Posted on 12:53 pm on September 1, 2014 by

Why is 29.97 Significant?

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If you’ve been following the recent run of monthly newsletters covering our latest tech for the video industry, you will have come across the title 29.97. Firstly, why the name? Well, it is common knowledge in our industry that the video on your televisions in North America play at 30 frames per second (fps) – unless you reside in Europe where it is 25fps.

Frame Rate Per Second

Unknown to many is the frame rate isn’t strictly 30fps – this is just an approximation, the exact frame rate is 29.97. Why this seemingly random number?

Frame rate describes both the speed of recording and the speed of play back. In order to make video play back at a fixed rate there needs to be some kind of timing circuit. Back in the early black and white days, engineers didn’t benefit from the high tech silicon based chips in which we implement today. They had to improvise by using the oscillation of AC electricity as the basis for the timing circuit.

Early television systems selected frame rates based on local electrical standards to avoid electrical interference with the picture. NTSC in the North America uses 30 fps based on 60 Hz electrical mains. PAL, used primarily in Europe, uses 25 fps based on 50 Hz electrical power.

However all this changed with the introduction of colour NTSC. The colour carrier signal was phasing with the sound carrier signal because they were very close in the spectrum. This ultimately distorted the picture and made it unwatchable. Engineers quickly fixed the problem by reducing the frame rate slightly by .03fps – which moved the two signals out of phase of each other.

This frame rate has become the standard for video broadcast ever since.


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  • Tim Burton

    Nearly.

    Cameras were locked to mains at studio so you didn’t get strobing under the lights (ever seen PAL cameras used in the US?) however the consumer sets don’t use the mains for lock, otherwise phase would be an issue under when it matched 1:1; how you can you guarantee studio and consumer are on the same mains supply? You’d get offset issues like an episode of the ‘Outer limits’. Video signal carries sync signal both horizontally and vertically. If the US sets were locked to their 60hz mains, they’d not be able to play a 59.94hz image.

    Interestingly enough we use GPS for genlock these days :)

    • http://aframe.com/ Aframe

      Thanks for the extra insight Tim.

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