VBR - What's the point?

Discussion in 'iPod' started by liamski, Feb 22, 2007.

  1. liamski macrumors member

    Joined:
    Jan 14, 2007
    #1
    Hi

    I'm relatively knowledgeable about audio and recording etc.... but I'm just not getting the point about encoding at VBR rather than constant bit rate.

    Can anyone share their experiences of VBR over CBR - is it worth it from a quality perspective?

    cheers...

    ps: I prefer CBR at 320kbps..
     
  2. net26 macrumors member

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    Aug 30, 2006
    #2
    all i think it does is encodes the sections of the song requiring a lower BR (or, rather NOT requiring the highest bitrate to still remain pretty much the same) to that lower BR, thus making the file smaller. that's the only difference - file size.
     
  3. Vlade macrumors 6502a

    Vlade

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  4. imacdaddy macrumors 6502a

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    #4
    VBR is only good when you are streaming over a network or internet where bandwidth fluctuates. Say you encode a video+audio stream at VBR 1000Kbps total (1Mbps) and your streaming server bandwidth is 100Mbps, you can have 100 connections to that video stream without any loss in quality. If the streaming server exceeds the number of concurrent connections, the video bit rate would be less to accomodate the number of connections...less than 1000Kbps. The quality of the video would be degraded. If you have too many concurrent connections, the client will experience "buffering" on their player.

    CBR is good when encoding videos which you will play back on a stand alone computer, a video player or a portable device.

    I would encode my videos in CBR to be played on an iPod or the coming :apple: tv. Even though Apple says videos will be streamed to the :apple: tv, from a Mac or PC, the bandwidth on a home network is big enough for CBR and some internet browsing/downloading. ;)
     
  5. Dunepilot macrumors 6502a

    Dunepilot

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    #5
    I think that it's primarily meant to offer you space savings where there is little high frequency information in the music, and where there are sections of silence in an audio track, I guess you'd make significant space savings by using fewer bits to generate that 'silence'.
     
  6. Vlade macrumors 6502a

    Vlade

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    #6
    Someone with experience or a reference to site please correct me if I'm wrong, but aren't all modern video compression formats VBR? I remember doing lots of AVI conversions a while back with Dr.Divx, and it was very variable, with some frames receiving almost no data, and some parts of the movie receiving TONS of data from keyframes. I was under the impression that mpeg2/mpeg4 were always VBR to a certain extent.

    And I thought that most video streams were CBR, and if you didn't have the bandwidth they would just have to buffer more or drop frames... I didn't think that most streams were smart enough to lower the bitrate/quality of the stream if the user didn't have enough bandwidth. Again someone correct me if I'm wrong about this
     
  7. Dunepilot macrumors 6502a

    Dunepilot

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    #7
    Yes, I think you're correct about video being VBR. I'd assumed that was the case when you see areas of darkness turning all blocky in DVDs after a few seconds of camera in a certain position.
     
  8. Krevnik macrumors 68030

    Krevnik

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    Sep 8, 2003
    #8
    Somewhat. Every video codec is VBR these days (except ones which offer virtually no compression, like DV). You can do things to configure how the bitrate will fluctuate, but they are all VBR.

    For streams, the guy doing the encode will (hopefully) set a /maximum/ bitrate, and an average. The average is what the guy wants the video to hover around, and the maximum is the rate it cannot exceed (a cap so that it will be safe to stream). When encoding for a home network (which has excess bandwidth), or for playback on a single computer, you tend to just set the average bitrate, looking for a particular level of quality.

    As for artifacts like blocks appearing in very dark/black frames, that is usually due to single-pass encodes. When using an average bitrate on a single pass, the code has 'guess' when to crank the bitrate back down, and sometimes does it too aggressively, taking away too many bits in parts of the scene. Two-pass or multipass (QT H.264 does /4-5/ passes for multipass encoding, insane, but the quality speaks for itself) analyzes where bits are getting used and takes that data for the final encode, making better use of the bits you have allocated to the video. An example is the black, blocky scene, will likely look better under a two-pass encode than a single-pass encode, and tends do a better job removing block artifacts in high-motion scenes.

    For audio, the only real difference for CBR/VBR, is a couple things:

    - CBR will let you KNOW how big your file will be. However, it can have the effect of 'clipping' audio in more complex parts, more than the rest of the song.

    - VBR will let you ESTIMATE how big your file will be. The upshot though, is that the quality will be more consistent through the song. It will sound bad/good/great throughout.
     
  9. Krevnik macrumors 68030

    Krevnik

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    #9

    I normally don't do something like this. But I urge people interested in video encoding to disregard this quoted post, as it is very misleading. VBR vs CBR has no bearing on the ability to stream a file, and in reality, VBR makes the job of streaming a file harder unless you put a maximum bitrate cap on it.

    VBR does not change the bitrate for streaming media if the server gets overloaded with clients, as claimed above. VBR is all about trying to get better quality in the same file size by letting simple scenes give up bits in the stream to the complex ones.

    If any CBR vs VBR recommendation comes out of this, it is the opposite: Use CBR for streaming (or VBR with a bitrate cap), and VBR for your videos on your PC.
     
  10. Rocksaurus macrumors 6502a

    Rocksaurus

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    #10
    I was always under the impression that VBR would give you an average overall bit rate, and it would encode areas of a song that don't require a high bit rate to sound good at a lower bit rate, and then encode areas of a song that do require a higher bit rate to sound good at a higher bit rate and when it was all said and done you had a file that was the same average bit rate and thus the same size as CBR, but sounded better due to the fact that VBR was upping the quality where the song needed it most.

    For the OP, who prefers 320 kbps CBR, if you're going to encode at such a high bit rate anyway, CBR probably is better. I may be wrong but I believe VBR has a more noticeable effect for the lower bit rate songs.
     
  11. BBC B 32k macrumors 6502

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    #11
    you will sure need a decent sound system to hear that 320 kbps mp3/aac.

    I pray you are not listening on a laptops internal speakers :D I have an old iMac hooked up to my Marantz amp and Mission floorstanders and there is bugger all improvement in the sound above 192 kbps. The weak link becomes the computers auidio components and the crap compression routines.

    Just turn on Bass Boost and drink some beer!
     
  12. Krevnik macrumors 68030

    Krevnik

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    #12
    Pretty much. The catch is that the bit rate is an /estimated/ average. So the file size isn't exact. You will get pretty close to the same bit rate as CBR, but it could also average out a little higher or lower, depending on the material. I have 160kbps VBR AAC files that actually average out to 167Kbps or so because of this.
     
  13. g3ski macrumors member

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    Jun 18, 2002
    #13
    Is this really a "catch"? Does anyone encode their music to fit an exact file size? ....my new Black Eye Peas song needs to compress to 6.193MBs or I cant use it!"

    VBR produces better quality audio files than CBR.
    VBR will take longer to encode than CBR.
    VBR and CBR files will be approximately the same file size

    At a bit rate of 320Kbps, the OP won't notice the difference. Bigger differences will be noted be codec choices (MP3 vs. AAC vs. Ogg) and bit rate (128 vs 192 vs 256). 320kbps is going to sound almost exactly like a CD even on very good equipment. Only a true audiophile in a well-built audio room with very high-end equipment will be able to tell the difference. In 2000 or 2001, there was a big study testing the ability of average people and pros to tell the difference between MP3 and CD. Most couldn't tell the difference, even at 128Kb. At 256Kb even the best ears were often fooled.

    If you want to hear the difference between VBR and CBR, select a few different types of songs: bass heavy, light/easy/ballad, classical/opera. Encode each song at both 96bit CBR and 96bit VBR. At such a low bit rate you will be able to notice bad music and possibly the difference between CBR and VBR.

    -J
     
  14. g3ski macrumors member

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    Jun 18, 2002
    #14
    DV has 6-1 compression..... from wikipedia, "DV uses DCT intraframe compression......DCT compression is lossy".

    The CD native format (seen as AIFF files on a Mac) is lossless and pure; no compression, all of the data is there. Apple Lossless also has all of the actual music data, but compresses the music file at 2-1. In any timeslice of a song, there can information between 20Hz and 40,000Mhz. If there is silence in that timeslice, there is no useful information. If there is only a 20-100Hz bass thump over a 3 second period, there is a lot of empty space in the upper ranges. Lossless codecs dump that empty information without touching the parts that contain sound. MP3, ACC, and cell phones all "clip" part of the audio information - the worse the codec and lower the bit rate, the more the audio sounds like bad AM radio.

    -J
     
  15. liamski thread starter macrumors member

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    Jan 14, 2007
    #15
    INteresting debate... cheers

    FYI... I'm playing these mp3s in a club so I suspect quality is more of an issue.
     
  16. g3ski macrumors member

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    Jun 18, 2002
    #16
    Encoding at 256Kbps or better, no one in a club will be able to tell that it's not a CD. The crowd noise, acoustics, and bevs they sell at the bar will ensure that :p
     
  17. Fuchal macrumors 68020

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    Sep 30, 2003
    Location:
    Boston
    #17
    With today's modern VBR encoders, there's no reason to encode CBR at all. Instead of 320kbps, which wastes a lot of space on nothing, encode using LAME's V0 mp3 preset, which will give files of the same quality as 320 but in a generally considerably smaller file size.

    If you want more information, I suggest asking the smarties at http://www.hydrogenaudio.org
     
  18. Krevnik macrumors 68030

    Krevnik

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    Sep 8, 2003
    #18
    Not for music that you listen to no... but if you stream files, or have a fixed file size limit for video, then it can be a catch. For example, I am encoding a couple Blu-Ray discs at 720p right now, and because of various limits, I can't create an encode over 4GB when it is all said and done. So VBR does throw a wrench in this, as the best H.264 encoder I have access to, ironically, can't handle max size caps.

    I agree with this, with the exception that VBR takes longer to encode. Two-pass does take longer (but really only applies to video), but single-pass VBR encodes are just as fast as a CBR encode.
     

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