Does Video Frame Rate Affect File Size?

[Shelleen]

Sorry for the very basic question but in my limited research on this topic, I found contradictory information but I may not be understanding the information correctly. In a short test I tried, increasing fps (is that frame rate?) from 30fps to 60fps, had little affect on the resulting file size of the video. My test: A 5 minute video recorded at 720/1280, bitrate of 7500, 30fps, had a file size of 281,600. The same 5 minute video recorded at 720/1280, bitrate of 7500, 60fps, was 282,000KB. Almost no difference in file size. However I found several articles that state increasing the frame rate significantly increases the file size of the recording.

I'm probably not understanding something here, so if anyone can give me the correct information I would appreciate it. Thank you.

In one article it states: "Changing frame rates require a close examination of our storage, as file sizes increase significantly as frame rates increase." With this example:
24 fps video = 100% capacity and bandwidth increase
30 fps video = 125% capacity and bandwidth increase
50 fps video = 208% capacity and bandwidth increase
60 fps video = 250% capacity and bandwidth increase

 

[Lawrence_SoCal]

I'm not authoritative on this... so hopefully someone more knowledgeable will chime in.. but for a prompt response, hopefully this will help

depends on what you are recording. If there is motion, then by default, of course saving more info (fps) will require more data (file size). But modern codecs means if there isn't much (or any) motion in your test video, then fps may not make, if any difference in results encoded output. in part depends on things like into B-frames, and other settings.
but in general more data = more data. your results may be due to not actually having more data, despite more fps (ie no pixel deltas [movement/image change] to record)
for example - recording a minute of video of static Picture (not video) or PowerPoint, etc - fps won't make a difference as pixels aren't changing... so my suspicion is your results mean something different than what you interpreting them to be. So try your test again, making sure to use a high color, high motion (ie at least 60fps) source video as test source... and record/encode again

 

[Suslik V]

Bitrate has higher priority over other restrictions during encoding.
If your encoding rate control is CBR, then the file size depends only on duration of the recording.
If your encoding rate control is not CBR, then less fps video can has smaller file size.
CRF method of rate control (values about 18-23 for x264 encoder) is suitable for local recordings.

 

[Shelleen]

Thank you both for the replies. The test recording had a decent amount of movement and was in color. It was a clip from a soccer game. I think Suslik's answer was what I was experiencing. I had it set to CBR. So now that brings up another question.

1. If I use CBR is there any advantage of using 60fps over 30fps?
2. If I use CRF at say 20 for x264, would a Bitrate of 5500 Kbps and Buffer size of 3500 be acceptable if I'm using 30fps?

Thank you

 

[Harold]

using CRF as your rate control means that a quality target is set, you should NOT be manipulating bitrate numbers when using CRF as your rate control.

 

[Shelleen]

Thanks Harold. I forgot about that. So would this be good basic settings for recording medium/fast moving color videos?

VIDEO
Monitor: 1920x1080. 16:9
Output Resolution: 1280x720. 16:9
Downscale Filer: Lanzos
Common FPS Values: 30

OUTPUT - RECORDING
Type: Standard
Recording Format: mkv
Encoder: x264
Rate Control: CRF: 23, Keyrame 0s, CPU Preset: very fast, Profile: High, Tune: None
CPU: Very fast
Profile: High
Tune: None

 

[Harold]

CRF closer to 14 if you need more quality, cpu preset to ultrafast to reduce cpu usage at the cost of increased filesize
Unless you specifically need multiple audio tracks, you should use simple output mode, indistinguishable recording quality, software low cpu usage preset encoder (unless you have an nvidia video card, then choose nvenc h264 as your recording encoder).

 

[koala]

Video size is determined by the amount of pixels to be handled within some time period. If you double the resolution, you quadruple the amount of pixels in the frame so you get four times the video size. If you double the frame rate, you get double the frames as before, so you get double the the video size.

If you use a rate control that sets a bitrate, this bitrate is enforced no matter the resolution or the fps of the video. So if you have a bitrate of 5500 kbps, the video will always 5500 kb per second - a fixed size. To enforce a bigger resolution or fps, quality is tuned down to match the bitrate.
If you use a quality based rate control (CQP, CRF), the size on disks roughly follows the 1st paragraph, however the compression is better the higher the fps is or the larger the resolution, because the higher/larger it is, the difference between adjacent pixels is lower, so it can be better compressed. So you don't get 4 times the video size for doubling the resolution but perhaps 2-3 times. Or about 1.3 to 1.7 times the size if you double the fps.