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Topic: Is AAC transparent in 5.1 at high bitrates (Read 14632 times) previous topic - next topic
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Is AAC transparent in 5.1 at high bitrates

I'm currently building a library of 5.1 music from lossless, high-resolution sources.

To save space, I'm trying to determine what codec to use.

Is AAC in 5.1 transparent at high bitrates? I can't seem to find any data or studies.

Alternatively, I'm considering 16-bit, 48khz flac with noise shaping, or 20-bit without noise shaping.

My goal is to keep the music as close as possible to the limits of human hearing, without wasting space.

Thoughts?

Is AAC transparent in 5.1 at high bitrates

Reply #1 –
Quote
Is AAC in 5.1 transparent at high bitrates? I can't seem to find any data or studies.
Probably.  Most lossy-compression formats are darn-good at higher bitrates.

Quote
Alternatively, I'm considering 16-bit, 48khz flac with noise shaping, or 20-bit without noise shaping.
16-bits should be fine with or without dither & noise shaping.  Dither is one of those things that "everybody does", but at 16-bits dither is about 90dB down.  If you don't have a feeling for how quiet that is, try reducing the volume by 90dB in your audio editor and see if you hear anything.    (No cheating by turning-up the volume after reducing it by 90dB.)

20-bit seems non-standard and you'd have to make sure your player supports it.  Does FLAC support 20-bits?

Is AAC transparent in 5.1 at high bitrates

Reply #2 –
Quote from: DVDdoug on
Does FLAC support 20-bits?
FLAC supports 20 bits.

It also supports signalling zero-padding in the low-order bits separately from the compressed data to reduce overhead. If 20-bit FLAC is too much of a hassle, you can zero-pad the audio to 24 bits before compressing. FLAC will automatically detect and strip the empty 4 bits, and the result will be the same size as if you'd compressed it as 20 bits.

Is AAC transparent in 5.1 at high bitrates

Reply #3 –
20 bit Flac is legal, but unsupported in most players.  The reference flac tool doesn't support it, for example, and the last time I tried it didn't fail very nicely either.

Is AAC transparent in 5.1 at high bitrates

Reply #4 –
20-bits FLAC is uncommon, but are you serious that reference FLAC does not support it? http://xiph.org/flac/format.html#metadata_block_streaminfo suggests it does ... but I guess it needs a 20-bit infile then?

Anyway, FLAC specifies one wordlength for the file and one in each frame header (where a 000 in the latter means to inherit from the stream info). IIRC, putting a 16-bit signal in a 24-bit FLAC file (with the 8 LSB being zero, that is) doesn't increase filesize, and that is because the encoder can put each frame's wordlength to 16. So ... is there any point at all in using a 20-bit FLAC file?

Is AAC transparent in 5.1 at high bitrates

Reply #5 –
Quote from: lithopsian on
20 bit Flac is legal, but unsupported in most players.  The reference flac tool doesn't support it, for example, and the last time I tried it didn't fail very nicely either.

What "reference flac tool"?
This 20bit flac file was encoded by the official flac command line encoder. It can decode this file back to a 20bit wav, and then encode back again to a 20bit flac. Have tried version 1.2.1 and 1.3.1, both did work fine.
[attachment=8147:George.flac]
I'm also afraid you went too far by saying "unsupported in most players", since 20bit flac is supported by libavformat/libavcodec of ffmpeg/libav project, which means that quite many of players based on them should also support it (including direct show based players using lavfilters).
Other than that, players such as fb2k or winamp support 20bit flac, for example.

Is AAC transparent in 5.1 at high bitrates

Reply #6 –
Never worked for me.  I don't have the latest version to test with.

Is AAC transparent in 5.1 at high bitrates

Reply #7 –
Quote from: GWBasic on
Is AAC in 5.1 transparent at high bitrates? I can't seem to find any data or studies.

There was a blind listening test done by the EBU in 2007, see here (page 57 for AAC-LC).

This was at 320 kbps 5.1. At 448 kbps and above AAC-LC is fully transparent to my ears IIRC. Don't remember, but maybe this bit-rate is supported by Fraunhofer's AAC encoder in Winamp, so you might be able to give it a try.

Chris
If I don't reply to your reply, it means I agree with you.

Is AAC transparent in 5.1 at high bitrates

Reply #8 –
Quote from: lithopsian on
Never worked for me.  I don't have the latest version to test with.

As I have said, it did work on the version 1.2.1, which is 7 years old.

Is AAC transparent in 5.1 at high bitrates

Reply #9 –
Quote from: nu774 on
Quote from: lithopsian on
Never worked for me.  I don't have the latest version to test with.

As I have said, it did work on the version 1.2.1, which is 7 years old.


I've run with your file.  Crash, but it may not be specific to Flac.  The crash is an assert related to masking in the Linux kernel, as called from Alsa.  Don't know why.  I'll look into it further when I have time.

Is AAC transparent in 5.1 at high bitrates

Reply #10 –
Going back to the original topic, I would be careful about multichannel AAC.  I remember a few years ago using the Nero encoder to compress some 5.1 audio, with sufficient bit rate (500kb/s or more) and it came out with very, very ugly artifacts. I know about TOS#8, but I'm too lazy to dig up the original thread. But, one of the Nero AAC devs actually replied and said that the multichannel encoding was a "work in progress" at the time.

I don't know how Apple's encoder fairs, hopefully better. Anyone with more details on multichannel AAC, speak up!

Is AAC transparent in 5.1 at high bitrates

Reply #11 –
Quote from: slks on
I don't know how Apple's encoder fairs, hopefully better. Anyone with more details on multichannel AAC, speak up!

As far as I understand, nothing special is done for multichannel AAC. 5.1ch AAC is just encoded as Mono (center) + Stereo (L+R) + Stereo (Ls+Rs) + LFE.
In other words, channel coupling is always fixed. Just as if you split channels of input like this before feeding to the encoder, then concatenate the resulting AAC channel elements. It should need about 2.5 times higher bitrate than ordinary stereo.
I know that there's a CCE(coupling channel element) in the spec for more flexible channel coupling than this, but I haven't seen it actually in use.

Is AAC transparent in 5.1 at high bitrates

Reply #12 –
Quote from: slks on
I don't know how Apple's encoder fairs, hopefully better. Anyone with more details on multichannel AAC, speak up!

As far as I understand, nothing special is done for multichannel AAC. 5.1ch AAC is just encoded as Mono (center) + Stereo (L+R) + Stereo (Ls+Rs) + LFE.
In other words, channel coupling is always fixed. Just as if you split channels of input like this before feeding to the encoder, encode each of them separately, then concatenate the resulting AAC channel elements. It should need about 2.5 times higher bitrate than ordinary stereo.
I know that there's a CCE(coupling channel element) in the spec for more flexible channel coupling than this, but I haven't seen it actually in use.

[edit]Sorry, replied instead of editing the previous post.[/edit]

Is AAC transparent in 5.1 at high bitrates

Reply #13 –
Quote from: slks on
Going back to the original topic, I would be careful about multichannel AAC.  I remember a few years ago using the Nero encoder to compress some 5.1 audio, with sufficient bit rate (500kb/s or more) and it came out with very, very ugly artifacts. I know about TOS#8, but I'm too lazy to dig up the original thread. But, one of the Nero AAC devs actually replied and said that the multichannel encoding was a "work in progress" at the time.

I don't know how Apple's encoder fairs, hopefully better. Anyone with more details on multichannel AAC, speak up!


Yeah, I ended up getting some weird static in my right channel when I used AAC (7xx kbps in Handbrake on Apple). I don't know if it was equipment-related, but when I re-listened to the same material in 16-bit noise-shaped flac, there was no static!

The size difference wasn't phenomenal: 7xx kbps AAC was about 0.5 gigs for 1:15, and flac was 1.3 gigs for the same material at 16-bit, noise-shaped.

I'm going to stick with flac, mostly because it's 0% risk. It seems that lowering resolution to 16-bit, 48khz is the safest and most effective way to save space.

Thanks everyone!

Is AAC transparent in 5.1 at high bitrates

Reply #14 –
Quote from: C.R.Helmrich on
Quote from: GWBasic on
Is AAC in 5.1 transparent at high bitrates? I can't seem to find any data or studies.

There was a blind listening test done by the EBU in 2007, see here (page 57 for AAC-LC).

This was at 320 kbps 5.1. At 448 kbps and above AAC-LC is fully transparent to my ears IIRC. Don't remember, but maybe this bit-rate is supported by Fraunhofer's AAC encoder in Winamp, so you might be able to give it a try.

Chris


That was a really awesome read. After looking at the results, it's pretty clear that lossless is the right choice. It seems that the lossy codecs are really only useful when there's a bandwidth restriction.

After reading that, I'd consider DTS, but it's bitrate is so high that I'm not sure it's worth the hassle. Flac encoders are free.

Is AAC transparent in 5.1 at high bitrates

Reply #15 –
Quote from: GWBasic on
After looking at the results, it's pretty clear that lossless is the right choice. It seems that the lossy codecs are really only useful when there's a bandwidth restriction.

Yes, of course. When you can spend as many bits as you like, lossless compression is the best choice.

Quote
After reading that, I'd consider DTS, but it's bitrate is so high that I'm not sure it's worth the hassle. Flac encoders are free.

Note that, in principle, the AAC format also allows up to 1500 kbps for a 5.1 surround signal. Note also that on page 21 the report shows that AAC at 320 kbps (and WMA9 at 448 kbps) sounds better than DTS at 448 kbps.

Chris
If I don't reply to your reply, it means I agree with you.

Is AAC transparent in 5.1 at high bitrates

Reply #16 –
Wasn't opus pretty awesome with mutlichannel compression? The concern would be software/hardware support I guess.