AAC bitrate vs. AC3 bitrate

[Vetal]

I itried to reencode AC3 file to AAC file. Settings:

Input AC3 file: 57Mb @ 224Kbps, 2 Ch

Output file: 18Mb @ Adaptive Bitrate, 224 bps, 2Ch.
AAC Profile: HE-AAC+PS

--------------------

Quastions:
1. If 2 files of the same lenght, channel numbers and bit depth compressed. And they encoded with the same bitrate they must be about the same size? Why mine AAC is 3 times as small as AC3? Was it any mistake (I just beginning with AAC)
2. Do I significally loose in quality? I'm not pro and can't hear the difference. But I'm not pro listener.
3. What bitrate/profile sane-pro ( :) ) would chose to reencode NNN Kbps AC3? To keep as much information as possible without unreasonable overkill. I mentioned sane-pro to eliminate "Uncompressed" answer :)
4. What if there us 6Ch (5.1) AC3. What AAC profile and bitrate (ratio to source bitrate) should I choose?
5. Same as #4 for for DTS source?

Thank you!

[check]

1. If the number of channels is the same, and the length is also the same, the bitrate must be different is the file size is different.
2. There's no objective measure that can tell you. If your subjective opinion is that it doesn't, that's generally good enough :)
3-5. Personal preference.

[Vetal]

1. If the number of channels is the same, and the length is also the same, the bitrate must be different is the file size is different.


Source file is about 33 min long. And AC3 properties shown ac 224kbps.

Anyway, as a failsafe,

Size = N_min * 60_sec * (bitrate_kbit * 1024 / 8) = 33 * 60 * (224 * 1024 /8) ~ 54 mb

So, AC3 size is right.

But why is AAC is 18 mb, if I set Adaptive bitrate 224 bps (via MeGUI). It using Nero encoder. 18 Megs gives me ~75Kbps according to formula. And 75Kbps < 224Kbps, that is my question

[tebasuna51]

Bitrate = Size_in_bits / TimeLength_in_seconds

Not matter the channel number or bit depth sample, equal length + equal bitrate => equal size.

Something is wrong in your files. Check the aac bitrate, 224 Kb/s is too much for stereo audio. If the timelength is the same the bitrate can be 70 Kb/s.

When re-encode always lose quality. If you listen the audio with simple headphones, TV speakers or so on, you never value the difference.

Like check say your questions are personal preferences involving the kind of audio source, the audio equipment to play and the need of save space.

To transcode to aac I think the better choice is select a quality based profile (let the encoder select HE or LC and the apropriate bitrate for the channel number).

From AC3/DTS make your test with quality between 0.2 to 0.4, for me less than 0.2 is bad and more than 0.4 don't save space then better preserve the originals.

Edit: I don't see your last post. Maybe MeGUI is not the adequate way to transcode audio. Use BeLight, BeHappy, Foobar or use the MeGUI threads to put your questions.

[Vetal]

Bitrate = Size_in_bits / TimeLength_in_seconds

Not matter the channel number or bit depth sample, equal length + equal bitrate => equal size.

Something is wrong in your files. Check the aac bitrate, 224 Kb/s is too much for stereo audio. If the timelength is the same the bitrate can be 70 Kb/s.

When re-encode always lose quality. If you listen the audio with simple headphones, TV speakers or so on, you never value the difference.

Like check say your questions are personal preferences involving the kind of audio source, the audio equipment to play and the need of save space.

To transcode to aac I think the better choice is select a quality based profile (let the encoder select HE or LC and the apropriate bitrate for the channel number).

From AC3/DTS make your test with quality between 0.2 to 0.4, for me less than 0.2 is bad and more than 0.4 don't save space then better preserve the originals.

Edit: I don't see your last post. Maybe MeGUI is not the adequate way to transcode audio. Use BeLight, BeHappy, Foobar or use the MeGUI threads to put your questions.

Thank you for a clear answer.

1. I tried foobar2000. It allows to select VBR Quality, but there is no LC or HE selection for Nero. How could I choose it?

2. File size confusion came from +PS (Parametric Stereo) part. I just read about it (http://forum.doom9.org/showthread.php?t=131918&highlight=Parametric+Stereo) and realized why it is so small

[check]

there is no LC or HE selection for Nero. How could I choose it?You shouldn't need to. The encoder will decide with good accuracy itself.

Actually, from your point 2 it sounds like you are seriously confused. The HE options trade off quality for compression. They don't just increase compression! You do not want to use them in general when your bitrate is over 96kbits (for stereo) or so, and PS should only be used at extremely low bitrates (say 30kbits for stereo).

[Vetal]

You shouldn't need to. The encoder will decide with good accuracy itself.

Actually, from your point 2 it sounds like you are seriously confused. The HE options trade off quality for compression. They don't just increase compression! You do not want to use them in general when your bitrate is over 96kbits (for stereo) or so, and PS should only be used at extremely low bitrates (say 30kbits for stereo).

Thank you. Very nice note indeed! Especially valid for people who can not dedicate much time in learning every detail. So, if you read doom9.org rather quick,
"LC means "low complexity" and HE means "high efficiency"

It is easy to make wrong conclusion that HE is superior AAC profile over LC (like x264 vs XviD). I think your point should be reflected in AAC FAQ.
It is a lots of entusiasts who would prefer bigger size for better quality, where LC is better. While subconciously HE inprinted in mind as "superior" over LC, it is harder to get right conclusion later on. And I was one of them.

[check]

I think your point should be reflected in AAC FAQ.Well, it's here at least :):
http://mewiki.project357.com/wiki/Computer_movie_files/Audio#AAC

[shon3i]

The HE options trade off quality for compression. They don't just increase compression!You simply wrong, all lossy encoders (audio and video) trade off quality for better compression :). LC-AAC is not good in multichannel encoding without some high bitrate 256kbps and higher, where is 256kbps not always good. From last AES ABX test with E-AC3 @ 384, AC3 @ 448 and HE-AAC @ 160kbps, and HE-AAC shown, that have very similar quality for most listener's.

[Tirade]

From last AES ABX test with E-AC3 @ 384, AC3 @ 448 and HE-AAC @ 160kbps, and HE-AAC shown, that have very similar quality for most listener's.

Is this for stereo only or does that also apply to multi-channel?


LC-AAC is not good in multichannel encoding without some high bitrate 256kbps and higher, where is 256kbps not always good.

If your source is AC3 640K 5.1 and you are transcoding to AAC-LC for DVD backup, is quality .7 recommended? This seems to conflict what most people say about .35 being sufficient. Maybe .35 is only sufficient for stereo?

[shon3i]

Is this for stereo only or does that also apply to multi-channel?Only for multichannel. Stereo situation is very clear. There is no need to use HE-AAC over 96kbps, but DSBR sometimes can help on higher bitrates IMHO :)

If your source is AC3 640K 5.1 and you are transcoding to AAC-LC for DVD backup, is quality .7 recommended? This seems to conflict what most people say about .35 being sufficient. Maybe .35 is only sufficient for stereo?I realy prefer ABR/CBR multichannel encoding, and really i can't tell you what is most bitrate good. Better use your ears to find the golden mean :)

[check]

You simply wrong, all lossy encoders (audio and video) trade off quality for better compression .
Yes, to be more precise in what I was trying to say: HE lowers the maximum possible quality attainable in today's encoders, while simultaneously giving a disproportionate boost to compression efficiency.

LC-AAC is not good in multichannel encoding without some high bitrate 256kbps and higher, where is 256kbps not always good. From last AES ABX test with E-AC3 @ 384, AC3 @ 448 and HE-AAC @ 160kbps, and HE-AAC shown, that have very similar quality for most listener's.Your point being? As I recommended HE for consideration at bitrates under 96kbits for stereo, it should be reasonably simple to extrapolate that to 6 channel audio and see HE might be a good idea there too for bitrates under 288kbits.