Hydrogenaudio Forums > ?stereo and surround in compression?

Full Version: ?stereo and surround in compression?

Hydrogenaudio Forums > Digital Audio/Video > Movie/Multichannel audio

John Doe

Jan 26 2004, 19:17

Hi folks,

following problem: We wanted to watch a DivX-movie (in this case the disturbingly honest movie DOGVILLE), not marked as AC3, thru a Dolby Surround/Digital capable soundcard and a Doby Surround/Digital capable amp. WAS JUST STEREO

First of all: Will analog Surround Sound be kept in a compressed format such as mp3?
If not, why shouldn't it? From my knowledge its just a difference of wave-forms, that make the surround - that can't be wiped out by the compression, can it?
Is it possible, that a DVD doesn't have an analog surround channel but just the Dolby Digital sound?

I have a fight with my buddy here, who keeps telling me, that even simple Audio CD's (not tagged as Surround sound) would often be mixed in 5.1 just because his GREAT amp is mixing it somehow to the rear speakers as well...is it possible, that the amp just filters some frequencies more or less randomly for imitating surround sound?
(as an example: The Prodigy - Speedway, where the cars would come from the rear)

I'm running out of arguments here ... help me to help him, we're close to grab fire arms!!

Doctor

Jan 26 2004, 20:13

This exact topic has been discussed many times, please use search.

Good implementations of MP3, such as LAME, preserve the stereo imaging necessary for the surround information.

The amp may do tricks like that, however, people say that it messes up the overall sound.

Loke

Jan 26 2004, 20:32

Well.. I'm not quite sure I understand you. Analog surround sound will not be kept in a compressed format. The AC3 is decoded, first digital pcm, thereafter converted to analog sinewave.
And mp3 does not support surround sound by iso standard.
You can still hear artifacts from the AC3 encoding though, in the analog surround sound.

QUOTE

Is it possible, that a DVD doesn't have an analog surround channel but just the Dolby Digital sound?

DVD has never any analog sound on it, it's a digital-media, "Digital Versatile Disc"
You have to go to laserdisc's to get analog sound from optical discs.

Well, your buddy's amp is guessing when mixing to 5.1, but it may be good at it. I mean, why would we need ac3's with 5.1 sound if your amp could just get it right from 2channels music, and turn it correclty back into 5.1?
To get it correctly you need 5.1channels from the source and the format.

BTW: Johndoe's "ja takk til trøbbel" is not very good mastered...

John Doe

Jan 26 2004, 22:26

QUOTE

I'm sorry but sometimes you just don't know what to look for!!!
But now I read some threads and it seems that mp3 just is to lossless to keep the surround info properly...what a pity. I was hoping to have a great movie night

QUOTE

Well.. I'm not quite sure I understand you. Analog surround sound will not be kept in a compressed format. The AC3 is decoded, first digital pcm, thereafter converted to analog sinewave.

Shit there is more to keep in mind? Ok so Dolby Surround is digital as well without being AC3? But what's the frickin difference between Dolby Digital (thats AC3 right?) and Dolby Surround then??? I just learned that Surround is "hidden" in stereo (what means analog for me) and AC3 has 5or6 seperated channels. Damn, my brain's to tiny for that!

QUOTE

Well, your buddy's amp is guessing when mixing to 5.1, but it may be good at it

HA, so I'm right and hardly any Audio CD that sounds like 5.1 is recorded in multichannel format right? But which rules does the decoder follow when handling simple stereo files?

...and I'm really not related to Mr Johndoe in ANY WAY (if he cannot master correctly - - - or is he ritch?). I meant to be the nameless dead who doesn't know a thing!!

dreamliner77

Jan 26 2004, 23:35

QUOTE(John Doe @ Jan 26 2004, 11:26 PM)

But which rules does the decoder follow when handling simple stereo files?

Essentially filters specific frequencies to specific speakers with added delay.

k4dwi

Jan 27 2004, 00:40

depending on the encoder/codec used, surround may be wiped out. it's something that the encoder may decide will be inaudible, as it is encoding simply for stereo-playback.

newer stereo systems have Dolby Pro-Logic II, which is an analog version of AC3; basically it's a more advanced version of PL that uses a LFE channel and stereo rear's. PLII can take any stereo source and re-process it so it fills 5.1 channels. i don't have a system yet but i hear nothing but good reviews of PLII.

surround sound: digital is mixed per channel; each one gets a 'feed'. analog is mixed as a stereo track which contains certain sections of different phases; out of phase will be sent to the rear- basically anything not in phase but of enough distinction in stereo not to be sent to the center will be routed to the rear; center is made up of simultaneous L+R data.

the reason a compression method may kill the surround is because the audio is treated as simply L+R information. L+R is always sent to the center, but sounds on just L/R are sent to their respective channel. phase on a poor encoder makes no difference; it'd probably just be sent to the center because the distinction is not strict enough. this is all from my personal experience listening to various tracks that i know to be PL-compatible... the mp3's i find online (poor encodings) usually only work good in stereo, LAME using the absolute max settings (VBR 320, Normal Stereo, no low/hi-pass filtering, etc) and probably many alt- settings will probably not cause much harm to the audio but the encoder probably still sees just L+R, not the phasing.

one last thing, a normal audio CD (not Super Audio or anything new or not 100% compatible on all players) isn't mixed in 5.1- it's mixed in stereo. there are always only 2 channels for audio on a CD. even a mono recording is stereo- just both channels have the same data. again, this is all my personal understanding from reading mag's and boards like this one.

for reference, PL=Pro-Logic. also, if you're here your brain is NOT too small to understand the a/v world. just keep learning and don't worry about asking a dumb question. just try to apease the regulars here and at any board by using the search function before posting

edit in bold.

Loke

Jan 27 2004, 04:59

I forgot about PLII.
But it isn't full 5 channel, better call it 3.2, because the two rear speakers isn't full resolution.
And there are cd's that supports ProLogic II, therefore 5 channel mixed, allthogh you don't get the complete 5 different channels.

John Doe

Jan 27 2004, 09:30

Why the heck does everybody tell something different - here's a thread from Doom9 - the very site for "DVD backup"/compression.

It says that mp3 joint stereo with "--alt-preset standard" and "--alt-preset extreme" > 160kbps would preserve DPL!

JD

KpeX

Jan 27 2004, 09:41

Basically, any lossy compression that uses full stereo or M/S joint stereo will preserve Dolby Pro Logic information.

IS joint stereo such as in MP2 will destroy Pro Logic information.

Also keep in mind that Dolby Digital 5.1 can be downmixed to a Pro Logic (II) downmix in MP3 with BeSweet.

MP3 via lame definitely preserves Pro Logic information, this has been done for some time now.

As far as whether CDs can have Pro Logic: An amp can attempt to create Pro Logic (II) from any 2 channel source - it's simply a matter of applying the upmix mathematical formulas. With non-Pro Logic sources, however, your results will vary depending on the audio source and the amp used.

tigre

Jan 27 2004, 09:56

QUOTE(John Doe @ Jan 27 2004, 05:30 PM)

Why the heck does everybody tell something different

Lame --alt-presets (and probably most lossy encoders) aren't designed for extensive use of DSPs on playback. Applying DSPs that mess with differences between channels (simple example: 'Karaoke' effect = substract channels) can lead to audible artifacts that are not audible on plain stereo playback. AFAIK no extensive testing has been performed on this with lame, so it's hard to make generally valid statements.

There are several related threads you'll find using the search, e.g. for "surround AND mp3"; any date, one of them even in the FAQ, maybe you'll find something useful there ...

2Bdecided

Jan 27 2004, 10:42

I'll ask one more time...

(just like I did here)

Please put http://www.hydrogenaudio.org/forums/index....showtopic=12004 in the FAQ. It's more complete than the others.

The following with respect to musepack q5 still stands:

QUOTE

In the mean time, if anyone can find a sample which gives audible artefacts using a real surround decoder and sensible speaker placement, I think Frank might be interested.

I'd extend it to lame aps too for the benefit of those here who are still interested. Rather than all this endless speculating, can someone with a correctly set-up surround system actually do some tests please? Anyone?

EDIT: because it you find a real problem, then there's just a chance of tweaking the psychoacoustics to fix it, and allowing this tweaking to be triggered by a special switch. But if no one finds any real problem samples...

Cheers,
David.

PowerPigg

Jan 27 2004, 10:52

In short, John Doe, you are right and he is wrong. Now you don't have to shoot each other.

Analog surround sound uses phase shifting tricks to save the information into a stereo track. Surround receivers use this info to recreate the surround effect. Unfortunately, they will do this for all stereo files they are asked to decode, even if they are not supposed to. With normal stereo files, yes, they may be able to pull phase shifted waveforms into the rear channel, but this will only trash the stereo image and give the track a "spaced out" sound that should not be there. Live performances recorded as normal stereo tracks can really get killed by this.

Also, it's possible that that car sound in the Prodigy soundtrack *was* encoded as a surround effect by design...

PowerPigg

Jan 27 2004, 13:21

One more thing I forgot to add to my previous post: I wanted to add to k4dwi's comment about the effect of compression on surround info. Whatever it might be in LAME that destroys surround information (and I'd be surprised if someone proves that it does), it would have *nothing* to do with use of joint stereo encoding, but rather with limitations in the psychoacoustic model that would be in place even if normal stereo was used.

Remember that joint stereo encodes the stereo channels as mid (L+R) and side (L-R) signals. L-R contains the complete surround content of the track, since L-R is exactly how Dolby surround extracts it.

My point is we should not derive from k4dwi's comments that joint stereo is "bad" and that it lies behind any possible problems encoding surround content properly. LAME's implementation of joint stereo long ago proved to be properly implemented and necessary to derive the best sound quality from a music sample at a given bit rate, and inherently *must* contain all surround information before any lossy compression is applied.

That being said, there are better choices than MP3 for encoding surround movie content. Other lossy encoders like Ogg Vorbis implement lossless coupling at higher quality settings, making the whole surround problem moot, and multi-channel implementations of such encoders exist for encoding discrete surround tracks, but not for MP3 (and I doubt any will pop up).

QuantumKnot

Jan 30 2004, 20:08

Just a minor question. What are the main differences between Pro-logic I and pro-logic II?

When I do DVD ripping and convert the AC3 file to a stereo wav to encode to Vorbis, I always use surround 2 as the downmix (Prologic II) but my amplifier is marked as just Dolby Prologic. Does that mean it's pointless to use surround 2 downmix when I dont have a PL-II decoder? And do I lose surround effects by doing this (playing PL-II on a PL decoder?

John Doe

Jan 30 2004, 22:46

couldn't describe it myself perfectly, so I googled and found the following:

from 5dot1.com:

QUOTE

What's the difference between Pro Logic, Pro Logic II, and Dolby Digital 5.1 Decoding?

Dolby Pro Logic is a matrix decoder that decodes the four channels of surround sound from a Dolby Matrix encoded stereo track, such as VHS HiFi tapes, Stereo CDs, and some TV shows. Dolby Surround is a matrix encoding process that in essence combines the Left, Center, Right, and Surround channels onto a single stereo track. A Pro Logic decoder then extracts the four channels on playback. If you listen to a Dolby Encoded soundtrack on a normal stereo system you may notice some sounds appearing to come from beyond the speakers or even from behind you.

Dolby Pro Logic II is an advanced matrix decoder that derives five-channel surround (Left, Center, Right, Left Surround, and Right Surround) from any stereo track. It works on any file even if it hasn't been encoded in the Pro Logic II format. On encoded material such as movie soundtracks, the sound is more like Dolby Digital 5.1 (see below), while on unencoded stereo material such as music CDs the effect is a wider, more involving soundfield. Among other improvements over Pro Logic, Pro Logic II provides two full-range surround channels, as opposed to Pro Logic’s single, limited-bandwidth surround channel. This is very similar to Circle Surround.

Dolby Digital 5.1 is the latest in multi-channel delivery. 5.1-channel soundtracks can be heard on most movies either on DVDs or in theatres. Dolby Digital 5.1 is also being offered through Window Media 9, digital cable, digital broadcast TV (DTV), and satellite transmissions. Dolby Digital is just that: Digital. It needs to be decoded by a Dolby Digital Decoder. It does not use the same Dolby Surround encode/Pro Logic decode process. Though you can listen to Dolby Digital 5.1 Encoded soundtracks in Dolby Pro Logic via the analog outputs on most DVD Players. Dolby Digital 5.1 is a discrete system that keeps the multiple channels fully separated throughout the encoding and decoding processes. In addition to having full-range Left, Center, Right, Left Surround, and Right Surround channels, Dolby Digital 5.1 soundtracks carry a sixth (“.1”) channel recorded with low-frequency effects (those bass rumbles and booms you feel as well as hear in a well-equipped cinema).

Seems a quite easy to understand description. Couldn't make it better myself!

JD

QuantumKnot

Feb 2 2004, 18:41

Thanks for the info.

Is Prologic 2 backwards compatible with Prologic 1? I only have a Prologic 1 decoder at the moment so will I get surround effects if I play a file that is encoded as Prologic 2?

Artemis3

Feb 3 2004, 02:06

I think this reference to the meanings of the various Dolby Technologies is in order.

And yes, lossy compression "may" compromise some of the tricks used by Dolby to squeeze more channels over less. In the case of Mp3, it is supposed to preserve them using joint stereo and stereo, what lame uses. The FHG encoders can use something called "intensity stereo" (for low bitrates) which does destroys the information.

All CDs are stereo, but some could be mastered using one of the 3 Dolby Surround Technologies: Dolby Surround, Dolby Surround Pro Logic, and Dolby Surround Pro Logic II. Such CDs normally show the Dolby logos (see reference). You need to have a decoder with the appropiate logo, normally built in the pre-amp or amp to hear the extra channels (and the extra speakers).

None of the Dolby Surround (2 ch) technologies have the "LFE" channel.

Dolby AC3 (aka Digital) uses discrete channels, but each channel is lossy compressed.

Loke

Feb 3 2004, 04:07

QUOTE

All CDs are stereo, but some could be mastered using one of the 3 Dolby Surround Technologies: Dolby Surround, Dolby Surround Pro Logic, and Dolby Surround Pro Logic II

You're reference to dolby seems to say something different.
Only the encoding of the "multichannel onto two tracks" prosess is called "Dolby Surround".
The decoding however is called "Dolby Surround Pro Logic" & "Dolby SurroundPro Logic II".

So there isn't 3 different technologies, just 1 for encoding, and 2 for decoding....

Fro Dolby's:
"Dolby Surround Pro Logic is the technology that decodes program material encoded in Dolby Surround"

And:
"Dolby Surround Pro Logic II is an improved matrix decoding technology that provides better spatiality and directionality on Dolby Surround program material"

That is, both only DECODING

tigre

Feb 3 2004, 06:02

I've split the post by John Doe containing questions about how to ABX mp3s converted to surround to this new thread.

edit: link fixed

John Doe

Feb 3 2004, 07:19

QUOTE(tigre @ Feb 3 2004, 04:02 AM)

I've split the post by John Doe containing questions about how to ABX mp3s converted to surround to this new thread.

Hope you don't mind tigre but the link is not the right one. The 2. thread is here.

JD

This is a "lo-fi" version of our main content. To view the full version with more information, formatting and images, please click here.