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Can someone explain Bit reservoir for me?
djchristian
post Jan 29 2013, 17:04
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I try to struggle to understand what this does. I am relatively new on this area

http://wiki.hydrogenaudio.org/index.php?title=Bit_reservoir

What does it do really? Does FLAC have Bit reservoir too?

Thanks in advance!
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mjb2006
post Jan 30 2013, 05:25
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FLAC doesn't have it because it's not part of the spec for FLAC. It's an MPEG audio thing (MP3, AAC).

It's not just for CBR, but that's where it's used the most, because you get the most benefit there.

Since you've already seen the wiki article, I'm not sure how we can explain it any more simply, but I'll try.

An MP3 is segmented into frames, each one corresponding to the same amount (time-wise) of audio.
Each frame has a fixed size (data-wise), depending on its bitrate. You can think of it like different-sized hopper cars in a train. They only come in certain sizes (e.g. 320, 256, 224, 192 ...).
The audio data spit out by the encoder varies in size for each frame, because some sections of the music will just be simpler to encode and will compress better than others.
So, each hopper usually goes not-completely-filled, but occasionally some need to be overfilled.
If the file is going to be VBR, the bitrate can vary from frame to frame (different-sized hoppers can be used). That's usually sufficient to avoid wasting space in frames.
If the file is going to be CBR, though, the bitrate has to stay the same from frame to frame (the hoppers must be the same size), which pretty much ensures some space will be wasted.
Someone thought, "when a frame isn't filled up, why not use the leftover space to hold some of the data for the next frame?"
So every frame has a pointer that says how far back that frame's audio data really begins. This distance is the "reservoir".
It's like there's a reserve of space in the preceding frame(s) which can be used to overfill the current frame, if it ever needs it.
In effect, it's like a limited form of VBR. It lets some frames have more than their alotted amount of data.
The downside is that the reservoir can't grow very big, and once it's used up (by overfilling frames), it has to grow again.

This post has been edited by mjb2006: Jan 30 2013, 05:29
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