why odd frame rates like 23.976 ?

[lovelove]

Hi. I was wondering why odd frame rates are used like
23.976 (=24 × 1000/1001)
59.94 (= 60 x 1000/1001)
?

[hello_hello]

It relates back to analogue TV days and the refresh rate used.
NTSC Lines and refresh rate (http://en.wikipedia.org/wiki/NTSC#Lines_and_refresh_rate) (second paragraph).

[lovelove]

thank you.

The NTSC field refresh frequency in the black-and-white system originally exactly matched the nominal 60 Hz frequency of alternating current power used in the United States. Matching the field refresh rate to the power source avoided intermodulation (also called beating), which produces rolling bars on the screen. When color was later added to the system, the refresh frequency was shifted slightly downward to 59.94 Hz to eliminate stationary dot patterns in the difference frequency between the sound and color carriers

I did suspect that.
But with analogue TV days now long gone by, why do we still complicate our lives by encoded digital video in odd frame rates?

[mpucoder]

Because analog tv is NOT gone. Yes all high power television stations in the US broadcast in digital, and all sets sold must receive digital. But there is a huge installed base of analog receivers that will take time to be replaced. We can't just stop supporting them overnight and force everyone to buy new receivers. And there are still analog broadcasters (low power stations do not have to convert to digital yet).
Also there is a huge library of material recorded in NTSC, digitizing it works better at its native refresh rate.

[lovelove]

that makes sense. very interesting read, :thanks: you
so odd frame rates won't disappear in the foreseeable future, right?

The NTSC field refresh frequency in the black-and-white system originally exactly matched the nominal 60 Hz frequency [...] which produces rolling bars on the screen. [...] was shifted slightly downward to 59.94 Hz

I get that. But why didn't they just use 59 Hz then instead of 60 Hz?
That would have eliminated the beating/intermodulation and still have provided a "round" framerate of 59 fps (so that each second of film always has an equal amount of frames).

[Sharc]

For easy technical implementation. The 59.94 are locked to (i.e. derived from) the 60Hz power frequency: 59.94006 = 60x1000/1001.
1000 and 1001 are integer numbers which are more easy for technical (hardware) realization.

[lovelove]

I know that 60 x 1000 / 1001 ~ 59.94006 (see OP!)
so what?
how is *1000/1001 easier than *59/60 ?

And even if it were easier (which is still unclear) then it's not a
good argument to do it because it's easier, when it was easily
foreseeable that odd framerates bring along all sort of problems.

[mpucoder]

It is a little more complicated than 1000/1001, which is an approximation. For a good explanation of how the horizontal rate was lowered, resulting in a lower frame rate, see http://en.wikipedia.org/wiki/NTSC
The biggest considerations were to eliminate the existing audio interference with the video, add color, and maintain compatibility with monochrome sets.
One thing not explained in that article is why the particular ratio of chrominance subcarrier to horizontal line rate. It is 455/2 - 455 is the product of three prime numbers (5*7*13), virtually eliminating the chance of harmonic interference.

[Gavino]

It is a little more complicated than 1000/1001, which is an approximation. For a good explanation of how the horizontal rate was lowered, resulting in a lower frame rate, see http://en.wikipedia.org/wiki/NTSC
Isn't 59.94 the approximation, and 1000/1001 the exact ratio?

Reading that article, I see the line frequency was changed from 15750 Hz to 4.5MHz/286, which is a factor of 4500/(15.75 x 286), or exactly 1000/1001.

[mpucoder]

I never realized it was exact since the ratio is arrived at in a round about way based on other factors. But you're correct, it is exact.