MPEG frame rates

Chienworks wrote on 10/14/2005, 12:40 PM

Someone please refresh my memory. Why is it that we can't specify arbitrary frame rates for MPEG encoding? Is there any reason i can't create a 15fps MPEG file? I used to do this often with ULEAD software (*shudder*). Vegas only seems to allow 23.976, 24, 25, 29.97, and 30 for MPEG. Oddly enough, with AVI and and WMV i can type in pretty much whatever i want.

Comments

rs170a wrote on 10/14/2005, 2:15 PM

What frame rates are permitted in MPEG?

A limited set is available for the choosing in MPEG-1 and the currently defined set of Profiles and Levels of MPEG-2, although "tricks" could be played with Systems-layer Time Stamps to convey non-standard picture rates. The set is: 23.976 Hz (3-2 pulldown NTSC), 24 Hz (Film), 25 Hz (PAL/SECAM or 625/60 video), 29.97 (NTSC), 30 Hz (drop-frame NTSC or component 525/60), 50 Hz (double-rate PAL), 59.97 Hz (double rate NTSC), and 60 Hz (double-rate, drop-frame NTSC/component 525/60 video).

Only 23.976, 24, 25, 29.97, and 30 Hz are within the conformance space of Constrained Parameter Bitstreams and Main Level

Thanks to MPEG's top_field_first and repeat_first_field, it is technically possible to have somehow irregular coded frame rates and still have a constant display frame rate. But watch out for VBV compliance!

For more info , try the MPEG Pointers & Resources site.

Mike