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Video Sync help — Parallax Forums

Video Sync help

BeauHoundBeauHound Posts: 6
edited 2007-10-10 00:14 in Propeller 1
I need to create frame pulses at different speeds which stay in phase with the propeller composite out. For example output a 23.98hz pulse output which stays in phase with the 29.97fps video ouput. The video output is used to genlock other hardware. Can I use the VSCL frameclock to detrimine timing? Any ideas on how to do this? Thanks.

Comments

  • BeanBean Posts: 8,129
    edited 2007-10-09 23:28
    I don't see how it would even be possible to keep a 23.98 Hz signal in phase with a 29.97 Hz signal ???

    Bean.

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  • Graham StablerGraham Stabler Posts: 2,510
    edited 2007-10-09 23:48
    me neither, two signals of differing frequency will have a constantly varying phase, they will drift past each other and there is nothing you can do about that.

    Graham
  • deSilvadeSilva Posts: 2,967
    edited 2007-10-09 23:50
    The video driver - ALL video drivers - generate the sync signals in a very specific routine. This is not exactly very tight timing, you can add any additional sync you want. In fact even the HUB gets informed during field and frame retrace.

    Beans question however seems unquestionable smile.gif
  • BeauHoundBeauHound Posts: 6
    edited 2007-10-09 23:50
    what i meant is for the 23.98 to derived from the same counter. 23.98 is 4/5 29.97.
  • deSilvadeSilva Posts: 2,967
    edited 2007-10-09 23:59
    BeauHound said...
    what i meant is for the 23.98 to derived from the same counter. 23.98 is 4/5 29.97.

    Alright, so you need 4 signals during 5 frames or 2 during 5 fields or 1 each 2.5 fields.

    So you should count the fields and output alternately at the start or in the middle of filed (or skip one).

    I think there should be even time to output the line number to the HUB, so you can even handle that in SPIN with a jittter of around 20 us.

    But the line retrace is a little bit tighter as it toggles the colour burst... I shall have a look tomorrow, if you have further questions.
  • HarleyHarley Posts: 997
    edited 2007-10-10 00:03
    My calculator 'says' you have 1.249791493 cycles of the 23.98 Hz signal during the 29.97 fps timing. No relationship. Not even a 4:5 ratio.

    But almost close. Was that a slide-rule calc? turn.gif

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    Harley Shanko
  • BeauHoundBeauHound Posts: 6
    edited 2007-10-10 00:14
    Thanks harley but I are taklking about rounded frame rates 30 ->29.97 24->23.98 video ->film. There is a relationship if you have ever seen a film on a tv. All films are transferred at 23.98(23.976) not 24 because they are displayed at 29.97 with a 3:2 field pulldown. Blame color tv for the number misunderstandings.
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