user5349: I'm trying to select the correct capture options for ffmpeg and I'm confused by this kind of listing:
[decklink @ 00000000010564c0]   17      3840x2160 at 24000/1001 fps
[decklink @ 00000000010564c0]   18      3840x2160 at 24000/1000 fps
[decklink @ 00000000010564c0]   19      3840x2160 at 25000/1000 fps
[decklink @ 00000000010564c0]   20      3840x2160 at 30000/1001 fps
[decklink @ 00000000010564c0]   21      3840x2160 at 30000/1000 fps

Based on the fact that 24 and 30 have two entries and 25 only has one, I assume that the two 24 options are true 24fps and 23.9fps, and the two 30 options are true 30fps and 29.97fps.
But which is which?  And why is this kind of notation used?

user17880: The original framerate for broadcast television was 60 fields (half of a frame) per second in the U.S., and timing was based on the oscillation of AC power (so, clearly, European television needed a different framerate under the oldest technology).
However, when color was added to NTSC, the color signal interfered the audio signal. By this time, more advanced means of timing were available, so the quick fix was to reduce the frame rate ever so slightly. Color was done slightly differently in PAL and this adjustment was not necessary so there is no 25000/1001.

user1871: Those are rational numbers. So, 24000/1000 = 24.000 and 24000/1001 = 23.976. These are in fact the exact representations i.e. 23.976 is an approximation but 24000 divided by 1001 isn't.
As for why 24000/1001 and not 24/1.001, it's because that's the smallest set of numbers that represent the value as integers. Else, a floating point variable would be needed for the denominator.

user17685: Yes, 24000/1000 is 24 fps and 24000/1001 is 23.976fps. Some refer to the X/1001 frame rates as "drop down" (as in "dropped down from the integer") but this is easy to confuse with "pull down" which often refers to the cadence of frames when fitting 24 or 25 fps material into a 30 fps program.
You can also think of these notations as 24,000 divided by 1,000 equals 24 and 24,000 divided by 1,001 equals 23.976023976 repeating. This notation is also the exact same thing as saying 24 divided by 1 equals 24 and 24 divided by 1.001 equals 23.976023976 repeating. It is, of course, quite ridiculous to articulate the repeating decimal values.
The 1001 options and their resulting notation are a legacy from analog color television systems. As Mulvya points out "it's because that's the smallest set of numbers that represent the value as integers". These standards are still around today mainly for backwards compatibility (with black and white television sets) and 'cause that's just the way TV grew up. Prior to color television, the NTSC frame rate was actually exactly thirty frames per second.
As for the 30000/1000 and 30000/1001 notation it comes from the NTSC video standard. "30"fps NTSC is actually displaying 30/1.001 frames per second (sometimes referred to as "29.97" or "30DF" for short, but to be precise 29.97002997... repeating frames per second). Due to this slight difference (.01%), NTSC uses either Drop-Frame (DF) or Non Drop-Frame (NDF) time-code counting. Drop-Frame "drops" frame counts in a pattern such that every ten minutes the time-code duration will correspond to the actual playback duration. No image frames are dropped and this was mainly a consideration for broadcasters fitting their programs with commercial breaks. Non Drop-Frame on the other hand more accurately represents a count of the total number of frames in a program (for those who enjoy counting in base 24:60:60:30...)
As for 25000/1000 there's no 25/1.001fps standard because 25fps is a PAL standard and PAL video systems were engineered using 50Hz AC power which did not have the same issues as NTSC when color came about.

5349: <p>I'm trying to select the correct capture options for <code>ffmpeg</code> and I'm confused by this kind of listing:</p>

<pre><code>[decklink @ 00000000010564c0]   17      3840x2160 at 24000/1001 fps
[decklink @ 00000000010564c0]   18      3840x2160 at 24000/1000 fps
[decklink @ 00000000010564c0]   19      3840x2160 at 25000/1000 fps
[decklink @ 00000000010564c0]   20      3840x2160 at 30000/1001 fps
[decklink @ 00000000010564c0]   21      3840x2160 at 30000/1000 fps
</code></pre>

<p>Based on the fact that 24 and 30 have two entries and 25 only has one, I assume that the two 24 options are true 24fps and 23.9fps, and the two 30 options are true 30fps and 29.97fps.</p>

<p>But which is which?  And why is this kind of notation used?</p>
 17880: <p>The original framerate for broadcast television was 60 fields (half of a frame) per second in the U.S., and timing was based on the oscillation of AC power (so, clearly, European television needed a different framerate under the oldest technology).</p>

<p>However, when color was added to NTSC, the color signal interfered the audio signal. By this time, more advanced means of timing were available, so the quick fix was to reduce the frame rate ever so slightly. Color was done slightly differently in PAL and this adjustment was not necessary so there is no <code>25000/1001</code>.</p>
 1871: <p>Those are rational numbers. So, <code>24000/1000 = 24.000</code> and <code>24000/1001 = 23.976</code>. These are in fact the exact representations i.e. 23.976 is an approximation but 24000 divided by 1001 isn't.</p>

<p>As for why <code>24000/1001</code> and not <code>24/1.001</code>, it's because that's the smallest set of numbers that represent the value as <strong>integers</strong>. Else, a floating point variable would be needed for the denominator.</p>
 17685: <p>Yes, <code>24000/1000</code> is 24 fps and <code>24000/1001</code> is 23.976fps. Some refer to the X/1001 frame rates as "drop down" (as in "dropped down from the integer") but this is easy to confuse with "pull down" which often refers to the cadence of frames when fitting 24 or 25 fps material into a 30 fps program.</p>

<p>You can also think of these notations as <code>24,000 divided by 1,000 equals 24</code> and <code>24,000 divided by 1,001 equals 23.976023976 repeating</code>. This notation is also the exact same thing as saying <code>24 divided by 1 equals 24</code> and <code>24 divided by 1.001 equals 23.976023976 repeating</code>. It is, of course, quite ridiculous to articulate the repeating decimal values.</p>

<p>The <code>1001</code> options and their resulting notation are a legacy from analog color television systems. As Mulvya points out "it's because that's the smallest set of numbers that represent the value as integers". These standards are still around today mainly for backwards compatibility (with black and white television sets) and 'cause that's just the way TV grew up. Prior to color television, the <a href="https://en.wikipedia.org/wiki/NTSC" rel="noreferrer">NTSC</a> frame rate was actually exactly thirty frames per second.</p>

<p>As for the <code>30000/1000</code> and <code>30000/1001</code> notation it comes from the NTSC video standard. "30"fps NTSC is actually displaying 30/1.001 frames per second (sometimes referred to as "29.97" or "30DF" for short, but to be precise 29.97002997... repeating frames per second). Due to this slight difference (.01%), NTSC uses either Drop-Frame (DF) or Non Drop-Frame (NDF) time-code counting. Drop-Frame "drops" frame counts in a pattern such that every ten minutes the time-code duration will correspond to the actual playback duration. No image frames are dropped and this was mainly a consideration for broadcasters fitting their programs with commercial breaks. Non Drop-Frame on the other hand more accurately represents a count of the total number of frames in a program (for those who enjoy counting in base 24:60:60:30...)</p>

<p>As for <code>25000/1000</code> there's no 25/1.001fps standard because 25fps is a <a href="https://en.wikipedia.org/wiki/PAL" rel="noreferrer">PAL</a> standard and PAL video systems were engineered using 50Hz AC power which did not have the same issues as NTSC when color came about.</p>
