RealAudio, HE-AAC, 22050Khz!?

[Toll]

Recently, I compressed my AV file to rmvb.

I accidently found that I converted AV file's audio to HE-ACC format and I only got 22050Khz.

I am a little confused.:confused:

It is better than AAC format, right?
But It is lower than AAC's result(44100Khz).
Why doesn't HE-AAC format has 44100Khz or higher?

Is anyone have the same experience? I wish that someone may help me to solve this doubt.

[bond]

:search: :readfaq:

but to make it short: everything is correct
to play your file with full quality you need a decoder which handles he-aac of course, like coreaac

[SeeMoreDigital]

Originally posted by Toll
Recently, I compressed my AV file to rmvb.

I accidently found that I converted AV file's audio to HE-ACC format and I only got 22050Khz.

I am a little confused.:confused:

It is better than AAC format, right?
But It is lower than AAC's result(44100Khz).
Why doesn't HE-AAC format has 44100Khz or higher?

Is anyone have the same experience? I wish that someone may help me to solve this doubt. Welcome to the wonderful world of AAC-HE!

As bond says everything is correct. In my experience I've found that when generating AAC-HE, encoders like Nero, look at the sources sampling frequency and (more often than not) halve it, ie: 44.100kHz becomes 22.050kHz or 44.000kHz becomes 22.000kHz!

I'm not 100% sure but I think AAC-HE only has two sampling frequencies. Unlike AAC-LC which can use quite a few (8.000, 11.025, 12.000, 16.000, 22.050, 24.000, 32.000, 44.100 and 48.000kHz).

Cheers

[P0l1m0rph1c]

HE-AAC isn't limited to 2 sampling rates. It simply halves the sampling rate of the original audio.

Why?

It's the SBR technology. An AAC file with SBR only encodes the lower frequencies of the audio. The higher frequencies are computed, using the SBR algorithm. So, there you have why the sampling rate is halved.

It's not really a sampling rate limitation...

[SeeMoreDigital]

Originally posted by P0l1m0rph1c
...An AAC file with SBR only encodes the lower frequencies of the audio. The higher frequencies are computed, using the SBR algorithm. So, there you have why the sampling rate is halved.

It's not really a sampling rate limitation... That's good to know. Thanks!

[damrod]

so he-aac works like mp3pro??

i inderstand better no ;-))

[bond]

Originally posted by damrod
so he-aac works like mp3pro??exactly, both use the same technology: SBR "Spectral Band Replication"

but in contrary to mp3pro, he-aac is an official part of a mpeg standard

[KpeX]

Check out the AAC FAQ #2 (http://forum.doom9.org/showthread.php?s=&threadid=68300#post424070), especially the second paragraph, for more technical explanations of SBR / HE AAC.

[Toll]

Thanks, you guys really help me a lot...:)