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afterdawn.com > forums > digital video > video to dvd > bitrate?
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Bitrate?
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rjessa
Member
_ 		7. February 2006 @ 17:37 _ Link to this message    Send private message to this user   
What is bitrate and how does it affect the video quality? Could someone provide a more detail explaination?
Thanks
[ + quote]
nomad311
Junior Member
_ 		17. February 2006 @ 18:30 _ Link to this message    Send private message to this user   
[q]Bitrate
Bitrate or Bit Rate is the average number of bits that one second of video or audio data will consume. Higher bitrate means bigger file size and generally better video or audio quality while lower bitrate means lower file size but worse video or audio quality. Some bitrate examples in common video and audio files:
MP3 about 128 kbps (kilobits per second)
VCD about 1374 kbps
DVD about 4500 kbps
DV about 25 Mbps (megabits per second).[/q]

having said that bitrate is directly related to quality in 2 ways
there is CBR(constant bitrate) that means the entire video is encoded at a constant bitrate ...this is a very wasteful way of encoding because if the CBR is 4500bps then a completely black frame (actually a second of black frames ...but for some reason i always say frame so bare with me) take up 4500bits and a picture of a building would too ...now it would make much more sense to have say 4300bits from that black frame and improve the quality of that bulding right?!
thats where VBR(variable bitrate) comes in it does just this ...the black screen will take up say 500bits (although most encoders have a minimum bitrate set to like 2000 ...for speed is my guess) and the buuilding can have up to 9000bits (this is the highest allowed in a dvd complient MPEG-2 vided)

now i called CBR wasteful at first because it will either make a file that could have been much smaller to be quite larger ...or limit the quality of a dvd because to increase the length of the video you have to decrease the bitrate

when encoding with VBR you set an average bitrate and the result is that complex frames get lots of bits and simple ones get few , but the encoder attempts to maintain an average close to what you set to give predictable filesizes therefore you get better quality doing multiple scans so that the bits can be moved around to a more efficient layout with each pass (4 is said to be the point where more passes simply arent worth the time for possile improvement)

...the newest encodeing method is OPV (aka roba) ...not sure what it means or how it works but it provides quality comparable to 4 passes of VBR in one pass CCE (cinema craft encoder) is said to be the best at this ...or any form of MPEG-2 encoding in terms of the fundamental quality for amount of time trade off ...some encoders are faster at the cost of quality and others are slower (by a lot) with comperable quality ...TMPGEnc comes in a fairly distant second overall (but about as popular)

hope that answers your question ...but i never was good at essay questions so i just start goin on about whatever i can pull out at the time

later
-nomad311
[ + quote]
my lil guide to usin your tv as a monitor:
http://nomad311.f2g.net

-enjoy
rjessa
Member
_ 		18. February 2006 @ 02:07 _ Link to this message    Send private message to this user   
Thanks ever so much. That makes a lot of sense, and puts the whole thing in perspective.

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