# [FFmpeg-user] Converting a 23.98p source to 29.97i ProRes w/interlaced 3:2 pulldown?

Moritz Barsnick barsnick at gmx.net
Mon Oct 26 21:39:22 CET 2015

```On Mon, Oct 26, 2015 at 09:42:43 -1000, Mel Matsuoka wrote:
> > I believe (strongly) that what you like as output
> > framerate is not 29.97 but 30000/1001
>
> I guess I'm looking for the "why",  rather than the "what", as far as
> this syntax is concerned.

> In NTSC broadcast land, 29.97 frame rate is *always* defined and
> referred to as just that: 29.97. No professional video tool that I'm
> aware of requires the user to define that frame rate with the level
> of precision that dividing 30000 by 1001 gives you.

>From analog times. I think you can read some here:
https://en.wikipedia.org/wiki/NTSC#Lines_and_refresh_rate
though I'm sure there are better "sources" and explanations.

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
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 [...]

> And I suppose this begs the question, where do the numbers 30000 and 1001 come from to begin with?

>From the same page, different section:

To make the resulting pattern less noticeable, designers adjusted the
original 60 Hz field rate down by a factor of 1.001 (0.1%), to
approximately 59.94 fields per second.

So that's just how it was defined, in analog times: A factor applied to
the synchronous rate of 30 or 60 Hz. (I wish they had solved it
differently. What a pain these numbers are. But sort of appropriate for
the land[s] of inches per yard per mile. I digress.)

Moritz
```