Frame Rates versus Encoded File Size

[Jim Norman]

I have some 1920X1080 video that was shot at 59.94 frames/sec.   I assumed that if I exported at a frame rate of 29.97, the file size would be half as large as if I exported at 59.94, but the estimated file size in the Export window does not change as a function of frame rate?  What am I missing here?

[Averdahl]

The size of a file is related to the bitrate and not the frame size. To get a smaller file you need to lower the bit rate.

But keep in mind that you will loose all the fluid motion from the 59.94 footage if you export to 29.97. You will degrade your footage big time by reducing the frame rate.

[R Neil Haugen]

The important factor is the bits per second. More so than anything.

Neil

[Ann Bens]

Filesize = bitrate x duration.

[Jim Norman]

Thanks for the responses.  I understood the relationship between bitrate and size, but assumed that the more frames, the larger the file.

I just ran a test a 59.94, 29.97, 23.976 and 10 frame rates per second and found that all were the same file size.  In looking at the files with Mediainfo, I found that all of these files had 4 reference frames per second regardless of frame rate.   I assume that these are the same as I frames whose contents would account for the majority of file size.  I am assuming that for 59.94, you would have approximately 4 I frames and 56 non I-frames which would account for a small amount of the file size.   I am also assuming that for 29.97, you would have 4 I-frames and 26 non I-frames, but these non I-frames would contain twice as much data as the 59.94 non I-frames since they cover twice as much video time as the 59.94 non I-frames, resulting in the same file size for either frame rate.

Regarding using a 29.97 encoding frame rate for 59.94 frame rate video, for .H264 MP4s, I incorrectly assumed that the I was improving the quality.    However, for 1080 Blu-Ray disks, Premiere CC does NOT allow me to encode my 1080p 59.94 video with a higher frame rate than 29.97 and even then, it is listed as interlaced (1080i).  If I downsize to 1280X720, then I can keep the 59.94P frame rate.  I assume that this limitation is because of the Blu-Ray spec.  Is there any workaround to allow 59.94?

[Averdahl]

Is there any workaround to allow 59.94?

For Blu-ray authoring i always export 1080 59.94p to 29.97i, iow 1080i. That will look pristine and all smooth motion will be there as well. I did some tests some years ago and to my eyes 1080 59.94p > 1080i looked better than 1080 59.94p to 720p60. Run some small tests an see what you like best. 🙂

Yes, that is due to a limitation in the BD spec.

[Jim Norman]

Thanks for confirming that the 1080 29.97 frame rate is due to the BD spec.