1080i, 720p, 24P . . . oh my!

30 frames, 60i, 25 frames, 50i, 29.97, 23.98.  Somebody help!

Here are links to two in-depth articles on the web that I found that does an excellent job in explaining what all these terms all mean: http://tinyurl.com/ysxs4h   http://www.answers.com/topic/24p

But if you want a quick-read summary, I’ll give it try.  Please read on:

Let’s start by asking, what makes you sense motion on film or television?

Well, it’s a series of still frames projected fast enough so that you perceive the result as one continuous motion. 

How fast do the frames need to be projected?

Well, for most of the history of film, the projection rate has been 24 frames per second.

But when television came in, being an electronic medium, it made sense to the developing engineers to base the frame rate of that to match the electrical systems around the world.  And the electrical system of the US is based on 60 hertz, or cycles, per second.

If you have traveled internationally, you know that the electrical systems in some other parts of the worl are based on 50 cycles.

So you might think it would have made sense to have 60 frames of video in the US and 50 frames elsewhere.  But that was thought to be overkill and take too much broadcast bandwidth, so the developing engineers came up with an ingenuous way that called for the scanning of every other line of the picture frame. 

The result was that within the 60 cycles of every second, the cycles would alternate such that one would scan only the odd numbered lines of a frame and the next one would scan the even numbered lines.  That process is called “interlacing.”  Do the math, and the result is 30 complete frames a second, still faster than the 24 frames per second of film.   And you can see then that in the countries with 50 cycle electricity, the result was 25 frames a second.

The quest was then on over the years to try to get television to look like film.  Well, the advent of high definition television has made that goal finally achievable.

First, the shape of the high def 16×9 rectangular wide screen matches that of a theatre film screen.

Then the high resolution of HDTV (many more lines per frame than standard definition) now rivals that of film.

While some HD formats utilize an “interlacing” line scan approach (e.g. 1080i), others are able to use a sequential line scan approach that is called “progressive.”  The thought is that a “progressive” approach provides a even clearer picture quality.  An example of that is the HD format 720p.

A final step in striving to achieve a film look for video, was when modern camera technology allowed for the shooting of HD video at 24 frames per second.  These video cameras and recorders still work off of regular 60 cycle electricity, but they manipulate the picture digitally (via a 3-2 pulldown process — again, refer to above link for more details on that) with the result being that 24 video frames are recorded in one second. 

OK, that takes care of the 24.  But what is the “P” in 24P all about? (notice it is usually written as a capital “P” when used with 24)

The “P” here does indeed refer to a progressive scan rather than interlaced, but this is not to be confused with the the “p” and “i” in the scanning of the lines in the original frame.  24P means that the the resulting frames created in the “pull-down” process mentioned above are scanned progressively. 

To demonstate the point further, consider this — it is possible to have a 1080i HD tape that has been recorded  in 24P.

Is your head starting to hurt yet? 

Well, just one further thing to make it swell a bit more.  When color television came in, there were technical interference issues in introducing that element to the NTSC television signal such that the nice and logical 30 frames of video had to be slightly adjusted to 29.97 frames per second.  And brother, have we “non-rocket scientists” had to work hard in wrapping our heads around that one! 

So sure enough, as a result. in our NTSC world, for all intents and purposes, what we call 24P is really 23.98 frames per second.  (Now there are exceptions to this where true native 24P can be carried through in the production workflow to display – see above mentioned in-depth articles for more on that)

Whew.  I hope that helps make things at least a bit clearer.  (I am sure some of you are saying, “Yeah, sure, David.”  Oh well, I tried.)

Regardless, just know that we can handle your 24P tapes here at Video Labs, both in dubbing and editing.

Please contact me with any questions.  dryan@videolabs.net

I also welcome your comments below.

You may also want to check out that Video Labs also offers HD captioning: http://tinyurl.com/6bpxje

. . . and SpoTTrac encoding: http://tinyurl.com/6yq8zo