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Thursday 19.3.2026 / 03:39
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afterdawn.com > forums > dvd±r discussion > dvd±r for newbies > constant bit rate vs variable bit rate
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Constant Bit Rate vs Variable Bit Rate
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TedDavid
Newbie
_ 		3. June 2005 @ 05:25 _ Link to this message    Send private message to this user   
Can anyone tell me when Variable Bit Rate (VBR) encoding makes a difference? I know that it requires multipass transcoding, which certainly aggravates transcoding time. But, if I'm only interested in burning DVDs for a stand-alon player (i.e., not for a computer's DVD drive or for streaming from the internet) does VBR improve quality? Size?
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vurbal
Staff Member

2 product reviews
_ 		3. June 2005 @ 05:57 _ Link to this message    Send private message to this user   
Let's say you have a DVD that's 1 hour long with one minute of footage that needs a bitrate of 8500kbps to get good quality and a bunch of other material that only need 6500kbps. If you use CBR encoding you'd need to encode everything at 8500kbps to keep that one minute of video from looking like crap. If you use VBR encoding you might be able to use an average bitrate of around 6700kbps instead and still keep that one minute at a higher bitrate. That means that the CBR file with the same quality as the VBR file would be more than 25% bigger for no quality gain.

Obviously this is a very simplified example, but in general, when you use CBR everything has to be encoded at the same bitrate as the hardest scenes to encode, and if a scene doesn't require that much bitrate you'll just add padding to the file. It will be faster for the same quality but very wasteful.
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Rich Fiscus
@Vurbal on Twitter
AfterDawn Staff Writer
scf_au
AfterDawn Addict
_ 		3. June 2005 @ 06:58 _ Link to this message    Send private message to this user   
vurbal,
This is quite interestingly put.
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TedDavid
Newbie
_ 		3. June 2005 @ 09:30 _ Link to this message    Send private message to this user   
Sounds like its just a file space-saving mechanism, which is something one might be interested in for streaming video from a website, or if there's a concern about the DVD media having enough space. It also sounds like, when the various burning programs and stand-alone recorders give you a encoding options tied to DVD recording duration that its either CBR they're spec'ing or the 1-hour, etc., are just averages.

But, back to the other half of my question: does it improve or degrade video quality? I know for certain that single pass transcoding can save time, and that VBR has to use multi-pass. Am I sacrificing any quality by using CBR, single-pass MPEG encoders? Or am I simply allowing the encoder to crunch the video data to a smaller size, with no noticeable benefit or detriment to video quality?

Bottom line: If I'm simply interested in the best quality video for playback on TV, as opposed to on a computer, is there any reason I shouldn't use single-pass, CBR encoding?

Ted David
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squizzle
AfterDawn Addict
_ 		3. June 2005 @ 10:31 _ Link to this message    Send private message to this user   
to add on what vurbal said, the difference you will find will be on larger movies where that "excessive padding" you get with CBR causes the compressed output to be over 4.4GB. Then everything gets compressed more to reach your desired output size. This causes you to notice the flaws in certain spots. This is where the VBR method really outshines the CBR.
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Convert PAL to NTSC or NTSC to PAL------>http://forums.afterdawn.com/thread_view.cfm/167922
ScubaPete's guides------>http://www.dvdplusvideo.com/tutorial007.html
Bbmayo's guides------>http://home.comcast.net/~bbmayo/index.html

My ever-growing movie collection------>http://www.intervocative.com/dvdcollection.aspx/squizzle
scf_au
AfterDawn Addict
_ 		5. June 2005 @ 01:24 _ Link to this message    Send private message to this user   
squizzle,
More useful knowledge yet. Thanks.
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squizzle
AfterDawn Addict
_ 		5. June 2005 @ 08:37 _ Link to this message    Send private message to this user   
you're welcome. I was reading that after I posted it and I thought, I really hope people can understand what I was trying to say there. It looked kinda funny to me reading it after posting.
[ + quote]
Convert PAL to NTSC or NTSC to PAL------>http://forums.afterdawn.com/thread_view.cfm/167922
ScubaPete's guides------>http://www.dvdplusvideo.com/tutorial007.html
Bbmayo's guides------>http://home.comcast.net/~bbmayo/index.html

My ever-growing movie collection------>http://www.intervocative.com/dvdcollection.aspx/squizzle
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scf_au
AfterDawn Addict
_ 		6. June 2005 @ 05:17 _ Link to this message    Send private message to this user   
No, both your post and vurbal's post were explaining things from the user's perspective, rather than technical jargons that are commonly found. Both reminded me of something useful.
Cheers!
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