Latest post Mon, Apr 19 2021 6:21 AM by Pat Horridge. 3 replies.
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  • Sat, Apr 17 2021 5:23 AM

    Why 29.97 rather than 30 fps?

    Why use 29.97 rather than 30 fps (or 23.97 rather than 24)?

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  • Sat, Apr 17 2021 7:01 AM In reply to

    Re: Why 29.97 rather than 30 fps?

    Black and White TV in the 60Hz world was 30.00fps. When color was added in 1953, the frame rate had to be lowered ever so slightly to 29.97fps, due to potential interference between subcarrier frequencies.


    When a transmitter broadcasts an NTSC signal, it amplitude-modulates a radio-frequency carrier with the NTSC signal just described, while it frequency-modulates a carrier 4.5 MHz higher with the audio signal. If non-linear distortion happens to the broadcast signal, the 3.579545 MHz color carrier may beat with the sound carrier to produce a dot pattern on the screen. To make the resulting pattern less noticeable, designers adjusted the original 15,750 Hz scanline rate down by a factor of 1.001 (0.1%) to match the audio carrier frequency divided by the factor 286, resulting in a field rate of approximately 59.94 Hz. This adjustment ensures that the difference between the sound carrier and the color subcarrier (the most problematic intermodulation product of the two carriers) is an odd multiple of half the line rate, which is the necessary condition for the dots on successive lines to be opposite in phase, making them least noticeable.

    https://en.m.wikipedia.org/wiki/NTSC


    Film for cinema was shot and projeted at 24.000fps, and digital cinema still is. To convert 24fps film sources to 60Hz TV, a 3:2 pulldown was used, where the 24 original frames would be divided across the 60 fields of the video signal. When the signal was slowed down to 59.94Hz (29.97fps), they also had to slow down the 24fps film source down to 23.976, in order to maintain the same consistent 3:2 pulldown. Obviously, sound had to be slowed down by just as much, or you gradually lose sync.

    In the HD world, at some point, 23.976fps was also made available as an acquisition frame rate, in order to eliminate the need to slow down picture (and sound) from 24.000 to 23.976. Also, many consumer camera's with a '24p' shooting mode, actually shoot 23.976fps. Which is obviously a different framerate, and should always be labeled as such (but often isn't).

     

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  • Mon, Apr 19 2021 4:14 AM In reply to

    Re: Why 29.97 rather than 30 fps?

    Thanks for the explanation.

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  • Mon, Apr 19 2021 6:21 AM In reply to

    Re: Why 29.97 rather than 30 fps?

    Also worth noting that 23.976 is often referred to as 23.98 which is fine but if you do a speed conversion you must use the correct value 23.976

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