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Why lower bitrate get better?

The AAC code licenced to many groups to research, I think the nero's is the best one (perhaps?) 

but I have a question recently:

I use the encorder, which not the newest verson (about nero6?)encoded two mp4(AAC) files.

One is used 64K/s the other is 128k, they' re both excellent that I can't hear any noise or loss in the audio

BUT when I compare them in the simple frequencer in FOOBAR, I found ,

The 64k's is better than the 128k's...the 128k's had losen signal in high frequency part obviously...(but they

all much better than the same bitrate's mp3 format~That's everyone know  )

Why??? Is the encoder had done some special improvement in lower bit rate?

Why lower bitrate get better?

Reply #1
The 64kbps is not better.

Concretely, the 64kbps most probably is an HE-AAC (aacplus) encode, while the 128kbps is an LC-AAC one.

HE-AAC is not a better codec (better meaning more faithful to the original), it is a more efficient one (efficient meaning better quality/bitrate ratio).

The characteristics of HE-AAC (and what makes you see a quasi full-band spectogram) is that the higher frequency bands are actually not stored, but regenerated, either by duplication, applying envelopes, making guesses or other similar methods.

In the end: At the bitrates of HE-AAC, it is the preffered format. (vorbis comes close, but not quite)
At higher bitrates, you may preffer a codec that tries to keep closer to the original.

Why lower bitrate get better?

Reply #2
As, [JAZ] suggested, 64 kbps uses HE-AAC. This means that only frequencies up to approx. 10khz are saved, while the rest (from 10khz to 20khz) is 'artificially' recreated using technology called SBR (Spectral Band Replication).

128kbps saves frequencies up to approx. 16khz (depending on the encoder) and does not use SBR.

Consequently, although the 64kbps may seem to have higher frequencies preserved compared to 128kbps, 64kbps files are likely to have less "natural" sound in high frequencies (due to SBR) than 128kbps.


Why lower bitrate get better?

Reply #3
Thx very much! I've learned a lot~~

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