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do we really know how good/bad is AC3 encoding?, presuming best-available AC3 encoder
timcupery
post Feb 19 2010, 03:59
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So I've been trying to learn about how good can the AC3 format be. Mp3 is excluded from the MPEG-2 spec (presumably b/c timing - AC3 was more mature at the time). But more interesting is that mp3 is also excluded from the Blue-ray spec, while AC3 is included. This doesn't mean that the standard-makers thought AC3 offers better audio reproduction than mp3, it could just be that lots of blue-ray discs will include better picture but the original already-encoded AC3 audio. And AAC audio has a higher ceiling and was there included over mp3.

But this has gotten me wondering, how good, actually, is AC3 lossy encoding?
It hasn't shown up in listening tests, either because people don't take it seriously, or because the only optimized encoders are dolby-licensed and very expensive to purchase.

So I'm wondering - do we (by which I mean, "knowledgeable people here at HA) have a good sense of where AC3 encoding quality ranks?
It's quite possible there are test results that I simply haven't seen and couldn't find when searching thread subject-lines.


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timcupery
post Feb 19 2010, 15:48
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wow - 81 views and no responses. so does that mean we really don't know anything about ranking AC3 audio quality?


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viktor
post Feb 19 2010, 16:05
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QUOTE (timcupery @ Feb 19 2010, 15:48) *
wow - 81 views and no responses. so does that mean we really don't know anything about ranking AC3 audio quality?

that means whoever has read this topic doesn't know the answer or is not willing to answer.

this is a forum, not a chat, so be patient.

This post has been edited by viktor: Feb 19 2010, 16:06
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benski
post Feb 19 2010, 16:12
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QUOTE (timcupery @ Feb 18 2010, 21:59) *
Mp3 is excluded from the MPEG-2 spec (presumably b/c timing - AC3 was more mature at the time).

It is not excluded from MPEG-2. In fact, the MPEG-2 standard expands on MP3 adding the additional sampling rates now referred to as MPEG-2 layer 3. It is, however, excluded from the DVD spec. DVD is a subset of MPEG-2 and excludes a number of other things as well.

QUOTE
But more interesting is that mp3 is also excluded from the Blue-ray spec, while AC3 is included.

MP3 is excluded from Blu-Ray for the same reason it's excluded from DVD. Layer 3 is not capable of surround sound.
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Notat
post Feb 19 2010, 16:34
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MP3 was developed first, then AC3 (Dolby Digital) then AAC. MP3 is definitely the inferior codec in terms of quality/bitrate. Due to Dolby's involvement in AAC, many of the AC3 coding techniques are incorporated in AAC.
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lvqcl
post Feb 19 2010, 16:36
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EBU evaluations of multichannel audio codecs: http://tech.ebu.ch/docs/tech/tech3324.pdf

I found this link on http://forum.doom9.org/
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timcupery
post Feb 19 2010, 16:59
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@victor - I understand this ain't a chat. just figured that 81 views and no posts was a rather high ratio, indicating that not many people know info about AC3, which fits with the fact that I couldn't find anything with search.

@benski - thanks for the clarification. MP3 being excluded from DVD and Blue-ray spec on the basis of multichannel.

@Notat - development order is but one predictor of audio quality of a codec. (although it's good to know the order - I was wrong on that part). My real question in this thread is precisely about how AC3 would perform in blind listening tests. Your statement of "mp3 is definitely the inferior codec in terms of quality/bitrate" implies that AC3 would be better or at least the equal of any highly-developed more recent codecs (e.g., AAC) which do not have much of a leg up on mp3 yet although it's generally agreed that they have a higher ceiling.
Obviously mp3 audio quality could only be tested against AC3 in 2-channel music.


As to why AC3 made it into Blue-Ray: perhaps this was about quality. But these standards are HUGE business decisions with lots of money at stake, and committees aren't simply making decisions on the basis of functional quality.
I ran across this link
http://tech.mit.edu/V122/N54/54hdtv.54n.html
which highlights a conflict-of-interest in an MIT prof and audio expert casting the deciding vote to get AC3 into the U.S. HDTV spec. MIT apparently stands to make loads of money b/c a partnership with Dolby (who owns AC3) whereas Phillips got their standard (under the MPEG's patents) approved for Europe's HDTV spec. The prof apparently cast his vote not b/c the money that stood to be made by MIT, but rather b/c he wanted to support an American codec. Either way, it's an example of politics and business and money in such decisions.


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sauvage78
post Feb 19 2010, 17:38
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Not only AC3 is a bad codec but it is an awfull codec.

I made some tests on the castanest sample long ago, the results were that is was a terrible codec, & in fact one of the worst codec around. I don't have them anymore but I think the ABX logs are lost somewhere deep in the forums.

I still have a summary of my personnal result in a .txt that I keep if ever I encode a video:

CODE
Sonic Foundry Soft Encode  (Dolby V6.6.2) Soft Encode V1.0:
Castanet 2.0 Wav>AC3, 100% instantly ABXable up to 224Kbps. ABXable at 256Kbps, but not instantly.
aften V0.0.8:
Castanet 2.0 Wav>AC3, 100% instantly ABXable up to 320Kbps. Most likely ABXable at 384Kbps but not instantly.
ac3enc (Used by BeSweet) (from ffmpeg)  V1.20 (18-02-04):
Castanet 2.0 Wav>AC3, 100% instantly ABXable even at 256Kbps. Couldn't test higher.


I recall that Aften was one of the worst encoder I ever tested.

My conclusion was:
1: One must be deaf to encode to AC3 as a final codec.
(Specially re-encoding AC3 to AC3 is one of the biggest misstake you can do: it's pure massacre)
2: AC3 as a source is already so bad that it isn't worth encoding it.

The ironic thing with AC3 is that some people keep the AC3 stream untouched thinking it's "lossless".
It's exactly the opposite, you'd better leave it untouched because if you encode it once more it may fall in pieces.

IMHO it's a shame that disks with a "high-resolution" sticker on it are sold using this obsolete codec.
AC3 is one of the reason why I don't do video encoding at all, even blu-ray.

I hope the quad-hd successor to blu-ray will stop using it.

Edit: ... and if you wonder why there isn't more ABX test of AC3 & DTS, my answer is that it's because setting a AC3/DTS test is a real nighmare due to the limit of the available encoders:
most killer samples are 2.0 when most AC3/DTS encoders only supports 5.1. (Specially true for DTS encoders, most AC3 encoders supports 2.0)
Furthermore most AC3/DTS encoders prices make this test cost's increadibly high unless you're willing to broke your setup with the hundred of malwares that you will found bundle with the "free" (read "cracked") version of these encoders.

This post has been edited by sauvage78: Feb 19 2010, 18:20


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saratoga
post Feb 19 2010, 18:57
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QUOTE (sauvage78 @ Feb 19 2010, 11:38) *
Not only AC3 is a bad codec but it is an awfull codec.

I made some tests on the castanest sample long ago, the results were that is was a terrible codec, & in fact one of the worst codec around. I don't have them anymore but I think the ABX logs are lost somewhere deep in the forums.


Although AC3 is a pure MDCT codec like AAC, it screws up like MP3 by having strange block sizes. Long blocks are just 512 time domain samples, 256 samples for short blocks. And short blocks must come in pairs.

One of the new AC3 flavors (E-AC3?) apparently works around this by adding additional block sizes, but I don't know how well supported it really is or if its any good. I haven't seen a decoder source for it.

Anyway, the reason to include AC3 is that every multichannel receiver on earth supports it. If they'd made AAC mandatory and forced everyone buying bluray to buy a new receiver, people would have bought HD-DVD players instead. The fact that AC3 isn't a very good format on technical grounds isn't so important. You can use relatively high bitrates on Bluray since you're not hurting for space.
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rpp3po
post Feb 19 2010, 19:03
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Thanks for the insightful information. AC3's potential quality has always been kind of a blind spot for me.
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sauvage78
post Feb 19 2010, 20:16
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QUOTE
You can use relatively high bitrates on Bluray since you're not hurting for space.


The bitrate I have given are for 2.0, so AC3 is instantly 100% ABXable at bitrates that are considered high/to overkill for any other modern lossy codec.

The idea that high bitrate would help the quality of AC3 to be better is IMHO a myth.

Castanet is not the worst sample around anymore, Harlem & Autechre are worst, but AC3 Soft Encode which, back when I tested it, was supposed to be one one the best AC3 encoder around (maybe SurCode or Sonic Foundry are better but I got a nasty virus before I could try those... I had to re-install XP despite antivir), is already not transparent at 256Kbps on castanets.

If you consider that a killer sample is only 20 sec, but that a movie lenght is 1h30 to 2hours, this leads to the simple conclusion that the AC3 stream from blu-ray is likely not transparent, & it's worst for DVD as the bitrate is lower. AC3 is not robust enough to stand the shock, it's like encoding a CD at 128kbps MP3 & hoping that it will be 100% transparent. It might be only if you don't listen carefully.

One conclusion that you can draw for almost certain is that any movie soundtrack with applauds in AC3 (which means any live music) is not transparent no matter the bitrate. The sellers are not idiot, that's the reason why most live music DVD comes with a 1536Kbps DTS stream.

Here is an equivalency table between AC3/DTS 5.1 bitrate used in physical media & 2.0 bitrate used for music: (with my non-100%-scientific opinion next to it, but the extrapolation is still based on some ABXing)

CODE
DTS ------ 5.1 1536Kbps /5x2= 2.0 614Kbps Good, most likely near transparent.
DTS ------ 5.1 0768Kbps /5x2= 2.0 307Kbps Not Good, not transparent, most likely easyly ABXable.
AC3 BD --- 5.1 0640Kbps /5x2= 2.0 256Kbps Not Good, not transparent, most likely easyly ABXable.
AC3 HD-DVD 5.1 0504Kbps /5x2= 2.0 201Kbps Bad, not transparent, instantly ABXable.
AC3 DVD -- 5.1 0448Kbps /5x2= 2.0 180Kbps Bad, not transparent, instantly ABXable.


It gives you a better idea of how to compare 5.1 bitrate to 2.0 bitrate & will help you realize that high bitrate doesn't help AC3 much.

The fact that AC3 is already used at high bitrate by default is an hint of how awfull it must be at low bitrate. Overall AC3 doesn't even compete with a bad mp3 encoder.

... but there is one hope, I heard on doom9 that professionnal AC3 encoder were better than those available to mortal users like us ... (I also heard that hope is what dies last ...)

This post has been edited by sauvage78: Feb 19 2010, 20:40


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andy o
post Feb 19 2010, 20:22
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bluray audio is not much of a problem. Most blurays come with lossless audio now, and AC3 is mostly used for backwards compatibility (as it's the main reason why it was included in the first place) as a secondary track or embedded in the TrueHD track. I do wonder about the quality of E-AC3 (DD Plus) though.

If E-AC3 which was used in most HD-DVDs (though many also came with lossless audio) turns out to be "bad" as well, then maybe the audiophile paranoia that helped the higher-capacity bluray discs win probably had a good side-effect.
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sauvage78
post Feb 19 2010, 20:32
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I haven't ripped a blu-ray since ages ... but the problem I had with blu-ray was that usually the lossless stream was only the main english stream ... there wasn't a lossless stream for each langages & I am not a native english. (Worst secondary langages were often 2.0.)

You can hardly rip a blu-ray with a mono-core barton sempron 3000+ wink.gif so I gave up anyway.

This post has been edited by sauvage78: Feb 19 2010, 20:33


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andy o
post Feb 19 2010, 20:36
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Heretic! Are you watching dubbed movies? biggrin.gif

Actually many movies now come with multiple lossless tracks. If not, the original track often comes in lossless anyway. There are several awful discs out there though (Jackie Chan's Legend of Drunken Master is a relatively new one), but I think there are fewer and fewer nowadays.
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andy o
post Feb 19 2010, 20:39
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QUOTE (sauvage78 @ Feb 19 2010, 11:32) *
You can hardly rip a blu-ray with a mono-core barton sempron 3000+ wink.gif so I gave up anyway.

You don't need a powerful computer, just a big hard drive and the tools (which I think would violate TOS if I mentioned). There's no transcoding necessary, and there are now many (though mostly unreliable due to bugs) HDMI solutions that will bitstream the bluray codecs even from individual files.
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DVDdoug
post Feb 19 2010, 23:12
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I've never done any A/B or ABX tests and I can't comment on it's transparency, but to me, Dolby AC3 can sound fantastic! Most DVDs have AC3 and they certainly sound better than VHS tapes...

I have several concert DVDs and some of the best sound I've heard from my home theater system is from 5.1 channel AC3!!! Although it's an apples-to-oranges comparison, the AC3 surround from DVDs usually sounds better to me than stereo CDs. (I don't have a Blu-Ray player or a DVD-Audio player, so I can't play non-lossy surround sound.)

I've never heard any artifacts or anything "wrong" with commercially produced movies/concerts (unless it was something like a very old movie with a low-quality original soundtrack). I've ripped a couple of stereo AC3 tracks, resampled (from 48 to 44.1kHz), and burned CDs. I don't hear anything wrong with these CDs either.

When there's a choice between stereo LPCM and AC3 surround sound, I'll always select the surround sound. If there's a DTS track, I'll usually select it because I "know" it has a higher bitrate. The DTS usually sounds different (maybe a different mix, or different levels), but I won't say it always sounds "better".
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timcupery
post Feb 19 2010, 23:27
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@DVDdoug - it seems possible that both your experience and sauvage78's testing could be true. It's possible that AC3 could be a poor-performing codec in listening tests (where you're comparing to an original lossless source) AND that AC3 doesn't detract from the enjoyment of listening to 5.1-channel audio. In your case, you're not comparing to any source where you'd have the ability to notice huge differences. So as long as AC3 doesn't produce obvious artifacts (i.e., obvious without even comparing to the original, like the "smearing" sound in many crappy mp3 encoders) then it doesn't hurt your experience listening. But this doesn't rule out the possibility that if you could do blind tests on your 5.1 audio system, it may still be very easy to tell the difference between AC3 encode and the original lossless source.


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sauvage78
post Feb 19 2010, 23:48
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Well, I am convinced AC3 is an awfull codec but I have never experienced any trouble with any DVD or Blu-ray (but I don't test them all day long either), ... it's happy that they sound OK for casual listening when you see the prices ... the problem is not that it sounds awfull, on most movies it doesn't, I mean it's like the average Joe that thinks mp3 128Kbps is good enough for him ... my criticisms is beyond that point. If you're happy with AC3 & don't bother bringing the codec to its knees. Great.
The problem is more that it is definitly not as good as it could be ... what is the logic of using AVC for video & AC3 for audio when AAC 5.1 is around ? AC3 is definitly not of the same effiency in the audio encoding world as AVC is in the video encoding world. (Indeed I do understand that AC3 is like mp3 on dap, a codec that never dies due to its huge compatibilty.) IMHO, it's good to point out that AC3 is a very weak codec now, because if people don't realize it, in ten years from now we will have quad-hd holographic disk using H265/HVC for video ... and AC3 for audio which would be a shame.

Now I agree that the probability that you can detect an audio artefact while watching a movie is low, even for trained ears. I often listen to radio, but I rarely complain that it sounds bad ... what I mean is that, on a daily basis, if you don't have an original to compare, you just don't realize what you lose. No matter if you have gold ears or not ... or if you have a super home theater or not. I did my 2.0 AC3 listening tests with cheap headphones.

This post has been edited by sauvage78: Feb 19 2010, 23:55


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andy o
post Feb 20 2010, 03:15
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Bluray can now take DD, DTS, LPCM, DTS-HD HR, DTS-HD MA, Dolby TrueHD and DD Plus (though no discs that I know except for a Dolby demo disc come with DD Plus). There has been problems fully supporting all formats in players (even on the highest-profile bluray one, the PS3), and there still are after years. Even the 7.1 variants can cause trouble sometimes. There is no way another format would have been well received, and DD and DTS are essential for backward compatibility. If you wanted to play a bluray with an older digital receiver via SPDIF, you couldn't if they didn't include them, except for 2-channel LPCM. HDMI with the HDCP and general handshake crap are tough enough to swallow already.

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probedb
post Feb 20 2010, 22:10
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As others have said, pretty much all new BR discs use either Dolby TrueHD or DTS MA-HD which are both lossless. AC3 is just backwards compatible and occasionally offered by idiots like Warner Brothers on some discs like Fringe S1....what were they thinking!?

Andy, I'm not sure what you mean by problems? My PS3 plays everything back fine.

Also I believe DD+ was popular on HD-DVD but I've never seen a BR title that uses it.

In summary, I wouldn't worry about AC3. It's served it's purpose and is very good for movie-soundtracks which is it's primary purpose. People don't sit down to watch a movie thinking about whether they can hear pre-echo etc wink.gif
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andy o
post Feb 20 2010, 22:24
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As far as I can remember, the PS3 initially didn't decode DTS-HD. It had to get an update for that. Only the new Slim can bitstream the "HD" codecs. In the PC software domain, the situation is pretty bad still. PowerDVD 8 supported at one point 7.1 DTS-HD MA, but some builds later it all got screwed up, and on PowerDVD 9 they stopped supporting it at all. People say it's come back but meh, I don't care anymore. WinDVD 2010 can't decode any 7.1 audio at all, though I solve that by bitstreaming with the ATI 5770.

In short, the bluray spec is an unfinished mess. HD-DVD was better in almost everything except for capacity (which as an "advantage" was overblown and now makes for pretty bloated discs). Even if it supported the same audio codecs, it seems codec usage was more organized.

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starcy
post Feb 21 2010, 00:17
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DD + is not popular Blu-Rays, because is lossy and offer less bitrates than his competitor DTS-HI Res, and needs core which is aslo lossles. While DD+ should produce better quality at given bitrate (around 1.7mbps) than DTS-HI res (~2-6mbps).

AC3 generaly is afwull comparing to the AAC, DD+, MP3 (in stereo), and other losssy encoders, but not DTS! even at 1.5mbps. My ears clearly prefer AC3 at 640kbp at any DTS lossy (not DTS HI-Res)
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saratoga
post Feb 21 2010, 02:46
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QUOTE (starcy @ Feb 20 2010, 18:17) *
AC3 generaly is afwull comparing to the AAC, DD+, MP3 (in stereo), and other losssy encoders, but not DTS! even at 1.5mbps. My ears clearly prefer AC3 at 640kbp at any DTS lossy (not DTS HI-Res)


Isn't DTS Hi-res just the same as DTS except with more channels and higher bitrate? IIRC its backwards compatible, so I think its still the same old subband codee.
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starcy
post Feb 21 2010, 12:09
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QUOTE
Isn't DTS Hi-res just the same as DTS except with more channels and higher bitrate? IIRC its backwards compatible
No, DTS HI-Res aslo need core to be backward compatible.
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probedb
post Feb 21 2010, 13:31
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QUOTE (andy o @ Feb 20 2010, 21:24) *
As far as I can remember, the PS3 initially didn't decode DTS-HD. It had to get an update for that. Only the new Slim can bitstream the "HD" codecs. In the PC software domain, the situation is pretty bad still. PowerDVD 8 supported at one point 7.1 DTS-HD MA, but some builds later it all got screwed up, and on PowerDVD 9 they stopped supporting it at all. People say it's come back but meh, I don't care anymore. WinDVD 2010 can't decode any 7.1 audio at all, though I solve that by bitstreaming with the ATI 5770.

In short, the bluray spec is an unfinished mess. HD-DVD was better in almost everything except for capacity (which as an "advantage" was overblown and now makes for pretty bloated discs). Even if it supported the same audio codecs, it seems codec usage was more organized.


To be fair DVD was like this originally. I still have my first DVD player and it doesn't support DTS at all smile.gif MPEG Multichannel and DD only.
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