Skip to main content
Topic: How good is AAC VBR in iTunes 5? (Read 96277 times) previous topic - next topic
0 Members and 1 Guest are viewing this topic.

How good is AAC VBR in iTunes 5?

Tried an album and iTunes still lists it as a CBR bitrate. Am I missing something? I assume the new AAC VBR has been discussed, but I can't find the thread.

How good is AAC VBR in iTunes 5?

Reply #1
Well although the files are all showing as CBR, just did a test of Bowie's Hunky Dory encoding off a lossless rip and...

192 CBR 57.5 MB
192 VBR 58.4 MB

128 CBR 38.5
128 VBR 40.2

Something is happening.

How good is AAC VBR in iTunes 5?

Reply #2
I have tried it but have not decided if I should switch from the --alt-preset standard using EAC and Lame 3.96.1 to the iTunes AAC format.  Although iTunes displays the songs as being CBR, they are actually VBR if you encoded them with the VBR option.  After testing some songs, I was able to find out that sometimes a VBR file will be larger than a CBR file at the same bitrate and sometimes a VBR file will be smaller.  Apple has increased the quality of their mpeg-4 AAC encoder with the release of iTunes 5 and QuickTime 7 (thouth QuickTime 7 has been out for some time for the Mac OS).  I tested some 128kbps CBR files and they sound better than previously encoded files.

Though my "tests" are just with my ears and sound system and are not really scientific or accurate so take them with a grain of salt.

How good is AAC VBR in iTunes 5?

Reply #3
Maybe it's really ABR?

How good is AAC VBR in iTunes 5?

Reply #4
It might be a ABR, I don't know.  After I analysed the bitrate fluctuation, I saw that the bitrate did not really change much.  It seems to be more of a CBR when compared to the VBR functions of the Lame mp3 encoder (either version 3.96.1 or 3.90.3).  For example, a song with a total of 5000 frames is encoded with Lame 3.96.1 and the new iTunes 128kbps VBR AAC encoding.  With the Lame mp3 encoder, 1000 frames can have the 128kbps bitrate and another 1000 frames can have the 320kbps bitrate depending on the song complexity.  However, with the iTunes VBR AAC encoder, the bitrate is mostly situated at the 128kbps bitrate while the song may have 200 frames at the 112kbps bitrate and 200 frames at the 160kbps bitrate with hardly any (IF any) frames at the 320kbps bitrate.

I shouldn't forget to mention that the iTunes VBR AAC encoder can encode up to 192kbps VBR and cannot go past that bitrate.

How good is AAC VBR in iTunes 5?

Reply #5
I encoded a couple songs using the new VBR mode at 128 kbps and the normal mode at 128 kbps.  I opened these files in Foobar2000 and I noticed that with the VBR files the bit rate did flucuate higher than with the normal files.  iTunes AAC encoder doesn't ever encode at true CBR.  The normal mode seems to me to be an average bit rate mode where the bit rate flucuates but stays closer to the chosen bit rate at all times.  According to Foobar2000, it seems that the VBR mode gives the encoder a little more range of bit rates to choose from during encoding but overall the bit rate still sticks around the chosen bit rate.  For example, in normal mode you probably won't see the bit rate go much above 140 kbps on some parts with a chosen bit rate of 128 kbps but in VBR mode the bit rate may jump to something around 170 kbps or above on some parts.

How good is AAC VBR in iTunes 5?

Reply #6
I think this is really just ABR, apple's implementation of VBR.

I tried an album and the avg was 130. U need to use another program to see the bitrate (fb2k, etc) cuz Itunes just says 128 all the time.

How good is AAC VBR in iTunes 5?

Reply #7
Quote
iTunes AAC encoder doesn't ever encode at true CBR.  The normal mode seems to me to be an average bit rate mode where the bit rate flucuates but stays closer to the chosen bit rate at all times.
[a href="index.php?act=findpost&pid=325758"][{POST_SNAPBACK}][/a]

Nero also behave the same. True CBR should use the same bitrate even in silent part, shouldn't it?

So, I encoded 5 minutes silent wave file to 192kbps MP3, iTunes AAC and Nero AAC.

LAME : --preset cbr 192 : output bitrate = 192kbps, filesize = 6.86 MB
LAME : --preset 192 : output bitrate = 32kbps, filesize = 1.13 MB
Nero : CBR Stereo 192 kbps (LC-AAC) : output bitrate = 3kbps, filesize = 129 KB
iTunes : AAC 192 kbps CBR : output bitrate = 2kbps, filesize = 144 KB
iTunes : AAC 192 kbps VBR : output bitrate = 2kbps, filesize = 144 KB

How good is AAC VBR in iTunes 5?

Reply #8
I ripped a bunch of metal tunes, as this type of music is notoriously difficult to encode with quality, at AAC 128 and AAC 192 using the new VBR mode in iTunes 5. I am taking my high quality headphones with me today and will report on Friday how these songs sound on the iPod.

I do not understand all the complaints that this VBR implementation is actually ABR. What is wrong with predictable file sizes? ABR still allows the encoder to allocate more bits to the complex passages and less to the easy passages. So its not perfectly efficient like true VBR, it still greatly improves upon CBR in theory and iTunes AAC 192 sounded awesome in CBR mode! How much better will the VBR moe be? Can't wait to find out.

How good is AAC VBR in iTunes 5?

Reply #9
Quote
I do not understand all the complaints that this VBR implementation is actually ABR. What is wrong with predictable file sizes?
[a href="index.php?act=findpost&pid=325802"][{POST_SNAPBACK}][/a]


Some people would rather have predictable quality rather than predicatable filesizes.

How good is AAC VBR in iTunes 5?

Reply #10
Quote
So its not perfectly efficient like true VBR, it still greatly improves upon CBR in theory and iTunes AAC 192 sounded awesome in CBR mode!
ABR isn't anywhere near as efficient as true VBR. An typical song encoded using LAME will have bit rate distribution all the way from 32 kilobit samples to 320 kilobit sample, an ABR encode would have a range of maybe +/- 20kbps.

Edit: I just tested it with a song at 128kbps, lowest sample was 109 kilobit, highest was 147 kilobit, giving 19kbps range either side.

How good is AAC VBR in iTunes 5?

Reply #11
Apple's VBR AAC implementation may not be perfect but it is a step in the right direction for its AAC encoder.  At the very least Apple's "VBR" mode is a more lenient ABR mode that allows the bit rate to flucuate a little higher for certain songs.  Let's hope they continue to improve upon it.

How good is AAC VBR in iTunes 5?

Reply #12
TOS #8 Warning! The following statements are not supported by a scientific ABX study. If you require this condition to read anything in this forum, please read no further.

I have listened to several VBR encodings at 192 and 128 and compared them to sister CBR encodings at the same bit rate. The listening tests were conducted for the album Seether "Karma And Effect" on an iPod Mini using high quality full size headphones (not Sure canalphones). Apple Lossless encodes were used for comparison purposes as well.

192
As earlier posters mentioned, I cannot ABX between VBR and CBR. However, not necessarily a bad thing, as I have always maintained iTunes 192 AAC CBR is transparent to me. In fact, I have yet to see a post from a listener who claims otherwise at this bit rate. So in theory, bits are being re-allocated more efficiently, but at the end of the day - great sound in both CBR and VBR at 192.

128
Again, very difficult to ABX between VBR and CBR. My perception is that VBR is slighly better at this bit rate but not obviously so. Both modes are clearly inferior to the Lossless encoding. However, the sound quality is definitely acceptable for portable use (IMHO) unlike many MP3 encodings at 128.

I'm interested in other's opinions and if anyone has tested 160. For now, I think iTunes AAC 192 VBR is the way to go if you own an iPod and desire transparency in your music. It's approximately a 10-20% reduction in file size vs. LAME APS if you listen to modern rock and metal which averages about 215 in LAME APS but can go as high as 240 on some metal albums.

How good is AAC VBR in iTunes 5?

Reply #13
Quote
192
As earlier posters mentioned, I cannot ABX between VBR and CBR. However, not necessarily a bad thing, as I have always maintained iTunes 192 AAC CBR is transparent to me. In fact, I have yet to see a post from a listener who claims otherwise at this bit rate. So in theory, bits are being re-allocated more efficiently, but at the end of the day - great sound in both CBR and VBR at 192.
Based on your statement, I cannot help but think 160 VBR AAC might be the best choice.  Depening on how well the reallocation of bits is done, it could attain the the quality of 192 CBR with the luxury of 160-ish file size.

How good is AAC VBR in iTunes 5?

Reply #14
Quote
Maybe it's really ABR?
[a href="index.php?act=findpost&pid=325751"][{POST_SNAPBACK}][/a]

thats what it soulds like to me given the two examples
Chaintech AV-710

How good is AAC VBR in iTunes 5?

Reply #15
Quote
ABR isn't anywhere near as efficient as true VBR. An typical song encoded using LAME will have bit rate distribution all the way from 32 kilobit samples to 320 kilobit sample, an ABR encode would have a range of maybe +/- 20kbps.

Why ABR isn't always a 2 pass encoding ? That would allow a larger bitrate spreading, no ?

How good is AAC VBR in iTunes 5?

Reply #16
I fail to see the relation between abr and two pass encoding.

Lecture me please because i do not see the relation between both...

How good is AAC VBR in iTunes 5?

Reply #17
Quote
I fail to see the relation between abr and two pass encoding.

Lecture me please because i do not see the relation between both...
[a href="index.php?act=findpost&pid=326075"][{POST_SNAPBACK}][/a]

With CBR, you simply encode at a given bitrate.

With VBR, you have some given formula (in theory) by which you gauge quality, and let the bitrate flucuate as high as it needs to be to achieve that quality.

But with an ABR situation, you're trying to balance out the bitrate against the quality. You're shooting for the best quality you can, obviously, but without compromising too much on the bitrate. So by using a 2 pass mode, you can determine what the best tradeoff would be before actually encoding, sacrificing quality in some areas for others that need the bits more, and still coming as close to that bitrate as you reasonably can. Also, ideally, you want the average bitrate of a streaming audio to always remain under the bitrate you're setting, so as to not cause delays when streaming it at that bitrate. So if I want ABR at 160, I've got to stay under 160 average at any given point in the stream. So if I need to encode a section higher than 160, something prior to that 160 in the stream needs to be encoded lower in order to save those bits. This prevents my stream from falling behind realtime on playback, and reduces/eliminates the need for excessive buffering or lagging. With a 2 pass mode, I can figure out that out in advance and get a better stream out of the thing. Basically I can "look ahead" to determine how to handle each piece of audio.

How good is AAC VBR in iTunes 5?

Reply #18
Look at the bright side: we will have no problems deciding which VBR preset to use in listening tests against other codecs.

How good is AAC VBR in iTunes 5?

Reply #19
I've done some encoding at 160 cbr and 160 vbr, and i think that using vbr does improve quality noticeably from cbr at the same bit rate, but i have only tried 160kbps, 192 is harder to ABX vbr from cbr. But with 160 vbr, there's a little less HF smearing and 'shsh'ing than with 160 cbr, maybe with my metal music this is more noticeable, but i find the improvement quite nice.

EDIT: After listening to some vbr encoded songs in iTunes, the bitrate diplayed on iTunes appears to be the average bitrate, like Lame vbr bitrates appear on Winamp's tag editor. The 160kbps vbr encodes i did now show up in iTunes as 159, 162, et cetera, but only after hearing through the whole song does this happen.
we was young an' full of beans

How good is AAC VBR in iTunes 5?

Reply #20
Has anyone noticed that MrQuestionman displays the average bitrate of AAC vbr files?, and as i suspected, the average bitrates are around +-~3kbps of    the target bitrate for iTunes vbr AAC.
we was young an' full of beans

How good is AAC VBR in iTunes 5?

Reply #21
Quote
Has anyone noticed that MrQuestionman displays the average bitrate of AAC vbr files?, and as i suspected, the average bitrates are around +-~3kbps of     the target bitrate for iTunes vbr AAC.
[a href="index.php?act=findpost&pid=326559"][{POST_SNAPBACK}][/a]

That's a sick joke of a VBR mode, if you ask me. It would be really nice to have an efficient, "transparent in 99% of all cases" mode like Lame --preset standard or Vorbis -q6 also for AAC. I don't doubt that iTunes ""VBR"" 192 will be transparent in most cases, but it is simply a waste of bits if it also encodes very easy or near-mono music at that bitrate.
The worst  thing about this is, since Apple already markets this ABR-disguised-as-VBR mode as "VBR", we'll maybe never see a "true" VBR encoding mode for iTunes
Proverb for Paranoids: "If they can get you asking the wrong questions, they don't have to worry about answers."
-T. Pynchon (Gravity's Rainbow)

How good is AAC VBR in iTunes 5?

Reply #22
Quote
Quote
Has anyone noticed that MrQuestionman displays the average bitrate of AAC vbr files?, and as i suspected, the average bitrates are around +-~3kbps of     the target bitrate for iTunes vbr AAC.
[a href="index.php?act=findpost&pid=326559"][{POST_SNAPBACK}][/a]

That's a sick joke of a VBR mode, if you ask me. It would be really nice to have an efficient, "transparent in 99% of all cases" mode like Lame --preset standard or Vorbis -q6 also for AAC. I don't doubt that iTunes ""VBR"" 192 will be transparent in most cases, but it is simply a waste of bits if it also encodes very easy or near-mono music at that bitrate.
The worst  thing about this is, since Apple already markets this ABR-disguised-as-VBR mode as "VBR", we'll maybe never see a "true" VBR encoding mode for iTunes
[a href="index.php?act=findpost&pid=326590"][{POST_SNAPBACK}][/a]


When you check the VBR option you give the encoder freedom to use more bits when it's needed. If the variation is only +-3kbps it means that the music is not very difficult. One might argue that it should be able to go very low like LAME does, but that can lead to problems (such as the problem sample posted a few days ago).

On some songs, I've seen the average bitrate to be over 20kbps higher than the target.

How good is AAC VBR in iTunes 5?

Reply #23
Maybe this is Apple's first attempt at tuning a VBR scheme, and for this reason they have placed limits on how far the encoder can vary from the nominal bitrate setting? These limits may be in place because they are concerned that the tuning at this stage isn't of a very high quality.

In the files I have encoded I have noticed that there are many more that have an average bitrate above my nominal 160K setting, and only a few that are below it. It is possible that the files under 160K are under that figure more as a result of digital silence than because of being easy to encode.

Is there a frame analyzer for .m4a files so that one can see the smallest and largest frames?

On Apple's webpage there used to be a page to suggest the addition of features to iTunes. Does that still exist? I can't seem to find it. If it does, we should now ask Apple to offer an improved VBR system where the entire internals of the encoder can be accessed. I.e setting min and max frame size limits, and VBR encoding quality, and even the ability to configure the frequency filtering etc. Of course these level of complexity would have to remain hidden from the average iTunes user.

How good is AAC VBR in iTunes 5?

Reply #24
They may be afraid of skipping on the Ipods.

 
SimplePortal 1.0.0 RC1 © 2008-2020