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Topic: what does AAC object type.....mean? (Read 8036 times) previous topic - next topic
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what does AAC object type.....mean?

what does AAC object type.....mean?......ie.... (LC (Low Complexity, Main or LTP (Long Term Prediction) ??

is it about sound quality or compression ?

s8n

what does AAC object type.....mean?

Reply #1
Are you refering to MP4 file header object types ??

what does AAC object type.....mean?

Reply #2
Some are mostly related to sound quality, but there are other, more specific profiles.

LC - low complexity - became the "default" profile. It has high quality and is relatively easy on CPU consumption for both encoding and decoding.

Main and LTP - long term prediction - are LC + more techniques to make quality even better. But, as it seems, in the end these techniques don't make that much difference and require MUCH more CPU power. For that reason, they aren't widely supported.

LD - low delay - is used for telecommunications. The other profiles introduce delay of about 200ms to their encodes, and that would be annoying on a, EG, phone call. LD has a delay of 30ms.

SSR - scaleable sampling rate - is a technique to encode a stream in several layers, starting at very low bitrates and going all the way up to lossless. This allows a streaming server to dinamically serve different bitrates to clients based on their bandwidth. It just needs to "shave" the higher bitrates until it reaches one that can be safely streamed to the client.
In concept it's the same as Vorbis' bitrate peeling, but in implementation they are quite different.

There are also optional enhancement tools, most of them parametric: PNS, SBR (HE AAC), Parametric Stereo...

Regards;

Roberto.

what does AAC object type.....mean?

Reply #3
Quote
Some are mostly related to sound quality, but there are other, more specific profiles.

LC - low complexity - became the "default" profile. It has high quality and is relatively easy on CPU consumption for both encoding and decoding.

Main and LTP - long term prediction - are LC + more techniques to make quality even better. But, as it seems, in the end these techniques don't make that much difference and require MUCH more CPU power. For that reason, they aren't widely supported.

LD - low delay - is used for telecommunications. The other profiles introduce delay of about 200ms to their encodes, and that would be annoying on a, EG, phone call. LD has a delay of 30ms.

SSR - scaleable sampling rate - is a technique to encode a stream in several layers, starting at very low bitrates and going all the way up to lossless. This allows a streaming server to dinamically serve different bitrates to clients based on their bandwidth. It just needs to "shave" the higher bitrates until it reaches one that can be safely streamed to the client.
In concept it's the same as Vorbis' bitrate peeling, but in implementation they are quite different.

There are also optional enhancement tools, most of them parametric: PNS, SBR (HE AAC), Parametric Stereo...

Regards;

Roberto.
[a href="index.php?act=findpost&pid=235932"][{POST_SNAPBACK}][/a]



Predictions (Main / LTP) can sometimes improves coding efficiency by as much as 50% like in the case of Bi.wav which contains only tonal components and no block switching... I did a study of the efficiency of Main Profile..

I think this profiles stuff existed to limit the decoder complexity.. A person building a decoder only have to conform to a specific profile depending on his application.. instead of having to lump everything into a single decoder.. This is important especially in mobile gadget applications where there is a constrain on memory and computation requirements..

wkwai

what does AAC object type.....mean?

Reply #4
thanx for all that info guys......i know what i want now......only how does
....  -c <freq>  work in FAAC ? .....i dont want any loss to the original file....ie if the orginal file is 44.1khz the final AAC is 44.1khz.... also i dont any khz downgrading to make a smaller file........

and can FAAC do 48khz ?.......

s8n

what does AAC object type.....mean?

Reply #5
If you don't want any loss use a lossless codec like FLAC or Wavpack.
"To understand me, you'll have to swallow a world." Or maybe your words.

what does AAC object type.....mean?

Reply #6
I'm afraid i have lost contact to this audio compression format during the last months  and I would be glad if somebody could bring me up to date.
I haven't found much here, so if you could answer my quiestions maybe or post any link, i would be happy!

1) Aac is using VBR too like Vorbis (generally) does!?

2) What expression 'HE' is standing for?

3) HE-Aac files are always below 128kbps, right!?

4) HE-Aac files are using spectral band replication (SBR), the same technique that also in MP3pro is used?

5) I have read something about 'parametric stereo (PS)' - do HE-Aac files also contain this?

6) Does exist any despription (for nomally dying people) how all this features like PS work? - I just like to have an impression how the files are encoded i am listening to.



Quote
...
Main and LTP - long term prediction - are LC + more techniques to make quality even better. But, as it seems, in the end these techniques don't make that much difference and require MUCH more CPU power. For that reason, they aren't widely supported.
...

Quote
Predictions (Main / LTP) can sometimes improves coding efficiency by as much as 50% like in the case of ...

This is confusing me a bit...

7) ...however, does a Main/LTP-encoded Aac file probably cause any risks for decoding?


Thanks any reply!

what does AAC object type.....mean?

Reply #7
Quote
Aac is using VBR too like Vorbis (generally) does!?


Often.  Some encoders only do CBR though (iTunes).

Quote
2) What expression 'HE' is standing for?


I believe its High Effciency.

Quote
3) HE-Aac files are always below 128kbps, right!?


HE is recommended around 80k and below, but its not that common because many decoders do not support it.  Most just use LC for all bitrates (iTunes, Ipod).  Hopefully this will change.

Quote
4) HE-Aac files are using spectral band replication (SBR), the same technique that also in MP3pro is used?


Yes.

Quote
5) I have read something about 'parametric stereo (PS)' - do HE-Aac files also contain this?


They can, but as of now, theres no encoder available to make PS, just samples released by Ahead.  At least I haven't seen an encoder that can.

Quote
7) ...however, does a Main/LTP-encoded Aac file probably cause any risks for decoding?


Theres no risk in decodeing anything.  The standard spells out a correct way to decode files, and you are either right or wrong when writeing a decoder.  Its just other profiles consume more CPU time to decode, and thus require more expensive hardware on portables (and bigger batteries, etc).

what does AAC object type.....mean?

Reply #8
Quote
Quote
Aac is using VBR too like Vorbis (generally) does!?


Often.  Some encoders only do CBR though (iTunes).

[a href="index.php?act=findpost&pid=242967"][{POST_SNAPBACK}][/a]


iTunes/QuickTime actually use more of an ABR implementation at present...I don't know of any AAC codecs that do true CBR.

what does AAC object type.....mean?

Reply #9
Quote
Quote
Quote
Aac is using VBR too like Vorbis (generally) does!?


Often.  Some encoders only do CBR though (iTunes).

[{POST_SNAPBACK}][/a]


iTunes/QuickTime actually use more of an ABR implementation at present...I don't know of any AAC codecs that do true CBR.
[a href="index.php?act=findpost&pid=242969"][{POST_SNAPBACK}][/a]


Nope.  Its CBR according to one of the developers who wrote it:

[a href="http://www.hydrogenaudio.org/forums/index.php?showtopic=17537&hl=#]http://www.hydrogenaudio.org/forums/index....opic=17537&hl=#[/url]

Edit:  woah strangeness

what does AAC object type.....mean?

Reply #10
Quote
iTunes/QuickTime actually use more of an ABR implementation at present...I don't know of any AAC codecs that do true CBR.
[a href="index.php?act=findpost&pid=242969"][{POST_SNAPBACK}][/a]


Nero in CBR mode is pure CBR.

what does AAC object type.....mean?

Reply #11
Quote
Quote
7) ...however, does a Main/LTP-encoded Aac file probably cause any risks for decoding?


Theres no risk in decodeing anything.  The standard spells out a correct way to decode files, and you are either right or wrong when writeing a decoder.  Its just other profiles consume more CPU time to decode, and thus require more expensive hardware on portables (and bigger batteries, etc).
[a href="index.php?act=findpost&pid=242967"][{POST_SNAPBACK}][/a]


I would say that the "risk" of using Main/LTP is that portable devices probably never will support it. The extra memory/computing effort needed to decode them is extra painful for such a device.

Moreover, HE and PS are only defined with LC AAC.

what does AAC object type.....mean?

Reply #12
Quote
3) HE-Aac files are always below 128kbps, right!?


HE-AAC can be used at any bitrate. It's just harder to get a benefit from it at more than 128kbps, so it generally won't be used for those files.

what does AAC object type.....mean?

Reply #13
Cool, thanks for all the replies! - That brings some light in that prospering audio compression type.

Meanwhile i also have found a rough answer in the web about Parametric Stereo.


As i have seen in this parallel Aac thread a new decoder chip supporting the new codec features is out. But i guess it will take some months until it will be available in consumer devices.

 
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