Post Go back to editing

Composite sync generation on AD724

Category: Datasheet/Specs
Product Number: AD724

Hello. 

I have a couple of queries regarding AD724.

1) Is it possible to drive only hsync and vsync to AD724 while grounding the RGB inputs to generate a composite sync signal?  

2) Can a composite sync signal without color burst be generated from AD724? The datasheet provides details on providing FIN for color burst but I do not require color burst for the composite sync. 

3) Will the chip support non-standard horizontal timing? The vertical timing (and the number of lines) will be as per NTSC/PAL standard.

Regards,

Akhil A

  • Hi,

       Please find the below comments,

    1) Is it possible to drive only Hsync and Vsync to AD724 while grounding the RGB inputs to generate a composite sync signal ?  

         YES, AD724 requires explicit horizontal and vertical synchronizing signals for proper operation. This information cannot and should not be incorporated in any of the RGB signals. However, the synchronizing information can be provided as either separate horizontal (HSYNC) and vertical (VSYNC) signals or as a single composite sync (CSYNC) signal.

       Note:   RIN, BIN, GIN are analog inputs that should be terminated to ground with 75 Ω in close proximity to the IC. When properly terminated the peak-to-peak voltage for a maximum input level should be 714 mV p-p.

    2) Can a composite sync signal without color burst be generated from AD724? The datasheet provides details on providing FIN for color burst but I do not require color burst for the composite sync. 

          Please note, "Luminance" output can produces a monochrome signal with no ColorBurst.  The ColorBurst signal is generated in the chroma processing section and is included in the "Chrominance" output and note that you have to connect the external oscillator for the monochrome path to work properly.

    3) Will the chip support non-standard horizontal timing? The vertical timing (and the number of lines) will be as per NTSC/PAL standard.

          We don't find any 'Non Standard' related timing details in AD724 datasheet. Please try it at your end.

    Thanks,

    Poornima

  • Thank you for the reply. 

    1) If I need a composite signal with picture & sync but without color burst (from Luminance output), do I need to provide FIN?

    2) If I provide a regular horizontal and vertical sync as in VGA (which has no equalisation or serration pulses), will AD724 add equalisation and serration pulses during the vertical blanking interval in the composite/luminance output? 

    Regards,

    Akhil

  • HI,

      Please find the below comments,

    1) If I need a composite signal with picture & sync but without color burst (from Luminance output), do I need to provide FIN ?

           FIN pin does not required since the LUMA signal does not contain any of the subcarrier and this makes it virtually impossible to accurately measure the subcarrier frequency of these luminance signals with any oscilloscope technique.

           This FIN pin mainly we use it to operate the chrominance output with proper frequency so depending on the layout, some circuits might require a small capacitor from FIN (Pin 3) to ground to operate with the chrominance at the proper frequency.

    2) If I provide a regular horizontal and vertical sync as in VGA (which has no equalisation or serration pulses), will AD724 add equalisation and serration pulses during the vertical blanking interval in the composite/luminance output? 

            If no equalization or serration pulses are included in the HSYNC input there won’t be any in the outputs. Although sync signals without equalization and serration pulses do not technically meet the video standards’ specifications, many monitors do not require these pulses in order to display good pictures. The decision whether to include these signals is a system tradeoff between cost and complexity and adhering strictly to the video standards.

    Thanks,

    Poornima

  • But horizontal sync does not contain equalisation and serration pulses, only composite sync does. Isn't this restriction meant for composite sync given through HSYNC pin?

    If so, will AD724 add these pulses when separate and regular active low HSYNC and VSYNC are given? 

  • Hi,

       As stated earlier, Since some monitors does not require these pulses and the decision whether to include these signals is a system tradeoff between cost and complexity and adhering strictly to the video standards.

        As per Page 9 in AD724 datsheet, HSYNC and VSYNC are two logic level inputs that are combined internally to produce a composite sync signal. If a composite sync signal is to be used, it can be input to HSYNC while VSYNC is pulled to logic HI (> +2 V).

    Please note two sync inputs HSYNC and VSYNC drive an XNOR gate to create a CSYNC signal for the AD724. If the user produces a
    true composite sync signal, it can be input to the HSYNC pin while the VSYNC pin is held high.

    Thanks,

    Poornima