Friday, March 2, 2012

29.97 frames per second?

A helpful student recently pointed out that at the rate of 29.97 frames per second, it would take slightly longer than a second to complete scanning a full 30 frames.

Great observation on his part. But, he wanted to know why. ... Ouch.

To answer the question, we need a bit of a history lesson. When the NTSC (and others) were playing with adding colour to the TV signal in the 1950s, they only had 6MHz of space to play with. With the monochrome system already in place, they had to figure out a way to get the colour in there, within the same space, and make it compatible with the existing equipment.

For those that sat in high school math class thinking, "I'll never need math once I leave school," hang on ... you will.

NTSC B/W line frequency was 15,750 hertz. The folks that were working on this problem found that info modulated on the carrier frequency seemed to be grouping itself around the harmonics of the line frequency, leaving places on the carrier where no info was being carried.

What's a harmonic? A harmonic is a frequency that changes by adding the initial/fundamental frequency to itself, again and again. It would look like 1, 2, 3, 4 ... as one gets added into the mix each time. It's different than doubling ... which would look like 1, 2, 4, 8 ...

Empty spaces were found at the odd harmonics of half the line frequency (7,875 lines per second). Thus, info could be inserted in these spaces without causing interference. Inserting the colour info into these spaces meant that the new colour system would be compatible with the old B/W system.

But ... they chose the 455th harmonic as the color subcarrier frequency (reasons edited out for brevity). The audio carriers were set at 4.5MHz above the video carrier for all TV stations. This means that regardless the TV station's video carrier frequency, the audio carrier was 4.5MHz above it. As a result, they found that the color subcarrier placed in the 455th harmonic caused interference around 900 kilohertz that was visible as black and white wavy lines running through the picture.

To solve the problem, they slowed the existing line frequency by 16 lines to 15,734 per second. All TV sets were built to receive signal within 1% of the line frequency, so that 16 line adjustment wouldn't cause a problem.

This tiny adjustment produced a new frame rate of 29.97 frames per second.

Wow. That was a mouth full. There's obviously more to it than my simplified version. If you're interested, I could recommend a few good books on the subject.


1 comment:

Anonymous said...

That's actually fascinating. Please, recommend reading. I remember wondering why that was a.long time ago, but that was before Wikipedia, and I never looked in to it again. Sounds like a great example of engineering between the margins of the existing and the possible.