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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.
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5 Replies
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It’s the bit rate, not the frame size, that makes the difference.
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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. (??)
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OK. With a bit of thought I think I got it. 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. It may not really show it, due to pixellation, but on the other hand there may be some gain….I suppose it depends on the content.
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[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
http://www.johnrofrano.com
http://www.vasst.com -
Thanks John, for clearing it up and confirming.
Hehe! After I wrote my last post I _immediately_ thought “woops” and did a VBR render, two pass. I realised that “optimised” was what I wanted and I was was going for “best quality” or at least “most predictable quality”.
So many choices. So many targets. So many…..
And it is hard to make watchable video without lots of data!
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