Skip to main content ↑ ↓

Notice

Please note that most of the software linked on this forum is likely to be safe to use. If you are unsure, feel free to ask in the relevant topics, or send a private message to an administrator or moderator. To help curb the problems of false positives, or in the event that you do find actual malware, you can contribute through the article linked here.
Topic: What will happen when AAC-LC decoder decodes HE-AAC-V2 encoded stream (Read 5644 times) previous topic - next topic
0 Members and 1 Guest are viewing this topic.

What will happen when AAC-LC decoder decodes HE-AAC-V2 encoded stream

what will happen when AAC-LC decodes HE-AAC-V1,V2 encoded stream?
Below are some thoughts from me, please correct me if i'm wrong and please also add your valuable opinions if any, i'm newbie

Scenario 1: AAC-LC decodes HE-AAC-V1 encoded stream.
Since HE-AAC-V1 uses SBR, so the high band data is in the form of SBR data, but AAC-LC decoder can't uses these SBR data to reconstruct high band. finally the aac file's high band is not played out.

Scenario 2: AAC-LC decodes HE-AAC-V2 encoded stream.
Firstly same as scenario 1, high band data is abandoned, and besides, the sound played out can't be stereo again since PS technology makes the encoded data monaraul signal and AAC-LC can't reconstruct the stereo signal.

Am i right? i very much doubt i'm wrong. some expert correct me please.

some other questions:
1, what's the sample rate relationship between AAC-LC and HE-AAC?
2, the unit that SBR performs on is one single sample, right?

thanks a lot for your response!:)

What will happen when AAC-LC decoder decodes HE-AAC-V2 encoded stream

Reply #1 –
I will answer this post since it was first.  I think you are right on both accounts.  I know that you are right on the first scenario in that a LC-AAC only decoder will play a HE-AAC V1 file without the SBR data.

As for your other questions, I can only answer the first one.  From my experience, an LC-AAC only decoder will play the HE-AAC at half the sample rate.  So I can have a 48kbps HE-AAC file at 44.1KHz and iTunes (or QuickTime) will play it at 22.05KHz.  This is the same as mp3pro.  The last time I tried HE-AAC was back in 2005 or 2006 so things might have changed.  I don't think they have changed though as I recently downloaded a free audiobook and it is encoded using HE-AAC and iTunes still recognizes it as being 22.05KHz.

Either way, you are not going to get full performance when decoding HE-AAC files (either V1 or V2) using a LC-AAC only compatible decoder.  A 48kbps HE-AAC V1/V2 file will end up having less audio quality than a 48kbps LC-AAC.  I was told this here on HA a few years ago and think that it holds true today.

What will happen when AAC-LC decoder decodes HE-AAC-V2 encoded stream

Reply #2 –
The scenarios are correct. I have no idea why you even had a doubt about it. It is clear enough knowing what the HE modes are.  HE-AAC v1 encodes a stereo signal at half the sample rate, and HE-AAC v2 encodes a mono signal at half the sample rate*

Quote from: zbt on
some other questions:
1, what's the sample rate relationship between AAC-LC and HE-AAC?
2, the unit that SBR performs on is one single sample, right?


1) An HE-AAC file has half the sample rate when decoded by an AAC-LC decoder.
2) I don't understand that question. Do you ask if the SBR is applied sample by sample, or to an entire chunk or samples?


* ok, that was really a simplified explanation

What will happen when AAC-LC decoder decodes HE-AAC-V2 encoded stream

Reply #3 –
Quote
' date='Sep 2 2008, 18:03' post='585963']
1) An HE-AAC file has half the sample rate when decoded by an AAC-LC decoder.
2) I don't understand that question. Do you ask if the SBR is applied sample by sample, or to an entire chunk or samples?


thank you guys for your quick and detailed replies.

for the 2nd question, what i want to ask is that "Is SBR applied sample by sample and each sample's high band is encoded as SBR?".

And finally can i say: when decoding a HE-AACV2 file, AAC-LC decoder will generate about 1/4 decoded data compare with HE-AACV2 decoder.

What will happen when AAC-LC decoder decodes HE-AAC-V2 encoded stream

Reply #4 –
Quote from: zbt on
for the 2nd question, what i want to ask is that "Is SBR applied sample by sample and each sample's high band is encoded as SBR?".

Sample as in each one of the 44100 "dots" of a 44.1Khz signal? A sample alone has no frequency. Just an amplitude relative to full scale.
In order to get frequency information, you have to evaluate a group of samples (The more you have, the better frequency resolution, and the worse time resolution)

I am unsure if an SBR decoder first decodes the LC part at half-rate and then adds the SBR over, or if it generates a spectrum and then decodes at full-rate.

Quote from: zbt on
And finally can i say: when decoding a HE-AACV2 file, AAC-LC decoder will generate about 1/4 decoded data compare with HE-AACV2 decoder.

yes

What will happen when AAC-LC decoder decodes HE-AAC-V2 encoded stream

Reply #5 –
Quote
' date='Sep 3 2008, 20:36' post='586174']
Sample as in each one of the 44100 "dots" of a 44.1Khz signal? A sample alone has no frequency. Just an amplitude relative to full scale.
In order to get frequency information, you have to evaluate a group of samples (The more you have, the better frequency resolution, and the worse time resolution)

I am unsure if an SBR decoder first decodes the LC part at half-rate and then adds the SBR over, or if it generates a spectrum and then decodes at full-rate.


Thank you