Frame Dims and File Sizes

Sony Vegas Studio

I have taken some video and I want to test quality vs file size. So I am looking at bit rate and frame size, for any given codec.

My latest attempt is with mpeg-2. I am using the Main Concept mpeg-2 group, sub choice Program Stream PAL. I have tried DVD and mpeg-2 as the Output Types.

I have set the bitrate at 3,000,000 fixed, as I feel that a,fixed bitrate is the best when optimising.

So I then tried various frame sizes. Going from 640*360 TO 480*270. I had some other frame sizes and they were a lot larger. But suddenly these two were exactly the same file size...about 25MB for a one minute video.

Is there some sort of lower limit? I have always thought that all things being equal, frame size set file size.

Thanks for any ideas.

Thanks for the reply.

hmm...how come? I mean I can see that the bitrate will make a difference, but surely less pixels per frame should make a difference as well, given equal bitrate. (??)

[Nick White] "I have set the bitrate at 3,000,000 fixed, as I feel that a,fixed bitrate is the best when optimising."

That was a bad "feeling". ;-) A fixed bit-rate is the worst thing for optimization. You want to use a variable bit-rate (VBR) so that more bits can be used for scenes with more details. Adding Two Pass to VBR will give you the best optimization because the first pass creates a "bit-budget" to be used in the second pass that will be more accurate than a single pass can provide.

[Nick White] "The bitrate _will_ set file size, because the smaller frame size will attempt to have more info (less compression/loss) than the larger frame size, for a given CBR."

Yes, the frame size will essentially affect the quality for a given bit-rate. It will take more bits to represent a larger frame and so larger frame sizes will have less quality for the same bit-rate as a smaller frame size.

~jr

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