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AAC question ?

hi, i see a large number of 5.1 dvd/bd rips these days use this codec , im not real familiar with it so im guessing its origin was built in to quicktime , i do hear some good things about this codec but why would you encode using aac when there is no available hardware to decode it  , maybe its so you can watch/hear your movies on/thru your computer in someway ??  am i correct here or is there another explanation.
This is my first post here and i hope im within the rulebook but have used this forum many times  so keep up the good work.       

      john


edit / sorry should be in general forum

AAC question ?

Reply #1
Welcome to Hydrogenaudio.

AAC is not something new. It is false also that there isn't any hardware support. There are good chances that your mobile phone (if built during the last 3-4 years) has AAC support.


AAC is the codec used for MP4. It is the codec that iTunes Music Store uses since the beginning. Of course, all iPod's play AAC.

I would also like to clarify that AAC origins don't come from Quicktime. In fact AAC is just a more mature MP3. (mostly the same principles, but more tools and better tuned).


Now, onto video:

Standard DVD discs used the standard called MPEG-2, which used MPEG-2 video, and AC-3 audio (sometimes MP2 audio, the predecesor to MP3).

Then, if you've heard of DivX, that was originaly MPEG-4 video codec with MP3 audio (Sometimes AC-3 instead). (Nowadays it i H264 with AAC)*

Blu-ray discs use MPEG-4 video. Usually with AAC audio, although it can come with LPCM too.


(I am not a video expert, so if there's any error in the above sentences, feel free to correct me).



*Ok, in its origins origins, it was a tweaked Microsoft video with a tweaked wma audio.

AAC question ?

Reply #2
The main reason you encounter AAC audio in BD/DVD rips so often is that it's the standard audio codec for MP4 containers, mostly used along with the powerful H.264 video codec. It is often used in Matroska containers (*.mkv) as well due to its efficiency.
However, it's not used on Blu-ray Discs, which mainly use AC3, DTS or LPCM. There are some other Dolby and DTS standards involved, but no AAC.
Nothing is impossible if you don't need to do it yourself.

AAC question ?

Reply #3
thanks so much for the welcome guys, now, your absolutety correct i can force my cell to playback aac mp4s  as for ipod i never bought in to the concept which may explain my total ignorance ....... point is you cant play your pristine HD 5.1 BD mkvs/mp4 aac's back "properly" on your home theatre much less your phone unless you fix the mapping problems re. DTS and DolbyDigital  for starters , i guess stereo would be ok but i havent watched a stereo movie in probably twenty years .

                                        john

AAC question ?

Reply #4
Quote: [JAZ]
"Blu-ray discs use MPEG-4 video. Usually with AAC audio"

As Silversight said, usually Blu-ray comes with 5.1 lossless english & other usually langage 5.1 AC3 & sometimes 5.1 DTS (music) or 2.0 AC3 (comments). (I have seen E-AC3 once too)

I would love Blu-ray to use AAC natively as AC3 is one of the worst codec around, but AFAIK AAC is not even in the Blu-ray spec.

IMHO it is not worth it to transcode AC3 to AAC because the source is usually 5.1 640Kbps which means that it is (/5x2= 2.0 256Kbps) quality wise equivalent to 2.0 256Kbps.

AAC being really efficient (transparent) near 192Kbps VBR for 2.0, if you transcode AC3 5.1 to AAC 5.1, you will gain almost no size if you want to keep both 5.1 & transparency (In the 2.0 CD world it would be roughthly equivalent to a bad MP3 encoder at 256Kbps to an AAC encoder 192Kbps, which no one would do in the audio world)

... so transcoding AC3 to AAC is only for people who want to drastically reduce the audio bitrate with sure lost (like MP3 256Kbps to 128Kbps AAC or worst in the 2.0 CD world) but many people doesn't have the experience to understand it (or they just don't care about audio quality)

Actually the only clever AC3 to AAC encoding is IMHO when you convert 5.1 to 2.0.

But if the source is lossless, then AAC is a way better codec than AC3 at the same bitrate, so if you can't offer to keep lossless, but that your source is lossless, AAC is one of the best lossy codec around be it for 2.0 CD audio or 5.1 BR video.

For this use only Vorbis can rivalize but as nowadays most people use mp4 AVC (x264) they tend to naturally use mp4 AAC, even if they use the mkv container over mp4 for subtittles support.

The doom of Vorbis will slowly come from the video world, because Xiph doesn't compete with H264: once you start using AAC for video you naturally don't want to use Vorbis anymore for CD. That is what is slowly happening to me, I like vorbis & theora, but without efficiency these codecs are limited to streaming. (which is not the same use as optimized backup but is not a shame at all)

So if the question is why do these guys use AAC, the answer is because it's a good tool for the job if the source is lossless. Now if the source is lossy the "gain" is near to zero IMHO.

Also if the container is mkv, it is very likely that the AAC files are Nero AAC & not itunes. I don't even know if itunes/quicktime supports 5.1 AAC.
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AAC question ?

Reply #5
It is often used in Matroska containers (*.mkv) as well due to its efficiency.


Actually, the mkv container isn't anymore efficient than either the mpeg-2/VOB, mpeg-4, or m2ts containers.  mkv is often used simply because it is an open source container and allows for the pairing of mpeg-4 AVC video with Dolby Digital 5.1 (or DTS 5.1) audio along with non-hard coded subtitles.  Personally, most mkv advocates that I come across are using the container in a way that doesn't take advantage of its features (such as multiple soundtracks).  Additionally, all mkv videos I have come across will work perfectly in either a VOB or m2ts container.  However, a 9Mbps mpeg-4 AVC video with 640kbps Dolby Digital 5.1 audio will consume the same amount of space regardless of the container.  I am just not a big fan of a container that has little hardware support and doesn't offer me any benefits over something such as the m2ts container (which is natively compatible with my PS3).

AAC hardware support has increased over time though.  So I am not sure where the "why would you encode using aac when there is no available hardware to decode it" statement comes from.  You have iPods, the newer Zune players, many different cellphones, the Xbox 360, PS3, PSP, car CD decks, the Nintendo Wii, Nintendo DSi, Creative line of portable products (starting with the Zen and above), stand-alone Blu-ray players, and many more devices that all playback AAC audio (with many of the portable players, consoles, and stand-alone Blu-ray players handling AAC audio in a mpeg-4 video).  So why not use a format that works with many devices?*


*I consider AAC hardware support (in an mpeg-4 container for video playback) to cover "many" devices especially when three common devices in my house work with mpeg-4 and mpeg-4 AVC videos, in an mpeg-4 container, with AAC audio.

AAC question ?

Reply #6
The popularity of DTS/AC3/...->AAC conversion in the video "backup" scene has had a single cause: the possibility to create fully standard compliant ISO 14496-12/14 MP4-packages (with potentially great hardware support). Until 2008 the format didn't specify the inclusion of AC3 audio tracks, thus all attempts to do so anyway would have been non-standard.

For MKV that was never the case and didn't make much sense. You could always include the original (AC3) audio tracks without transcoding. The benefit of AAC compression is negligible, especially compared to the size of the accompanying video track in times of HD content. You have to keep AAC bitrates quite high so that transcoding artifacts don't become audible. Additionally you loose AC3's integrated support for dynamic compression in different playback environments, which can be optimized be the recording engineer.

I would always recommend keeping the original (lossy*) audio track of your choice in store it in a video container that supports this (as MKV and MP4) and just re-encode the video. Future developments are no problem since repackaging of your video and audio streams into a different container is a lossless process.

M2TS is great but doesn't allow the inclusion of subtitles.

Generally MKV is probably the best format right now for archival. Ad hoc you can always generate the target format, that the rest of your ecosystem supports - and that even losslessly.

* For lossless source tracks encoding to AAC is worth a thought.

AAC question ?

Reply #7
Actually, both the VOB and m2ts containers support non-hard coded multiple subtitles (along with multiple soundtracks).

AAC question ?

Reply #8
Oh, do they? That's interesting to hear!

I'll have a look at it again, then. Last time I checked there wasn't anything, yet. Is it cleartext only, as in MP4, or can you also preserve bitmap based subs?

But don't want to take this off-topic. Feel free to answer via PM, if you like.


Edit: PM thankfully received. I have been convinced that the m2ts format (and it seems also available tools) deliver anything I need nowadays. It's just as good if not better for archival as mkv.

 
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