LM1881 to convert C-sync to H-sync and V-sync

Discussion in 'General Electronics Chat' started by hang-on, Nov 24, 2014.

  1. hang-on

    Thread Starter New Member

    Oct 18, 2014
    25
    1
    Hi,

    I'm working with composite and RGB video signals from an old SEGA Master System. The video signals will eventually be upscaled and converted to VGA.

    This question is related to my growing aspirations to understand the LM1881 Video Sync Separator, which is part of the above project. I have read the datasheet, and also some documents on the composite video waveform, and I can undertand how I can feed composite video into LM1881 pin 2, and get 'cleaned' composite sync signals (vertical sync + horisontal sync) from pin 1, and isolated vertical sync from pin 3. From pin 5 the chip outputs "burst/back porch sync". From looking at the diagrams, I'm almost ready to believe that this burst/back porch signal could also be called horisontal sync. Is this correct? Or if this is not correct, what is horizontal sync then, and can one get it from the LM1881.

    My obsession with hsync and vsync stems from the VGA specs, that need separate sync signals to go with RGB. I know that the LM1881 is not an upscaler. I just want to understand a little better what is going on in the circuit, and how I could, if needed, produce hsync from a composite signal.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    The back porch is the portion of the signal from the rising edge of the horizontal sync pulse until the start of active video, and is used to reset the black level. It also contains the color burst signal which sets the zero reference for the color difference signals.

    A general method of separating signals is to compare their voltage levels with the surrounding levels. The following article and the references may be useful to you.

    http://en.wikipedia.org/wiki/Analog_television
     
    Last edited: Nov 24, 2014
  3. hang-on

    Thread Starter New Member

    Oct 18, 2014
    25
    1
    OK, thanks! In case of VGA, where we have RGB on separate lines instead of a color burst, the required hsync is then the portion of the composite sync signal from front porch to breezeway (terms from the datasheet). I'm trying to imagine what hsync looks like - or 'where' on the composite sync signal the hsync is located.
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    The hsync signal is the low going pulse that starts at the end of the front porch and ends at the beginning of the back porch. The vsync signal occurs twice per frame; once after the odd lines and once after the even lines.
     
  5. hang-on

    Thread Starter New Member

    Oct 18, 2014
    25
    1
    I think I got it now. Thanks for helping me out! :)
     
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