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Topic: do we really know how good/bad is AC3 encoding? (Read 52702 times) previous topic - next topic
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do we really know how good/bad is AC3 encoding?

Reply #25 –
Quote from: Mike Giacomelli on
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.

As said above, the DTS-HD formats have a "core" of legacy DTS. Similar to AAC-HD and MP3-HD as far as I understand. Maybe you're confusing it with "DTS 96/24"?

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

Yeah but at that time there was no "better" format to choose. I think a very good case can be made that HD-DVD was the better format for consumers (and the DVD people learned from their past mistakes), with bluray, I don't think one can make much of a case for consumers. HD-DVD was very much all finalized and even menus had similar interfaces. With bluray is like you're running a different program every time you run a different disc, on the whims of the idiotic studio.

do we really know how good/bad is AC3 encoding?

Reply #26 –
Quote from: andy o on
Quote from: Mike Giacomelli on
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.

As said above, the DTS-HD formats have a "core" of legacy DTS. Similar to AAC-HD and MP3-HD as far as I understand. Maybe you're confusing it with "DTS 96/24"?



Err if it has a backwards compatible core, then it very clearly is still a subband codec.  Its not like you can take an MDCT backwards compatible with a QMF

do we really know how good/bad is AC3 encoding?

Reply #27 –
well maybe you're right. I'm not sure how technically correct is "just the same as DTS except with more channels and higher bitrate", though.

do we really know how good/bad is AC3 encoding?

Reply #28 –
Quote from: Mike Giacomelli on
Quote from: andy o on
Quote from: Mike Giacomelli on
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.

As said above, the DTS-HD formats have a "core" of legacy DTS. Similar to AAC-HD and MP3-HD as far as I understand. Maybe you're confusing it with "DTS 96/24"?



Err if it has a backwards compatible core, then it very clearly is still a subband codec.  Its not like you can take an MDCT backwards compatible with a QMF


I think it just has a core, it maybe an entirely separate track within the DTS-HD MA track rather than it being used as part of the main track.

Andy with regards the menus etc it's because of the silly Java thing, basically you can run what you want inside it, that's why there's no standardised way of returning where you left off as well. Most annoying!!

do we really know how good/bad is AC3 encoding?

Reply #29 –
yeah that's what I meant. BD-J is annoying as hell, and what's up with the loading times and screens.

AFAIK DTS-HD is an extension and the DTS track is a true "core". Dolby TrueHD is the one that includes an embedded DD track.

do we really know how good/bad is AC3 encoding?

Reply #30 –
A codec is what the specific implementation makes of it. The specifications (Dolby A-52) are as much only a frame for AC3 encoders like Aften, as ITU H.264 is a frame for MPEG-4 AVC video encoders like x264. Calling the whole AC3 format "the worst ever seen" is quite generalistic, nicely said...

As far as I read on the Aften home page, it does not yet implement a very complex psycho-acoustic model, and no good channel coupling either (based on version 0.0.8); but it is already very fast and very convenient to use (e.g. via WAVtoAC3encoder). And it is freeware. And being developed actively.

Commercial encoders (as used by Steinberg and Sony - who bought Sonic Foundry years ago) may probably have their advantages, quality-wise. I'll stay curious and watch this topic...

do we really know how good/bad is AC3 encoding?

Reply #31 –
Quote from: sauvage78 on
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: [Select]
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.



So, now being easily (or 'not instantly' or even 'most likely') ABX-able with a notorious killer sample, tested at less than maximum possible bitrate for the codec, with encoders that may be somewhat less than 'industry standard',  is now the standard for condemning a codec generally?

Please.  This is just overwrought, as well as a possible TOS violation.

And that leaves out the multichannel factor...some audio artifacts that are apparent in careful 2.0 listening become far less detectable in surround, according to Floyd Toole in his recent book.  (I don't recall if he was necessarily talking about lossy codecs, but still.)

do we really know how good/bad is AC3 encoding?

Reply #32 –
krabapple:
Quote
tested at less than maximum possible bitrate for the codec


The bitrate given are for 2.0 so IIRC I tested the codecs at the maximum available bitrate for 2.0. So higher bitrates means 5.1. As I said the ABX logs are lost somewhere deep in the forums, but they are here. Anyway, I repeat AC3 is a terrible codec, I am 100% sure I can prove it again & not only with castanet.

I have read several times about people telling that the quality of aften AC3 encoder is "great", I can tell you one thing: those people don't have a clue of what they are speaking about ... now I don't care what you think about my results ... I have easyly published around a hundred of ABX log on HA at various bitrates, on various codecs & with various samples. You are free to follow the opinion of newbies that have never done any serious listening test ... more sometimes serious users disagree with me, that doesn't matter, test for yourself & do your own opinion ... only then you might realize how awfull AC3 is. If you're happy with it, great. Like ignorance, deafness is a bless sometimes ... 

"with encoders that may be somewhat less than 'industry standard'" ==> This is pure speculation, there is no evidence that professionnal hardware encoders are better than those publicy available.
Without any independant test, this is only a rumor.
Furthermore, an end-user like me has no way to test those supposed "better" professionnal encoders. I tested the best encoders that I could easily found at that time. One of the reason why there is so few data about AC3 quality is the price of encoders (& their awfull interface).

You might think that my test is crap, but I broke my system with virus to get those encoders (I am not even afraid of malwares as I am an advanced user, I just low level formatted as a basic format wasn't enought ...), so I doubt any average/newbie users will be willing/able to run a better test, knowing how hard it is to collect the necessary encoders ... it's either he will broke is system ten time (Despite antivir/kapersky/avast) or will get very poor before he can even run the test ...
My very bad opinion about AC3 is also due to the amount of time wasted just to realize how bad it was ... I don't like re-intalling my system, nowadays that it is rock stable I never re-install unless I buy a new HDD ... so this test was a special moment for me ... I wouldn't have re-install my whole system just to run a cheap & crap test ... I have high trust that my result are good for myself, I don't care if others disagree as long as they cannot scientifically prove me wrong.

I tried to get my hands on every other non-free AC3 encoders but I didn't succeed (or I succeeded but the interface didn't accept 2.0, I honestly don't recall), but based on comments from doom9 users (which are very not scientific on this particular matter), Soft Encode is not particulary better or worst than other non-free encoders which were all at that time based on Dolby V6.6.2 ... so one thing that is true about "professionnal" encoders is that non-free AC3 encoders are slightly better than freely available AC3 encoders ... this might lead to think that maybe non-free professionnal hardware encoders could "maybe" be better than non-free professionnal software encoders. First, this is pure extrapolation IMHO. Second, what you call "better" might be a very slight "better" IMHO.

"become far less detectable in surround" ... my intuition tells me that this has a lot of chance to be bullshits ... maybe in some rare case a light artefact might be masked but heavy artefacts have very low chance to be masked. ... if it was the case you couldn't ABX 2.0 ... because it often happens that the artefact is only on the right or left channel ... & the non-affected channel doesn't mask the artefact much ... unless it is a very light one.

About my comments:
Quote
aften V0.0.8:
Castanet 2.0 Wav>AC3, 100% instantly ABXable up to 320Kbps. Most likely ABXable at 384Kbps but not instantly.


Do you know what this means to be instantly ABXable up to 320Kbps in my mouth ??? It means that I can ABX it in 2 seconds ... which means that I can reach 20/20 100% probability of not guessing in 40 seconds which is less than 1 minute ... if you think that this is a "good" result you seriously needs to do some ABXing by yourself ... I am not even sure that I can reach this result with Blade mp3 or VQF ... I am sorry if it wasn't clear for you but "Most likely ABXable at 384Kbps but not instantly." means that it was already the worst codec I ever tested at 320Kbps so I give up & didn't tested at 384Kbps because it was not worth it ... it was simply competiting for the crown of the worst codec on earth ... (even an experimental codec like CELT might beat it ... )

Anyway fell free to disagree, I am waiting for your ABXing logs.

do we really know how good/bad is AC3 encoding?

Reply #33 –
Quote from: LigH on
A codec is what the specific implementation makes of it. The specifications (Dolby A-52) are as much only a frame for AC3 encoders like Aften, as ITU H.264 is a frame for MPEG-4 AVC video encoders like x264. Calling the whole AC3 format "the worst ever seen" is quite generalistic, nicely said...

Sure, codecs can vary within a given format. But some formats are more limited than others, as well.

Quote from: sauvage78 on
"with encoders that may be somewhat less than 'industry standard'" ==> This is pure speculation, there is no evidence that professionnal hardware encoders are better than those publicy available.
Without any independant test, this is only a rumor.

Without testing, saying that the professional hardware encoders are better, is only a rumor. But assuming they're comparable to more easily-available encoders is also unverified.

Although it's true that if the companies wanted to produce encoded samples for use in blind tests, they could.
God kills a kitten every time you encode with CBR 320

do we really know how good/bad is AC3 encoding?

Reply #34 –
@ sauvage78:

You cannot compare "more or less no psychoacoustic model" implemented into the specific Aften encoder against tweaked psychoacoustic models implemented into the specific LAME MP3 encoder or into the aoTuV Vorbis encoder, and call it a fair comparison. Psychoacoustic models are specific to encoder implementations, not to the file format specifications. As well as channel coupling, blocksize switch strategy, bit reservoir allocation, ...

AC3 is a format, a specification - but no implementation. What do you call "the codec" exactly?

If you want to prove that the AC3 format is worse than others, then compare only the ATSC A/52 specs against the ISO/IEC 11172-3, ISO/IEC 13818-3, ISO/IEC 14496-3, Xiph Vorbis, Sony ATRAC3 and similar specs. Without even mentioning "Aften", "ac3enc" or "Soft Encode". Compare block sizes, spectral subband separation, bit allocation ... all that technical stuff which must be common for all AC3 encoders, regardless of additional tweaks.

And if that was not your intention: A more or less "fair" competitor against the Aften AC3 encoder would at most be the BladeEnc MP3 encoder, if you are looking for specific implementations instead, because what Aften still lacks about channel coupling and psychoacoustic model, BladeEnc lacks as well (being only able to encode Separate Stereo, but no Joint Stereo, for example).

But whatever you do: Compare apples with apples, not apples with oranges. Or bicycles with racing bikes.

do we really know how good/bad is AC3 encoding?

Reply #35 –
Every AC3 encoders I tested is very bad to awfull, so, so far, every implementations I tested is bad, that's why I concluded that AC3 as a codec is overall bad. Indeed you can still think that, maybe, with the existing specification, you could code a decent AC3 encoder if you were very skilled, but honestly with the preliminary bad results of 3 available implementations, it's like believing in in miracle ... Why would you be so clever that the implementation of all others developers would sound crap but not yours ? Anyway even if I know that the AC3 specifications are weak due to the age of the codec, I am not a technical guy so I am not discussing specifications here.

I am discussing about efficiency, which mean for me ABXable results at a specific bitrate with defined samples, I really don't see why in this context I couldn't compare AC3 to whatever codec I want to compare it to.

I don't see why comparing AC3 with more recent codecs with more advanced specifications would be being unfair with AC3. The evolution of codecs is relentless, I don't see why I should be nicer with AC3 than with any other codec.

Sorry, but I compared rotten apples with rotten apples (AC3 implementation X vs AC3 implementation Y) & rotten apples with fresh oranges (any AC3 implementation Vs. aotuv or nero AAC) ... it doesn't change anything: AC3 is awfull by itself ... I don't need to compare AC3 to anything except the original to know that it is bad ... no AC3 implementation I tested is transparent at 224Kbps for 2.0 (worst at such a high bitrate it is usually far from being transparent) ... I don't see how such a bad ABXing result could be read as being a good result ...

My test was not meant to be a reference, there is plenty of point where you can criticize it, I mean how could I seriously make a definitive judgement on a codec with only a single sample ? I can't, & I don't even pretend that I can. Also, there are several existing non-free implementations that weren't tested at all. But all I say is that within the context of my test 3 AC3 encoders results were bad when other codecs can shine in the same conditions. (Nero AAC & aotuv) ... now if you think that aften is a good codec well prove it.

A fair comparison is a comparison made to the best of your knowledge with the most scientific aprroach possible ... which in the end means that you & every honest people can re-produce the results ... I can re-produce my result if I want (& for exemple I know that /mnt is an honest ABXer because I was able to re-produce his results, so I hope anyone can re-produce mine) & I could even produce a  larger test (more samples) if I would think it would be of any interest. Sadly, my preliminary results just killed all my interest in AC3. I have better things to do than testing an obviously obsolete technology. I'd rather test the next aotuv version than waste my time with plenty of non-free & patented codec that sounds crap & can only be aquired freely illegally bundle with nasty malwares (free AC3 encoders seems weaker).

I admit my test is too short & specific to draw any serious conclusions (scientific), but at the same time I cannot do as if I didn't spend a few hours testing AC3 ... because many people that obviously never spent a few hours testing AC3 are trying to teach me what to think about AC3.

The truth lies somewhere between the lines, on the one hand there are serious hints (ABX logs on castanet, old technology) that might lead you to think that AC3 is a bad codec, but jumping directly on this conclusion is a shortcut, on the other end ignoring them is honestly leading nowhere, it's lying to yourself by omission.

I challenge anyone to prove that any AC3 implementation can rivalize with a modern codec, because I am sure that I can prove (providing that I grab the encoder) that none can achieve this. This is not being unfair, this is just being honest.
I can understand that people use AC3 due to compatibility with the device they own, but I cannot let people say that AC3 rivalize with modern codecs.

It gives to newbies the feeling that encoding a lossless blu-ray stream to aften AC3 5.1 is a better option than transcoding it to Nero AAC 5.1 ... unless you own a AC3 device for your living room, this is just not true. ... and nowaday with the flourishing H55 mini-itx motherboards even if you own an AC3 device in your living room I tend to think that it is also not true.

In the end, except for the force of habbit, there is no real reason to use AC3 in 2010.
The only good reason to use AC3 in 2010 is to keep an original AC3 stream untouched ... because it could fall to dust if you re-encode it ...

do we really know how good/bad is AC3 encoding?

Reply #36 –
Well, of course AC3 has its limits. Mind you, it had to be decodable quickly with the semiconductor integration level of the early 1990s.

And only because currently available home encoders do not yet implement all possible features doesn't mean that there will never be more complete and complex implementations. Let's wait for "fresh apples" ... (like Aften 0.1.x?) - it may never be as good as LAME, but surely better than now. And then it will also support E-AC3, according to their To-Do list.

Furthermore, the AC3 format is widely supported and compatible. How many A/V receivers do you know which are able to decode multi-channel AAC correctly, already?

do we really know how good/bad is AC3 encoding?

Reply #37 –
Quote from: sauvage78 on
In the end, except for the force of habbit, there is no real reason to use AC3 in 2010.


Yes there is, it's called compatibility. Can you imagine what would have happened if both Blu-Ray and HD-DVD just said, nope we're not supporting any old codecs any more? No-one would have adopted either format.

do we really know how good/bad is AC3 encoding?

Reply #38 –
Quote from: probedb on
Quote from: sauvage78 on
In the end, except for the force of habbit, there is no real reason to use AC3 in 2010.


Yes there is, it's called compatibility. Can you imagine what would have happened if both Blu-Ray and HD-DVD just said, nope we're not supporting any old codecs any more? No-one would have adopted either format.
Considering the average purchaser of HD-DVD and Blu-Ray had to purchase a new player for those discs, would the vast majority have even cared if AC-3 wasn't supported? Not likely. All Joe Public wants to know is: how do I hook it up, does it look better, which disc/format do I need to purchase for my player. That's about it. Outside of those basic questions, it becomes bithead territory. The player itself could be backwards compatible, but the discs didn't need to be.

do we really know how good/bad is AC3 encoding?

Reply #39 –
Quoting myself:
Quote
It gives to newbies the feeling that encoding a lossless blu-ray stream to aften AC3 5.1 is a better option than transcoding it to Nero AAC 5.1 ... unless you own a AC3 device for your living room, this is just not true. ... and nowaday with the flourishing H55 mini-itx motherboards even if you own an AC3 device in your living room I tend to think that it is also not true.


Using ac3 for compatibility in 2010 is a dying false-good reason ... with a Zotac H55-ITX WiFi (6 sata for 9+ tera octet video storage) & a Lian Li PC-Q08 you can decode whatever codec you want, almost wherever you want ... it is not as if AC3 compatible device would be DAP, I understand that mp3 doesn't die due to compatibility with all existing DAP. A mini-itx solution is not nearly as portable as a mp3 DAP (specially with 6 HDD inside...). But IMHO things are different for AC3.

AC3 compatible devices are not DAP, a mini-itx computer will easyly replace your divX (or whatever video codec)+AC3 decoding device in your living room ... nowadays a mini-itx solution with few HDD will not take more space in your leaving room than your old Divx/AC3 decoding device & will have the huge advantage to virtually decode every codecs (even not yet existing one+games) ... so in 2010 if you hang on to AC3 it's because you hang on to an old device that you already own... a 2010 newcomer has no reason to use an AC3 encoder, he only needs a AC3 decoder for backward decoding compatibility.

Encoding to AC3 in 2010 is a waste of time & audio quality & HDD space.

do we really know how good/bad is AC3 encoding?

Reply #40 –
Quote from: sauvage78 on
Quoting myself:
Quote
It gives to newbies the feeling that encoding a lossless blu-ray stream to aften AC3 5.1 is a better option than transcoding it to Nero AAC 5.1 ... unless you own a AC3 device for your living room, this is just not true. ... and nowaday with the flourishing H55 mini-itx motherboards even if you own an AC3 device in your living room I tend to think that it is also not true.


Using ac3 for compatibility in 2010 is a dying false-good reason ... with a Zotac H55-ITX WiFi (6 sata for 9+ tera octet video storage) & a Lian Li PC-Q08 you can decode whatever codec you want, almost wherever you want ... it is not as if AC3 compatible device would be DAP, I understand that mp3 doesn't die due to compatibility with all existing DAP. A mini-itx solution is not nearly as portable as a mp3 DAP (specially with 6 HDD inside...). But IMHO things are different for AC3.

AC3 compatible devices are not DAP, a mini-itx computer will easyly replace your divX (or whatever video codec)+AC3 decoding device in your living room ... nowadays a mini-itx solution with few HDD will not take more space in your leaving room than your old Divx/AC3 decoding device & will have the huge advantage to virtually decode every codecs (even not yet existing one+games) ... so in 2010 if you hang on to AC3 it's because you hang on to an old device that you already own... a 2010 newcomer has no reason to use an AC3 encoder, he only needs a AC3 decoder for backward decoding compatibility.

Encoding to AC3 in 2010 is a waste of time & audio quality & HDD space.


So compatibiliy with mainstream surround sound equipment is a waste of time?

do we really know how good/bad is AC3 encoding?

Reply #41 –
As I already explained twice IMHO yes, with recent hardware you can easyly achieve surround sound more efficiently. Encoding to AC3 can only have some meaning if you already own an AC3 based surround solution & don't intend to upgrade.

People are still acting as if backward compatibility with AC3 in the video device world was as important as the backward compatibility with mp3 in audio device world (specially DAP) ... IMHO nowaday due to mini-itx solutions this is not true anymore. Maybe you just didn't realized yet that starting with the H55 chipset there are plenty of mini-itx motherboards & cases that are suddenly out to invade your living room. I could name you almost 10 motherboards/case like those. This is a recent hardware fashion but this is potentially a bulldozer for old living room divx devices that are now almost good for the delete bin.

You could defend your position in say 2000 (not sure of the exact timing) when no LCD TV had HDMI & when no mini-itx motherboard could replace your divX/AC3 device for the decoding ... in 2010 your argument about the importance of the backward compatibility with AC3 falls flat IMHO. You're just no up to date from an hardware point of view.

Encoding to AC3 in 2010 is inertia. I know that all the warez scene is going AC3 & that there are plenty of mkv (hd or sd) with AC3 around ... that doesn't mean that AC3 is a good codec as a final target. It only mean that keeping the original AC3 stream is the least worst option.

There is a big difference between keeping the original AC3 stream because you have no choice (which is clever) & encoding a lossless stream to AC3 (which is definitly not wise IMHO). The problem is that newbies don't understand this big difference. They don't understand that many people use AC3 as if it was lossless because it IS the original stream despite being lossy, they think that the AC3 stream they get is, like a mp3, a second hand encoding ... so they think that AC3 is a good codec because everyone use it which is a miss-understanding of what an AC3 stream is & from where it comes from.

What I am fighting against is not people playing back original AC3 streams, but people thinking that because AC3 is widely accepted AC3 would de facto be a good codec & that it would be a clever choice to encode a lossless stream to AC3 in 2010 ... IMHO, this is a shortcut & a lie simply.

do we really know how good/bad is AC3 encoding?

Reply #42 –
Is encoding 2.0 sources to AC3 really all that widespread?

Most peoples' encounters with AC3 will be via 5.1 'Dolby Digital' mixes on their DVDs.

do we really know how good/bad is AC3 encoding?

Reply #43 –
Quote
So compatibiliy with mainstream surround sound equipment is a waste of time?

Quote from: sauvage78 on
As I already explained twice IMHO yes, with recent hardware you can easyly achieve surround sound more efficiently. Encoding to AC3 can only have some meaning if you already own an AC3 based surround solution & don't intend to upgrade.


It's lucky you don't work for any major consumer electronics company then

do we really know how good/bad is AC3 encoding?

Reply #44 –
Quote from: clintb on
Considering the average purchaser of HD-DVD and Blu-Ray had to purchase a new player for those discs, would the vast majority have even cared if AC-3 wasn't supported? Not likely. All Joe Public wants to know is: how do I hook it up, does it look better, which disc/format do I need to purchase for my player. That's about it. Outside of those basic questions, it becomes bithead territory. The player itself could be backwards compatible, but the discs didn't need to be.


I guess that's true but I think what sauvage78 is saying that you shouldn't be encoding to DD at all anywhere.

Most BR/HD-DVD players will reencode tracks into full bitrate DTS for compatibility so I agree with you on that  I do remember the amount of  going around when I got my HD-DVD player and they were outputing DTS when selecting the DD+ soundtracks.

do we really know how good/bad is AC3 encoding?

Reply #45 –
Quote from: timcupery on
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.


I'm under the impression that like MP3, AC3 is a coding format that can be improved by improving the encoder. I'm further under the impression that people who license the latest-greatest AC3 coder from Dolby Labs have been getting something that has gone thorugh a number of generations of improvement.

It remains for someone to compare productions  that have been encoded using the latest-greatest AC3 encoder, and the source material to that encoder.

do we really know how good/bad is AC3 encoding?

Reply #46 –
The spec's bad choice of the very long minimum block size of 256 samples makes it pretty hard to improve things as transient handling above a certain point. Even if you employ the best programmers on earth.

do we really know how good/bad is AC3 encoding?

Reply #47 –
Quote from: googlebot on
The spec's bad choice of the very long minimum block size of 256 samples makes it pretty hard to improve things as transient handling above a certain point. Even if you employ the best programmers on earth.


That would be one of those assertions that needs to be supported by some real world data. Where's the AC3 listening test performed with the latest-greatest encoders from Dolby?

Otherwise, you're basically messing with TOS 8.

do we really know how good/bad is AC3 encoding?

Reply #48 –
Quote from: Arnold B. Krueger on
That would be one of those assertions that needs to be supported by some real world data. Where's the AC3 listening test performed with the latest-greatest encoders from Dolby?

Otherwise, you're basically messing with TOS 8.


At 48 kHz, a block size of 256 samples means that you have one frequency domain representation every 5.3 ms. But research, for example about the 'precedence effect', has shown that the auditory system can discern arrival times of wave fronts up to a difference of 2 ms.* AC3 is unable to satisfy such a capability with 5.3 ms long samples. As said, no programmer or "latest-greatest encoder" from Dolby can go beyond that.

In contrast, at the same sample rate, AAC can switch to blocks of 2.6 ms length.

* "Spatial hearing - the psychophysics of human sound localization" by J. Blauert, MIT Press

do we really know how good/bad is AC3 encoding?

Reply #49 –
Quote from: googlebot on
At 48 kHz, a block size of 256 samples means that you have one frequency domain representation every 5.3 ms.

Unless you switch to 2x128 which is also possible.

Quote from: googlebot on
But research, for example about the 'precedence effect', has shown that the auditory system can discern arrival times of wave fronts up to a difference of 2 ms.

Ok.

Quote from: googlebot on
AC3 is unable to satisfy such a capability with 5.3 ms long samples. As said, no programmer or "latest-greatest encoder" from Dolby can go beyond that.

This is nonsense. You're implying that a codec using a filterbank with 256 banks isn't able to represent arbitrarily small time shifts. This is wrong for the same reason why this is wrong: people claiming -- just because PCM is sampled -- it's not able to represent sub-sample time shifts. Yes it is. And so is AC3. Sampling just limits the signal bandwidth, nothing else.

The only thing noteworthy is that this time/frequency partition affects the way how quantization noise is distributed (i.e. "pre-echo"). You can fight pre-echo in three ways (or a combination of them) in AC3:
(1) switch to short blocks (2x128 instead of 1x256)
(2) use noise shaping in the encoder
(3) higher bit rates

Cheers,
SG