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Topic: MP3 repacker (Read 678138 times) previous topic - next topic
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Re: MP3 repacker

Reply #600 –
Would you say that MP3packer converts a 320Kbps MP3 into something similar to a V0 MP3?

Re: MP3 repacker

Reply #601 –
@xabih Yes. Imagine an MP3 as a train of boxcars on a railroad, whizzing past you, the player/listener who reconstructs a (typically) 29-ms snippet of audio from each boxcar's cargo. In a VBR MP3, the boxcar sizes change to match however much cargo there is; some snippets require more and some require less, in order to maintain a consistent quality level. In a CBR MP3, the boxcars are all the same size, but they are usually not completely filled, because the next step up in quality would result in too much cargo. This means the quality varies from car to car. Modern CBR encoders, however, use an MP3 feature called the bit reservoir, which allows empty, wasted space in one boxcar to hold some overflow from the next car or two. This allows those cars to have higher quality, but it's definitely not as flexible as VBR.

MP3packer just puts a CBR train's cargo into a VBR train. The cargo doesn't change, so the audio is exactly the same. There is no change in quality at all. The goal is just to save space. So to answer your question, yes, it's similar. A typical 320 kbps MP3, even if it uses the bit reservoir, can waste a lot of space; they're really only about 260 to 300 kbps most of the time, although some frames might go higher.

As for similarity, yes, similar is the right word. Although the repacked MP3 has identical audio to the one it was made from, it's probably not going to be a match for the audio produced by encoding the same source material directly to a VBR MP3: 1. A particular MP3 encoder will use exactly the same algorithm when it originally encodes CBR vs. VBR. For example, LAME used to have separate psychoacoustic models for CBR and VBR. Before LAME 3.99 came out, a LAME-encoded 320 kbps MP3 run through MP3repacker would not be audibly identical to a LAME V0 MP3, even though superficially they'd both be VBR with a high average bitrate. 2. VBR strives for consistent quality, whereas CBR strives for consistent data flow (boxcar/frame sizes); CBR 320 can be (objectively, probably not perceptually) higher quality than the best VBR, just by virtue of the quality being allowed to exceed the target in some frames. So if you are comparing CBR 320 (repacked into VBR or not) to a native VBR V0, it won't be exactly the same audio.
 

Re: MP3 repacker

Reply #602 –
Quote from: mjb2006 on
A typical 320 kbps MP3, even if it uses the bit reservoir, can waste a lot of space; they're really only about 260 to 300 kbps most of the time, although some frames might go higher.
[...]
Before LAME 3.99 came out
[...]

Anecdotally there isn't that much to be saved for a CBR encoded "by a recent LAME".

But ... try! (1) make a copy of a selection of files, (2) remove all tags to get an apples to apples comparison, (3) run it.
Then think over how much/little it is.
Also, a bit-compare with foobar2000 is probably a good idea - while compliant files will be identical (except sometimes there will be "differences" in the -150 dB range because round-off in floating-point math), but mp3packer isn't necessarily forgiving about files with issues.