Questions about bit rates

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    • #40959

      Wondering about bit rate settings for DV and DVDs

      What is the highest bit rate that can be burned to a DVD? Am I right that it’s just over 8000 depending on the player? What about those Super Bit DVDs you can buy?

      What is the bit rate of an uncompressed AVI file captured from a DV tape?

      What actually is bit rate? How does it relate to resulotion or lines?

      If I’m converting old analog tapes, is it a waste of space to use a bit rate higher than 6000?

      When should I use VBR rather than CBR?

      In certain places on my DVDs I notice some bitmapping, or “blocks” on the screen. Can I avoid this by playing with the bit rate?

      Thanks for all and any input. Sorry if this is in the wrong forum.

    • #174735

      I can help to answer a few of your questions. As far as bit rates go for dvd, I believe it is a codec thing so… Since you are capturing DV from tape uncompressed it doesn’t have a bitrate. Mpeg2 bitrate however for a dvd does depend on a couple of factors.. first, it depends on how much video you need to fit onto a dvd. It also depends on your audio format. If you use AC3 encoding, you can have a larger video bitrate than if you use uncompressed pcm audio, because the uncompressed audio uses up some of the dvd players available bitrate.

      I would agree that you shouldn’t go over 8000 for video bitrate or you could start running into some problems with certain dvd players.

      If you are converting analog tapes, it is best to capture them to dv tape and save that as your master since it is the highest quality available as uncompressed.

      When your video editor encodes to mpeg2 it has to compress the uncompressed file. To do this most efficiently and effectively there are a couple of considerations you should know about. First you asked about variable bit rate VBR and constant bit rate CBR. VBR scans your video for motion, in areas where there is little motion it can use less bitrate, in areas where there is a lot of motion it must use the highest bitrate available to get the best quality overall. Generally, in most cases it is best to use VBR. Also you must consider using multipass encoding.. It takes twice as long to encode the file, but basically it uses the first pass to scan the video for motion, and the second pass to encode. This assures the highest possible quality when compressing the video.

      Hopefully some of that helped.


    • #174736

      Thanks for the info, very useful.
      Unless disc capacity is an issue, wouldn’t 8,000 kbps at CBR be the optimal setting? Shouldn’t I use VBR only when capacity is an issue, i.e. fitting more than 1 hr on a DVD? Or is there something else in VBR that enhances high-motion areas?


    • #174737

      From my experience, I agree with you that unless size is an issue do constant bit rate, I think the dvd players like it better that way too. I don’t know of any other visual advantage of vbr.


    • #174738

      The blocking (artifacts) usually is a result of a slow bitrate being used for the action going on or a poor encoder was used.

      The explanation given already about VBR is very good. Here are the details. When using VBR you would set a MAX rate, a MIN rate and a TARGET rate. The first pass does nothing more than analyzes the amount of data changing between each frame and makes notes to itself. (analogy of course). The faster the action, the more data will be changing between each frame. Because of this, the encoder will see that and change to a faster bitrate. It will go all the way to the MAX setting that you gave it. If the action is slow, the encoder will see that too because the data isnt changing that much or as fast so it lowers the bitrate needed. This is where you gain huge compression. Thus the term VBR.

      Once the first pass is done, the encoder will go back and start to actually compress the data based on the notes it took earlier from the first pass. Thats why it takes twice as long to do a 2-pass VBR encoding job. The advantage is that you get maximum compression with maximum quality. The only disadvantage is that it takes a long time. The solution to that is to encode overnight like I always do. The amount of compression and the quality can be tweaked by just adjusting the MIN, MAX and TARGET rates within your encoder settings. (That is if you can)

      CBR just takes the data and compress it to the setting you told it to do. There is no need for 2 passes because you made the encoders mind up already. The advantage is that it encodes faster, but your compression will be poor. As far as quality, if youre around 6000 to 7000 it will be about as good as you can get or notice anyway. It also depends on whats on the movie file too. If youre filming a high school play you could probably get away with 4000 maybe even 3000 and it would look fine. Doing a NASCAR race with those low settings would be a different story. There you better take it up to the ceiling which is 8000.


    • #174739

      Thanks for all the info. Wow, what a great forum! Can’t believe it took me a year of subscribing to the magazine before realizing it… (hint to the editors, maybe publicise the forums more…).

    • #174740

      Just a couple things to add and correct that the previous guys posted. DV (avi) video that you capture from your camcorder is COMPRESSED video. And yes it does have a bitrate (standard DV is about 25Mb/s – so roughly 4MB/s). If this was uncompressed the fastest computers would never be able to keep up with the video, editing would be impossible and you’d NEVER find a big enough hard drive (lol).

      As far as DVD bit rates go, the maximum bitrate is (and cannot exceed) 9.8Mb/s (or 9800Kb/s). This means that the total bitrate for both video AND audio can never exceed 9800Kb/s. So let’s say your video was running at 6500Kb/S CBR and your audio was AC3 at 192Kb/s. You’d be sitting at a total of 6692Kb/s and well within the specified range. One thing to keep in mind though is that most set top DVD players have trouble playing anything with a bitrate of 7.5Mb/s or greater (due to the cheaper drives used to keep costs down). This is why many times you’ll experience trouble playing your DVD in your set top, but it’ll play fine in your computer. I usually keep all my projects under 7Mb/s (the video usually VBR with a min of 4-4.5MB/s and max between 5-6.5MB/s). You can usually get by with lower bitrates if you have a high quality encoder. I use and recommend Canopus Procoder 2.0. It does an outstanding job. Many of my 4-5Mb/s encoded videos look better than TMPGenc at 8Mb/s

      There’s my 2 cents for the day


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