/vga.shtml
and i notice it talks about a composite sync embedded in the green channel.
is this absolutely necessary for proper vga operation??

thanks in advance.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
I hear and I forget. I see and I remember. I do and I understand
-Confucius

Comments

  • deSilvadeSilva Posts: 2,967
    edited 2007-10-18 22:06
    AFAIK it's an: either - or
    And not all monitors accept a composite sync...

    BTW: Very nice graphics on that link...
  • Lee MarshallLee Marshall Posts: 106
    edited 2007-10-18 23:48
    would it work if, instead of doing the front/back porches with the color wires(R,G,B) at ~0.3v and dropping to 0 on sync, they were at 0v all the time?

    it just seems that without the "optional" 470ohm resistors on the RGB DAC on the protoboard, the color lines would only be capable of two different levels.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    I hear and I forget. I see and I remember. I do and I understand
    -Confucius

    Post Edited (Mr Crowley) : 10/18/2007 11:55:12 PM GMT
  • deSilvadeSilva Posts: 2,967
    edited 2007-10-19 00:13
    There are no "porches" in the sense as on a TV signal with VGA, where it is different from a "black" luma level; there is just "low colour" on all three color lines, or maybe even not. They have no effect when they are out of the painted pixel area of the screen.

    There is either on digital line for each color, or (optionally) two, so you have either 8 colors (including "black" and "white") or 64.
  • Ken PetersonKen Peterson Posts: 806
    edited 2007-10-19 00:39
    I have recently successfully modified the VGA driver to generate a composite sync for an analog RGB LCD display. It was easier than I thought it would be.

    VGA has RGB + Hsync and Vsync. Analog RGB (with composite sync) is the exact same signal, but with the Hsync and Vsync lines XOR'd together. It is almost trivial to do this in software with the VGA driver - two lines of code changed in the ASM section.

    @deSilva: There are "porches" with RGB and composite sync. These porch widths are critical in my experience. It can make the difference between having a stable image and having garbage. There is a fine line between having everything displayed on the screen without being cut off, and having the whole thing get scrambled. [noparse][[/noparse]edit] the porches are only significant on the sync line, the RGB lines only have blanking intervals, not porches[noparse][[/noparse]/edit]


    @Mr. Crowley: some types of RGB mix the sync signal with the green. It's called Sync on Green, or SoG. This is often done to save a wire.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔


    The more I know, the more I know I don't know.· Is this what they call Wisdom?

    Post Edited (Ken Peterson) : 10/19/2007 11:22:31 PM GMT
  • Lee MarshallLee Marshall Posts: 106
    edited 2007-10-19 01:22
    thanks for replies, i assume that the monitor i have doesnt need SoG as i read somewhere that this is on older monitors.
    i would also like to know: what is the way in which the videoscale reg is implemented, what if i were to change its contents during a video cycle(whilst the generator was shifting out pixels). would it immediately change, or would it wait for the end of the cycle, then act on the new settings?

    i need to be able to increase the pixelclocks/frameclocks from 1 to 4, as soon as the waitvid command hands over the video data, not before/after.
    my plan is to use 4x4 two-color tiles each using 1 bit of memory on a 1024x768 screen, giving an effective resolution of 256/192 making the whole screen take up 6KB.
    i know i could probably tweak the current vga driver for this, but...well....as the sig says.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    I hear and I forget. I see and I remember. I do and I understand
    -Confucius

    Post Edited (Mr Crowley) : 10/19/2007 12:43:45 PM GMT
  • Ken PetersonKen Peterson Posts: 806
    edited 2007-10-19 23:19
    IF you set videoscale to 4, then each pixel is shifted out over four clocks instead of every clock. This has the effect multiplying the horizontal size of each pixel. I don't think it has an effect on the blanking areas, at least in the VGA driver, because it is changed in real time by the software to determine the width of the front/back porches and sync pulses.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔


    The more I know, the more I know I don't know.· Is this what they call Wisdom?
Sign In or Register to comment.