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MPEG2 rendering file size not what it should be

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Spencer Mullen
MPEG2 rendering file size not what it should be
on Feb 14, 2015 at 3:59:47 am

Hi, I'm using Vegas to convert my video files, importing a MPEG2 file that is 8.0GB at 7550kbps according to Gspot codec info, and windows file details. I render it out at 4000kbps and get a file that is 9.5GB. According to all the math, that should be just over half the size of the original. What am I not looking at? The resolution is the same, SD 720x480 29.97. The only thing I'm changing is the bitrate. Why is this not working like it should? I know I'm missing something really simple, but can't put my hands on it.


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John Rofrano
Re: MPEG2 rendering file size not what it should be
on Feb 14, 2015 at 12:54:19 pm

From what you've told us your assumption is correct. The only thing that affects file size is the bitrate and halving he bitrate should halve the file size everything else being equal.

Why don't you post a screenshot of what Mediainfo thinks the file specs are and a screenshot of the video tab of your MPEG2 render template. Maybe we can see something that would be causing this?

I agree with you that it doesn't make sense.

~jr

http://www.johnrofrano.com
http://www.vasst.com



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Sorin Nicu
Re: MPEG2 rendering file size not what it should be
on Feb 14, 2015 at 1:52:03 pm

Is the original file encoded 2-pass VBR (and therefore the value would be an average bitrate)?
Is the output file 1-pass CBR (and bitrate would be constant, maximum)?
Then all makes sense...
Encode your new file with VBR too.


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Graham Bernard
Re: MPEG2 rendering file size not what it should be
on Feb 14, 2015 at 2:12:24 pm

Hm.... Is the rendered file the same Play length as the original?

Grazie

Video Content Creator and Potter
PC 7 64-bit 16gb * Intel® Core™i7-2600k Quad Core 3.40GHz * 2GB NVIDIA GEFORCE GTX 560 Ti
Cameras: Canon XF300 + PowerShot SX50HS Bridge


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Spencer Mullen
Re: MPEG2 rendering file size not what it should be
on Feb 15, 2015 at 6:31:33 am

The two files are the same length. To be honest, I never thought about CBR vs VBR, I thought that while VBR varies, it averages out to a bitrate over the file, which in theory should be the same as a CBR of the same throughout.

It seems that Mediainfo is reporting a different bitrate than Windows and what Vegas renders out as according to the settings in Vegas. Windows reports the new file as 3500/160=3670 total (which is what I set in Vegas) but Mediainfo reports it as 4411kbps with an audio stream of 160kbps, Why would setting it at 3500 in Vegas produce an output file of 4411?

8557_vegassettingmismatch.jpg.zip

Pic attached is my settings and the resulting output file in Mediainfo.


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Spencer Mullen
Re: MPEG2 rendering file size not what it should be
on Feb 15, 2015 at 7:14:57 am

I just did a render of the same file. Same settings, only thing I changed was the bitrate VBR and CBR, set to be 3500. The output file were both over 3500 by a significant amount.

8558_vbrvscbr.jpg.zip


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Sorin Nicu
Re: MPEG2 rendering file size not what it should be
on Feb 15, 2015 at 2:02:21 pm

The VBR saves space versus CBR when the material has portions with fast action, many small details alternating with portions of slow movement and less details. Noise on image it is seen as "detail" and affects negatively the compression.
Now, of course that if you use CBR and VBR wit same rate will average at the same size.
The idea is that VBR can be set at a lower average rate, since the resulting maximum rate will still be sufficient for scenes with heavy movement or details.
In your case a VBR of 1500-2500 might result in same video quality as CBR 3500.


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