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iTunes AAC bad quality encodes

After reading about the listening test here on HA praising 128kpbs iTunes AAC and such I decided to do my own listening test with real music I listen to.

I'm not very experienced with this, so I started at 64kbps to spot problem regions under the assumption that these will remain problem regions with similar artefacts at higher bitrates. Besides lots of other things I noticed that the stereo image was not correct, but thought this was normal for 64kbps (maybe its using is, or whatever).
However, to my surprise the stereo image problems remained up to 128kbps "VBR" (I was going in steps) and even at 192kpbs "VBR". I can't imagine no one noticed this untill now. Even 192kpbs is a piece of cake to ABX cause of this.

To make sure it's not the decoder I also decoded using iTunes with the same results.

Original sample can be downloaded here.
(Since I'm new to this: should I have uploaded the encoded version, too?)

The thunder right at the beginning is not correctly positioned. Also the second, higher frequency synth sound (the one that is percusion-like used and to the right, first used after the main synth "melody" begins) sounded strange at 128kpbs (and is also not correctly positioned) -- didn't bother to check at 192kbps.

Granted, the problem is not an anoying one -- I probably couldn't tell which is which without direct comparison, but anyways I'm less than impressed by iTune's performance.
I wouldn't have thought that AAC is so easy to ABX even at 192kbps VBR:
Code: [Select]
foo_abx v1.2 report
foobar2000 v0.8.3
2006/02/09 00:02:12

File A: file://H:\test\thunder.wav
File B: file://H:\test\_\_\_ 00 - _.m4a

00:02:13 : Test started.
00:02:42 : 01/01  50.0%
00:02:49 : 02/02  25.0%
00:02:55 : 03/03  12.5%
00:02:59 : 04/04  6.3%
00:03:13 : 05/05  3.1%
00:03:20 : 06/06  1.6%
00:03:26 : 07/07  0.8%
00:03:30 : 08/08  0.4%
00:03:45 : 09/09  0.2%
00:03:50 : 10/10  0.1%
00:03:54 : Test finished.

----------
Total: 10/10 (0.1%)


iTunes version used: 6.0.1.3

Can anyone confirm my findings? Is this a known problem?

Edit: wording, last sentence
The idea of achieving security through national armament is, at the present state of military technique, a disastrous illusion. -- A. Einstein

iTunes AAC bad quality encodes

Reply #1
What hardware were you listening on? Headphones, loudspeakers, soundcard...?
Not really a Signature.

iTunes AAC bad quality encodes

Reply #2
I was listening using onboard sound (NForce4 w/ ALC655) connected to an amplifier and Beyerdynamic DT801 headphones.

Edit:
Out of curiosity just did a test with 320kbps AAC: 10/10.. This can't be an encoding artefact, can it? I guess it's a strange bug or something (or the thunder is a killer sample, don't know)..
However the stereo image on the synth sound is correct at 320kbps (or I'm tired). It may also be some encoding artefact that I perceive as a changed stereo image. I think it was more subtle at 320kbps, but still no problem.

Edit2:
I just tested LAME 3.97b2 -V5 --vbrnew (112kbps according to foobar): It was harder for me to ABX. Stereo is correct, but the synth sounds sound different (stopped at 6/6 because I'm tired).

Edit3:
It was -V5 --vbr-new, of course.
The idea of achieving security through national armament is, at the present state of military technique, a disastrous illusion. -- A. Einstein

iTunes AAC bad quality encodes

Reply #3
How about trying another AAC encoder like Nero?

iTunes AAC bad quality encodes

Reply #4
Quote
Edit2:
I just tested LAME 3.97b2 -V5 (112kbps according to foobar): It was harder for me to ABX. Stereo is correct, but the synth sounds sound different (stopped at 6/6 because I'm tired).
[a href="index.php?act=findpost&pid=362877"][{POST_SNAPBACK}][/a]

Reading that makes me wonder if maybe you are doing something wrong during encoding to iTunes AAC, maybe transcoding? something else?...
we was young an' full of beans

iTunes AAC bad quality encodes

Reply #5
I started with a wav I ripped from the originial CD for this listening test. I used foobar with itunesencode to encode this wav to AAC.
Later I cut the wav to 30s and compressed it with flac to upload it in the upload forum.

I did not transcode from a lossy source or whatever. Also in foobar's diskwriter prefs nothing is enabled that would change the sound (dsp, rg, dither etc.).
Besides I used the same procedure with the other encodes.

I just tested with Nero AAC (some old version I happen to have installed):
internet profile - stereo image is ok. I didn't try to ABX as I'm really tired now. That's also why I will retry the Nero test tomorrow.

It would be nice if someone with experience (no pun intended *) and good ears would try the sample I uploaded. At 128k it's absolutely apparent (to me).

*The sample happens to be from the album "Experience"

Edit:
Just retested iTunes and Nero (despite being quite tired ):

Nero:
Even at internet profile didn't have any stereo problems. I didn't listen for other artefacts.

iTunes:
At 128k VBR very very obvious stereo positioning problems.
At 192k VBR still obvious stereo positioning problems, easy to ABX, but less pronounced than at 128k.
At 320k subtle stereo positioning problems. 100% ABXable, but not that easy.

Just noticed there is a newer iTunes version out. Updated and retried with iTunes v6.0.2.23 (QuickTime 7.0.4): exactly the same.
The idea of achieving security through national armament is, at the present state of military technique, a disastrous illusion. -- A. Einstein

iTunes AAC bad quality encodes

Reply #6
An update:

Because no one answered wether they hear the same problems with iTunes encodes, I asked my brother to do an ABX test. I wanted to know if I'm the only one noticing this.

This was the first time he did such a test an he's not really interested in this stuff (nor does he listen to music with headphones regularly) -- read: he's absolutely unexperienced. He did pass the 128k VBR test with 10/10 and confirmed the wrong stereo positioning.

Since the last post I also tested this with lots and lots of other samples and this problem was easily heard with all of them.

Just to show you how easy it is for me to ABX at 128k VBR:
Code: [Select]
foo_abx v1.2 report
foobar2000 v0.8.3
2006/02/10 22:29:35

File A: file://H:\test\thunder.wav
File B: file://H:\test\thunder_128k.m4a

22:29:35 : Test started.
22:29:53 : 01/01  50.0%
22:29:55 : 02/02  25.0%
22:29:57 : 03/03  12.5%
22:29:59 : 04/04  6.3%
22:30:02 : 05/05  3.1%
22:30:04 : 06/06  1.6%
22:30:05 : 07/07  0.8%
22:30:07 : 08/08  0.4%
22:30:08 : 09/09  0.2%
22:30:10 : 10/10  0.1%
22:30:12 : Test finished.

----------
Total: 10/10 (0.1%)


.. 1-2 secs per vote. Just need to listen to X to immediatly decide.

This is a serious bug in the iTunes encoder. Although the effect gets less pronounced with increasing bitrate it is still there at 320k.

I will upload a 128k encode here so it's easier for others to test.
The idea of achieving security through national armament is, at the present state of military technique, a disastrous illusion. -- A. Einstein

iTunes AAC bad quality encodes

Reply #7
I can confirm the stereo positioning problem, but not the artifact on the synth sound.

Code: [Select]
foo_abx 1.3 report
foobar2000 v0.9 beta 13
2006/02/10 16:57:34

File A: F:\Samuel\thunder.flac
File B: F:\Samuel\thunder.m4a

16:57:34 : Test started.
16:57:52 : 01/01  50.0%
16:57:58 : 02/02  25.0%
16:58:04 : 03/03  12.5%
16:58:10 : 04/04  6.3%
16:58:16 : 05/05  3.1%
16:58:21 : 06/06  1.6%
16:58:27 : 07/07  0.8%
16:58:34 : 08/08  0.4%
16:58:37 : 09/09  0.2%
16:58:41 : 10/10  0.1%
16:58:43 : Test finished.

----------
Total: 10/10 (0.1%)
we was young an' full of beans

iTunes AAC bad quality encodes

Reply #8
Quote
I can confirm the stereo positioning problem, but not the artifact on the synth sound.
[a href="index.php?act=findpost&pid=363395"][{POST_SNAPBACK}][/a]


Thanks for testing. I'll upload a 192k encode, just in case you want to test that one, too.

The synth sound sounded strange to me at a first glance, but I can't be sure of this (it was just meant as a side note). Because the positioning is so obvious, I'm not under DBT-condition wrt the synth sound anymore so it's pointless for me to really investigate.
The idea of achieving security through national armament is, at the present state of military technique, a disastrous illusion. -- A. Einstein

iTunes AAC bad quality encodes

Reply #9
Quote
Thanks for testing. I'll upload a 192k encode, just in case you want to test that one, too.
[a href="index.php?act=findpost&pid=363408"][{POST_SNAPBACK}][/a]

I actually did my own encode and tested both yours and mine, they were the same, sorry for doubting you earlier.
we was young an' full of beans

 

iTunes AAC bad quality encodes

Reply #10
I also can confirm strange behavior of iTunes (6.0.3.5) aac encoder. The killer sample is glockenspiel. Even at 320cbr. There are two distinct ringing artifacts in the right channel. The sample is uploaded here.
keeping audio clear together - soundexpert.org

iTunes AAC bad quality encodes

Reply #11
Did you also try CBR mode in iTunes? Something tells me that CBR mode is alot more optimized than VBR encoding when it comes to the Quicktime/iTunes AAC encoder.

Downloaded the samples and will start ABX'ing in a few hours (kinda busy now).
myspace.com/borgei - last.fm/user/borgei

iTunes AAC bad quality encodes

Reply #12
I didn't try this sample but I can say I did an abx 8/8 on a song Hush by Tool about a mnoth ago.
The comparison was between MPC q5 and iTunes AAC 192 VBR.
In my opinion AAC was worse on handling the snares, chrashes and similar drum kit parts.

iTunes AAC bad quality encodes

Reply #13
Quote
Did you also try CBR mode in iTunes? Something tells me that CBR mode is alot more optimized than VBR encoding when it comes to the Quicktime/iTunes AAC encoder.

It is 320 CBR.

Quote
Downloaded the samples and will start ABX'ing in a few hours (kinda busy now).
[a href="index.php?act=findpost&pid=365038"][{POST_SNAPBACK}][/a]
You don't need to ABX them - artefacts are very very audible. Just listen them.
keeping audio clear together - soundexpert.org

iTunes AAC bad quality encodes

Reply #14
drezon,
I could try this with iTunes5 if you'd like.  If so, please upload the original same in wav format.

iTunes AAC bad quality encodes

Reply #15
This might seem like a bad question, as I am new to AAC... but in iTunes, under the AAC custom settings --> channels, are you forcing stereo or using the "Auto"? Does this make a difference?

iTunes AAC bad quality encodes

Reply #16
Quote
drezon,
I could try this with iTunes5 if you'd like.  If so, please upload the original same in wav format.
[{POST_SNAPBACK}][/a]


Just go [a href="http://www.hydrogenaudio.org/forums/index.php?showtopic=41275]here[/url], and download the FLAC file. Decode it then use the WAV 

iTunes AAC bad quality encodes

Reply #17
Quote
This might seem like a bad question, as I am new to AAC... but in iTunes, under the AAC custom settings --> channels, are you forcing stereo or using the "Auto"? Does this make a difference?
[a href="index.php?act=findpost&pid=365067"][{POST_SNAPBACK}][/a]


The 'auto' option will just choose whether to encode the file as mono or stereo depending on the input file. It shouldn't make a difference on the encoding (unless you encode a mono file as stero and vice-versa).


[ EDIT : Spelling ]

iTunes AAC bad quality encodes

Reply #18
Quote
Just go here, and download the FLAC file. Decode it then use the WAV 
Decode it with what?  I don't believe iTunes or EAC will do it, and  I'm not interested in installing software to read/convert FLAC for just this one-time listen.

iTunes AAC bad quality encodes

Reply #19
Quote
Decode it with what?  I don't believe iTunes or EAC will do it, and  I'm not interested in installing software to read/convert FLAC for just this one-time listen.
[{POST_SNAPBACK}][/a]
You can download it from [a href="ftp://ftp.tnt.uni-hannover.de/pub/MPEG/audio/sqam/gspi35_1.wav]here[/url].
keeping audio clear together - soundexpert.org

iTunes AAC bad quality encodes

Reply #20
I just tried this test using the thunder sample:
WAV
AAC 128kbps VBR (iTunes 6.0.3)
MP3 LAME -V 2

It appears that the AAC encoding is rendering the low frequencies of the "thunder" sound with less channel separation - more mono information. This seems to be much more of the effect, as opposed to shifts in the frequency domain.

I am sure that this is only detectable with headphones, as such low frequencies would always sound essentially mono over loudspeakers for obvious reasons.

LAME -V 2 does not appear to have an issue in my very short tests.

The artifact is not unmusical, and I have never noticed anything "wrong" with low-frequency AAC soundstages on typical material - even older recordings with "one channel bass" using headphones.

Interesting!

iTunes AAC bad quality encodes

Reply #21
Quote
You don't need to ABX them - artefacts are very very audible. Just listen them.
[a href="index.php?act=findpost&pid=365047"][{POST_SNAPBACK}][/a]

It's hard to perform any ABX test with the file (two samples merged into one file) you uploaded
But you're right, the artefact isn't very hard to spot. I could clearly hear one after the second glockenspiel note. It's a kind of "blip" occuring after the attack. You can also "see" the visual representation of the problem:


iTunes AAC bad quality encodes

Reply #22
Nice movie, guru. 
Glockenspiel turned out to be a problem sample for both CBR and VBR modes (all bitrates) of iTunes (6.0.3.5) aac encoder. BTW, this is the main reason why aac (iTunes 6.0) is just a little bit better than mp3 (Lame 3.97b2) in SoundExpert Multiformat Listening Test @128 kbps. Also it’s worth mention that wma 9.1 pro produced very annoying artifacts with glockenspiel sample in the test as well.
keeping audio clear together - soundexpert.org

iTunes AAC bad quality encodes

Reply #23
Quote
I just tried this test using the thunder sample:
WAV
AAC 128kbps VBR (iTunes 6.0.3)
MP3 LAME -V 2

It appears that the AAC encoding is rendering the low frequencies of the "thunder" sound with less channel separation - more mono information. This seems to be much more of the effect, as opposed to shifts in the frequency domain.

I am sure that this is only detectable with headphones, as such low frequencies would always sound essentially mono over loudspeakers for obvious reasons.
Is that all this is about?  And to think it had me worried.    I agree this isn't a problem unless you listen to music with headphones.  It is generally accepted that frequencies below 80hz are non directional -- that is the listener cannot discern from what direction they came.  I only use headpones when I absolutely have to so this is a non issue for me.

iTunes AAC bad quality encodes

Reply #24
It appears that the AAC encoding is rendering the low frequencies of the "thunder" sound with less channel separation - more mono information. This seems to be much more of the effect, as opposed to shifts in the frequency domain.


I don't think it has to do with low frequencies as I'm equally able to detect the wrong stereo image with most synth sounds (in the posted sample as well as other music I tried).

I also tried classical music and instrumental and there I was not able to hear wrong stereo image (with the small number of songs I tried at least).

The artifact is not unmusical, and I have never noticed anything "wrong" with low-frequency AAC soundstages on typical material - even older recordings with "one channel bass" using headphones.


Indeed it is not that bad, but a serious bug nonetheless. Esp. since it is even hearable at 320kb. Also at 128k it is so bad that I can identify the AAC encode without any reference (I just listen to a random permutation of orginial and AAC and at each stage say which is which).

Is that all this is about?  And to think it had me worried.    I agree this isn't a problem unless you listen to music with headphones. I only use headpones when I absolutely have to so this is a non issue for me.


I guess you have to be worried. I just ABXed the 128k encode with speakers -- the first time I ABXed with speakers btw.:
Code: [Select]
foo_abx v1.2 report
foobar2000 v0.8.3
2006/07/02 22:34:44

File A: file://H:\Flacs\The Prodigy\Experience\09 - Weather Experience.flac
File B: file://H:\test\The Prodigy\Experience\Experience 09 - Weather Experience.m4a

22:34:51 : Test started.
22:36:01 : 01/01  50.0%
22:36:28 : 02/02  25.0%
22:36:35 : 03/03  12.5%
22:36:50 : 04/04  6.3%
22:37:01 : 05/05  3.1%
22:37:08 : 06/06  1.6%
22:37:18 : 07/07  0.8%
22:37:25 : 08/08  0.4%
22:37:33 : 09/09  0.2%
22:37:43 : 10/10  0.1%
22:37:45 : Test finished.

----------
Total: 10/10 (0.1%)


Edit: Same score for 192k w/ speakers.
The idea of achieving security through national armament is, at the present state of military technique, a disastrous illusion. -- A. Einstein

 
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