AC3 quality at 256 kbps

[The Belgain]

Starting from the original AC3 from the DVD (Sleepy Hollow) which was 448 kbps 5.1 (2 channel track on DVD), I tried re-encoding the AC3 as 5.1 at 256 kbps using soft encode, and also using lame abr 128 (which averaged out at about 115).

I currently only have a 2 speaker setup to test these.

The AC3 file sounded noticeably worse than the mp3. I suppose this is not entirely surprising seeing as the lame has a higher bitrate per channel, but many people on this board have been saying that 256 AC3 will sound nearly as good as the original. How much channel coupling does AC3 use in 5.1?

What results have people been getting qualitywise?

[Awatef]

As you can see in Soft Encode, AC3 5.1 @256kbps has frequencies only up to 12KHz or something like this. That's why it sounds crap!

People saying that it sounds like the original are lying to theirselves.
If you want the truth, I don't even like AC3 5.1 @384kbps cause it has frequencies only up to 18KHz.
With at least 448kbps, one get full bandwidth till 20KHz

[MaTTeR]

Besides myself and Antimon I haven't really seen anyone else comment on transcoding down to 256kbps. I for one never said it sounded like the original AC3 though. What I have said is that it's pertty close with suprising results and very few people will hear the difference. You have to use common sense obviously. You cant take a 448kbps stream from a highly action packed film such as Matrix, Pitch Black or BlackHawk Down and transcode it to half it's original bitrate and expect it to sound just as well as the original;) Drama or comedy type flicks are prime canidates for transcoding downward though. I tried doing this on Pitch Black and could definitely hear some dyamics were lost but I epxected that out of that movie which I consider reference audio (great movie too :).

What movies, sound card and external amp system are you guys using to where you hear these artifacts?

FYI- The newer 4.0x InterVideo filters reportedly provide better sound quality in general.

[Doobie]

448Kb/s is overkill for AC3. Probably 320Kb/s is the optimal value. For most movies, 256Kb/s should be acceptable.

Why did 128ABR MP3 sound better than 256Kb/s AC3? For starters, the AC3 had a cutoff of 12.??KHz while the MP3 probably cutoff at around 14.??KHz.

Beyond that, consider how the audio was processed. Was the MP3 volume cranked up and the dynamic range compressed? This might make the same audio sound better to you. How were the channels combined for the MP3 (e.g. -3dB on the LFE) vs. what happened to the other 4 channels in your AC3 (considering you only have two speakers)? How were they combined on playback?

Also, give LAME some credit. They've been milking every bit of quality they can from MP3, including improved modeling, etc. The AC3 is CBR and otherwise hasn't been well milked for quality.

The reason to use AC3 is for 5.1 channels or to preserve the original audio. It would be interesting to compare these soundtracks with 5.1 speakers and then figure which sounds best. I'm betting on the AC3.

[Awatef]

@ Doobie
I don't agree with you, 448kbps is MINIMUM for AC3 5.1 (this means not even 96kbps per channel!), because only starting from that bitrate you get full bandwidth up to 20KHz, and that is equivalent to CD quality.
With 320kbps you get only frequencies up to 16KHz, which is equivalent to a normal tape. This is already far from CD quality.
And as said, I find 384kbps that is used on some DVDs really a shame, DVD authors that use such a bitrate are jerks IMHO! frequencies only up to 18KHz and in addition minor artifacts, this is really unacceptable!

And comparing MP3 and AC3 5.1 is non-sense anyway, if you wanna compare, compare MP3 and AC3 2.0 and MP3 will sound better anyway in that case.

[MaTTeR]

Originally posted by Awatef

And comparing MP3 and AC3 5.1 is non-sense anyway, if you wanna compare, compare MP3 and AC3 2.0 and MP3 will sound better anyway in that case.

This is assuming your talking about a stereo downmix though. If your looking for a surround downmix then it's going to be hard to get better surround FX during the transcode to MP3 because of the quality loss during the process. So since I'm always using surround sound in the home entertainment system then I'm always going to use original DPL/2.0 track.

If 448kbps isnt enough bandwith then jump onto the DTS band wagon. Once you watch a "SuperBit" DVD with DTS then your expectations of sound and video quality go up considerably. If someone tells you there isn't a difference in the AV quality on a SuperBit disc then send them to get some glasses and a hearing aid:D

[Awatef]

@ Matter
That's what I ment, comparing downmix.
Talking about genuine 2.0 AC3: I don't know if it's only on my system that way, but the damn original AC3s are mostly not loud enough.
That's why I prefer normalizing (a boost of about 9db is no rarity!)and reencoding to MP3, sounds IMHO better (well, more precisely: louder! :D)

[MaTTeR]

@Awatef

If your playing them through an external amp then maybe you need to turn the DRC up a little higher if your reciever allows it. I typically run my DRC setting at Normal but some movies require Max.

Also, I was thinking players like Power Divx 3.1x would allow some sort of on the fly DRC setting as well to boost dynamics.

[Doobie]

Originally posted by Awatef
448kbps is MINIMUM for AC3 5.1 (this means not even 96kbps per channel!), because only starting from that bitrate you get full bandwidth up to 20KHz, and that is equivalent to CD quality.
With 320kbps you get only frequencies up to 16KHz

Most movies don't have any appreciable audio above 16KHz and even if they did most people don't have any appreciable hearing above 16KHz. Not even 96Kb/s per channel? For our other readers who need a more common reference, you're complaining that it's not even 192Kb/s 2-channel(non-surround). You know what, most non-audio professionals wouldn't be able to tell a 128Kb/s AC3 movie track from the original DVD in a blind test, let alone 192Kb/s and no original reference track on an A/B switch.

Besides, I'm talking about one or two CD DVD rips where it's absurd signficantly degrade the quality of the video for an insignficant increase in audio quality. You seem to be talking about the tracks on the original DVDs where it's obvious "they" don't want "us" to have studio-quality audio.

[The Belgain]

A 16 kHz cutoff can make a noticeable difference as quite a lot of people can hear above this. Also I don't see why the extra 64 kbps can't be spared on a Dual-layered DVD holding 9.4 GB?

Also, anyone with decent ears will be able to tell the difference between a 128 kbps AC3 and a 192 AC3 (maybe not on speech-only parts, but certainly on bits with music).

It is true that it isn't really possible to have 5.1 sound on 1 cd rips while keeping good video quality (except on really short movies, ie less that 90 mins).

[Awatef]

@ Matter
I have a normal Stereo system connected with an analogic cable to my stereo sound card.
And... PowerDivX is crap! :D

@ Doobie
I agree with The Belgain, EVERYONE can tell the difference between 128kbps AC3 and 192kbps AC3, the difference is huge and very obvious, since 128kbps AC3 has a cut off at about 13KHz.
A Cut off at 16KHz is also clearly hearable for most people, and that's why most people can tell the difference between a 128kbps MP3 and the original CD (directly compared of course)
Just to let you know, I can hear without problems beyond 22KHz (I've done a hearing test).
And *ALL* movies (well, at least recent ones) have frequencies over 16KHz (stop talking without knowing what you're talking about!), you can clearly see that on a frequency analysis graph, like the one of Cool Edit. What do you think you have, a cheap type1 tape?!

[Beave]

@Awatef
How old are you? Under 15? Hearing degrades when you become older. Most people can't hear anything above 16-17 when they become older then 30. What you hear above that is actually the side frequencies those high noises create in the lower spectrum.
But whatever, if you are so much bothered by the low frequencies you will have live with it. And for the rest the AC3 at 256kps will be ok.

[Awatef]

@ Beave
No, I'm 21.
And what I hear is no noise, I here treble, nice cristal clear enjoyable treble!
I can even detect an 18,5KHz cutoff! beyond that I can't tell a difference anymore (I can't detect 19KHz cutoff or higher)
What people hear and not hear is another question, but I would rather estimate, most people can hear up to 18KHz.
(Re)talking about 256kbps AC3: there is a limit for everything! if you want a moderate bitrate, *MAY BE* 320kbps, this will be like 128kbps MP3 which is accepted by most people, but no bit lower than that! 256kbps has a way too low cutoff!

[MaTTeR]

@Awatef

I can't agree with a blanket statement like "for AC3, anything below 320kbps isn't acceptable". If that's the case then I suppose DSPguru just wasted some of his time writing the new transcoder.

Like I said before, it all depends on the movie and I find that most non-action type movies sound great at 256kbps.

I have a normal Stereo system connected with an analogic cable to my stereo sound card.

So how is it your testing lower AC3 5.1 bitrates with a stereo sound card? :confused:

[balbrain]

Yup I agree with Matter. I have tried both 448kbps 5.1 and the transcoded 2channel 256 kbps AC3 with a lot of movies. For movies like 'jurassic park' there are a few places where you can make out the diff, but not many. But for normal movies like 'the cider house rules' where FX are not that many (or nonexistent) you have to be an oscilloscope to catch any difference :) Incidentally for such movies 192kbps mp3 also seems the same. So I prefer to go with that over AC3 so that I can get higher video bitrate.

--balbrain

[Awatef]

So how is it your testing lower AC3 5.1 bitrates with a stereo sound card?

He?! what's wrong with that?! the whole talk is here about frequencies being cutted and muffled sound, and one can tell that with every good hifi system! no need for 5.1 system to test it, we aren't talking about surround effects!

[MaTTeR]

I've just spent 2 weeks in a theory class (4 more to go :-\)of sort and it does me absolutely no good hearing the lectures until I can get hands on experience and see how things work. Looking at numbers on paper and discussing concept is all well and good to give us an idea of what to expect but for me I do "listening" tests before drawing a conclusion. I see your point in listening for the muffled sounds but with 2ch your still not seeing the big picture IMO.

If someone tells me to use 5.1 Vorbis because it has better specs on paper (ie.better cutoff rates) then you can bet I'll do listening tests before taking the word of theory. Hope I'm not confusing the situation:D

[Awatef]

@ Matter
Of course I'm doing hearing tests, both on speakers and on headphones! :p
The sound being 5.1 or not doesn't have influence on the sound being muffled or not! a cutoff is a cutoff, with 2 or 10 channels!