5.1 or 2.0 source ???

[Soulhunter]

Hi everybody,

I remember DSPguru said something about this in the Audio FAQ ;
>
AC3->MP3. which AC3 stream should i decode ? 5.1 or 2.0 ?
The 2.0 stream has a better mix for 2 channels, than the 5.1 to 2.0 downmix features offered by AC3 decoding tools. therefore, it should be the best to work with the 2.0 stream.
Transcoding the 5.1 and downmixing it manually can only be useful for users who has a surround system that has Dolby ProLogic II decoder.
Some of the 2.0 tracks weren't downmixed for DPL2, so you must take the origianl 5.1 track and downmix it with a transcoder that capable of doing it.
<

Ok, maybe I should trust him because he is THE DSPguru... (because I think he knows much more about this stuff than me) But, I have some questions...

First I thought using the 5.1 source would be better bacause the 2.0 source has only a quality of 192kbps, and 5.1 has 448kbps (most time's)!
So, a 6ch 448kBit/s -> 2ch should sound better than 2ch 192kBit/s -> 2Ch ???
Yes&No -> Not fully sure... 2ch 192kbps has a better kbps/Ch ratio than 5.1, but you have some quality gain by 5.1 downmixing because the channels will be mixed together...!

Sure it depends on the output kBit/s you aim for (xxxkBit/s)... If you use a low bitrate (128kBit/s) it def. better to stay by the 192kBit/s source. But Im an HQ-Freak so I use higher bitrates... And yes, I could mux the original AC3-Source (I do it with 75% of my encodes), but I could also watch the original DVD... :D (Yap, I do it also... Some times...;))

Ok, lets say I aim for an Bitrate of 224kBit/s in MP3 (because for some reason I dont want to have AC3 ;)).

What's better higher kBit/s to Channel ratio with 192kBit/s 2ch source, or having more "Info" (6 Channels but lower kBit/s to Channel ratio) with a 448kBit/s 5.1 source ???
Yes, even better is a DTS source... (because it has a even higher bitrate) and I could mux the original AC3 source :D

But this problem is more fictional... Just something I want to have cleared out (just for interest/knowledge) :)

The main idear about all this is,
That its not fair just look on the kBit/s to say what quality it is... (as some of my PPL think) it also depends of the amout of channels. A 192kBit/s 2Ch source should give a better quality than a 448kBit/s 5.1 source because the 2Ch file uses more kBit/s per Channel!
Its the same with video bitrates... Bitrate means nothing, when its not compared against resolution. A 1000 Kbit/s video could look good at 512x384 pix. but looks crappy with a res. of 1024x768 pix. I think its same with audio bitrates and the amout channels!

But Im also not fully sure about this... Because,
A 2Ch source has to store the low freq. in thier L/R channels... A 5.1 source stores it in the LFE channel. So the 2Ch file has to use some of its better kBits/Channel ratio on this low freq. This scales the quality down... I think.

So enough for now, I just want to hear some opinions about this stuff...!

Bye

[KpeX]

IMO you're making it way more complicated than it needs to be. The 2.0 channel track is already in the format you want, and no conversions or downmixes are needed. Don't think that just because the 5.1 has more channels that it has more audio information. Don't want to put words into DG's mouth, but I think what he was getting at is that it's better and simpler to not have to downmix all those channels into 2. You can analyze the bitrate per channel all you want but I think you would be very hard pressed to notice any quality difference. Although you could always ABX it and check for yourself if you want. hth,

KpeX

[pacohaas]

Exactly, the 2.0 is usually the studio's own special downmix of that particular movie, so if you are to watch it on a 2 speaker setup, you should choose that while watching the DVD. Makes sense then to encode the 2.0 version if you are making a 2 channel mp3, mp2, ogg, etc rather than downmixing the 5.1 version yourself into 2 channels.

The kbps/ch thing does also have some bearing. You would certainly choose a 192kbps 2.0 stream over a 192kbps 5.1 stream.

[Soulhunter]

@KpeX
>
Don't think that just because the 5.1 has more channels that it has more audio information.
<

As I say...

>
Its not fair just look on the kBit/s to say what quality it is... (as some of my PPL think) it also depends of the amout of channels.
<

@pacohaas
>
You would certainly choose a 192kbps 2.0 stream over a 192kbps 5.1 stream.
<

Yap! Sure, but also a 192kbps 2.0 stream over a 448kbps 5.1 stream ???
That was the question... 96 kbps/Ch ratio with 192kbps 2.0 stream or ~75kbps/Ch ratio with 448kbps 5.1 ???


@All

DSPguru only said that the 2.0 is better to use because it has has a better mix for 2 channels... So what I want to know is, if it offers also higher quality ???

Sure I think the difference is so low that I will be not able to hear the difference...

But as I wrote before this is only a fiktional problem...

I only want to know what gives "theoretically" better quality !

Just because I had a discussion about this "unreal" problem with other PPL... Some of them said using 5.1 source is better, others (like me) said using the 2.0 source is better... Till I was not able find Info elsewhere, I thought its a good Idear to post it here, to clear this problem out.

PS: What do you think about downmixing a DTS 5.1 source to 2Ch...? This gives def. better quality for higher bitrate 2Ch files ("224kbps" or "256kbps") than a 192kbps AC3...

Bye