Decode 2.0 or 5.1 tracks?

[OxP]

Hi, when making a vbr mp3 and decoding the ac3 using azid, does it make a difference if i use the 2.0 track(if it exists on the dvd) or the 5.1 track? I mean, since the 5.1 is being downmixed, it would be more accurate to use the professionally mixed 2.0 track, or am i wrong?

[Taric25]

If your DVD has, say, Dolby Digital 5.1 EX vs. Dolby Pro-logic (2.0) it would be much better to use the 2.0 file since it was most likely recorded from a PCM then later compressed to 2.0. However, if you are comparring Dolby Digital 5.1 (EX) vs. Stereo, I would go with 5.1 since you would get better surround sound.

[ChristianHJW]

From my experience using the 5.1 AC3 track and downmixing to Stereo in Azid will be a very sensible thing to do if you are not after the Dolby ProLogic surround information, simply set Azid to 'Stereo' instead of 'Surround'. As i dont have any DPL equipment and for my normal 1 CD rips will use bitrates of 86 ... 112 kbps for sound ( in Lame using 'joint stereo' of course for these bitrates ) there is no sense in using the DPL information in my Stereo tracks.

If you take the 2 channel AC3 there is no way to remove the DPL info ..... so i dont take that one .... i decode the 5.1 to a 'simple Stereo' without DPL ....

[tangent]

If you're looking for mp3-vbr you're usually aiming for a bitrate around 192kbps... in which case it's better to just mux in the 192kbps 2.0 sound track and avoid the effort and quality loss of transcoding.

[ChristianHJW]

Its strange, when listening to Lame audio encoders i always have the feeling that movies will produce completely different bitrates for certain q values than audio.

My Lame VBR MP3 tracks, using old routine and q 1 ... 3 will usually result in bitrates of 160 ... 140 kbps ! Anybody to confirm this ?
Pls. dont tell me to use --VBR-mtrh ... i am not encoding 20 CDs per day, so i am absolutely not interested in the time my lame encodings may take ... i stick with old routine !

[OxP]

Yeah, i remember trying to do something with the matrix, and no matter what quality setting i put, the bitrate pretty much stayed the same...Also, i really doubt that many people try aiming for 192kbits for vbr mp3, i thought the point was to keep the size down. I read the other thread about vbr mp3+dpl. To get this straight, it seems senseless at low bitrates to utilize prologic because of some problems atleast from what ChristianHJW has said http://forum.doom9.org/showthread.php?s=&threadid=15328

Does anyone else find Dpl sounding bad at lower bitrates? If it is common then it seems useless to even downmix doesnt it? just extract the left and right channels and thats it?...its confusing...the audio size vs. DPL playback...even if this is an isolated problem, does it make sense to use the 2.0 DPL track instead of the 5.1 track?

[Taric25]

I still stand by my opinion. The ONLY reason I make VCDs and not SVCDs or DivX is because of one thing: compatibility. I lend my VCDs to my friends and they can play them almost anywhere. MPEG-1 video plays anywhere, so no CODECs are needed.

When I actually do make DivX, I always make DPL MP3s with Besweet (or Azid, before). The reason I don't use AC3s in my DivX is because only 2 of them at most have 5.1 equip. hooked to their computers, and Dolby Prologic II is good enough for them. So, I make DPL MP3s + DivX MPEG-4 (which I can get them to install if they don't already have it). This is because they wouldn't mind going through a wizard to install DivX4, but would definately mind to go through a process to properly decode AC3 on an old computer that doesn't even support 48kHz sound.

So, in conclusion, the only reason I bother to transcode 192kbs Dolby 2.0 AC3 into 192kbs DPL MP3 is for compatibility. My firends are able to view these with no problem. However, I hope to encode DivX + multichannel MP3s someday (even if most of my friends are just getting a downmixed output). ;)

EDIT - I hope my post makes more sence now that I have edited it.

[tangent]

@ChristianHJW
-q0 and -q1 are experimental stuffs and to date they have not shown any improvements over -q2 yet, and both of them still have issues. -q2 is still your best and safest bet.

@OxP
I mentioned that because VBR is only useful at bitrates of around 192kbps. VBR has issues below that and ABR provides better quality at those bitrates below 192kbps.

I would prefer to use the 2.0 DPL track instead of just encoding the L and R channels from the 5.1 track. Doing it the latter way may mean that your output track will be missing some important sounds (e.g. a gunshot from the rear). Anyway, DPL tracks are meant to be compatible with 2 channels.

@Taric
I apologize but I don't quite understand you. If you are making VCDs I understand you'll be transcoding but it will be to MP3 not MP2. As for DivX, if you mean compatibility with media player for AC3 tracks, there should be little problems (I've seen complaints that some people can't play back AC3 tracks, but I think it's mostly a case of missing acm/ds codec). Compatibility shouldn't be an issue with non-5.1 equipment since the player should downmix no problem if you are using a 5.1 track. In your conclusion, you said you are using AC3 2.0 tracks, so I don't quite understand the compatibility problem, since any player should be able to play that with the ac3 codec installed.

Multi-channel MP3s I believe is unlikely to come to light, but Multi-channel Ogg Vorbis is coming soon. Can't wait :)


EDIT: Ok, it makes sense now. I can agree with you on the compatibility issues :)

[ChristianHJW]

Originally posted by tangent -q0 and -q1 are experimental stuffs and to date they have not shown any improvements over -q2 yet, and both of them still have issues. -q2 is still your best and safest bet.

Sorry, my bad, i always mix the two. I dont mess around with the noise shaping and leave this at default ( q2 ? ), am talking about the VBR quality ( V 1 ..... V 3 = VBR quality ). They never give me bitrate above 170 kbps in an average using old routine on my movies ! Honestly, i cant remember getting a 192 kbps MP3 from any of my movie MP3 encodings. Strange ! For explanation : i set a very low minimum bitrate, my parameters are always -b56 or -b64 , plus a -B320 for maximum, so i let the codec decide what bitrates to use. A a result of that, in silent scenes etc. the lower bitrates are used a lot, which makes sense to me.


I mentioned that because VBR is only useful at bitrates of around 192kbps. VBR has issues below that and ABR provides better quality at those bitrates below 192kbps.

I use ABR for bitrates < 128 kbps as recommended by R3MIX forum members ( i remember Hans Heijden was amongst them ). They almost panicked when i told them i do VBR Lame at bitrate of 96 to 112 kbps ( being necessary for a 1 CD rip of 2hrs+ .... dont discuss with me if this makes sense .... a good 1 CD rip is sport ;) ) and convinced me its better to use ABR instead. So for now ABR ( old routine ) is the choice in most cases because if i do 2 CD rips i tend to use the 5.1 AC3 track for that . Only for very long movies ( 3 hrs ) or if high compatibility to a number of PCs is necessary i transcode the VBR MP3, V 1 ... 3 for 2 CD rips.

I would prefer to use the 2.0 DPL track instead of just encoding the L and R channels from the 5.1 track. Doing it the latter way may mean that your output track will be missing some important sounds (e.g. a gunshot from the rear). Anyway, DPL tracks are meant to be compatible with 2 channels.

Azid can make downmix to plain Stereo from a 5.1 track, the downmix matrix is very well described in the docs, so there is no information lost. Both recommended AC3 transcoding tools, BeSweet and HeadAc3he, are basing on azid.dll and will support the necessary switches.

@Taric .I apologize but I don't quite understand you. If you are making VCDs I understand you'll be transcoding but it will be to MP3 not MP2.

VCD norm dictates MPEG 1 layer II , this is MP2.

As for DivX, if you mean compatibility with media player for AC3 tracks, there should be little problems (I've seen complaints that some people can't play back AC3 tracks, but I think it's mostly a case of missing acm/ds codec).

Unfortunately this is not correct. The issues with AC3 sound are uncounted, PowerDivX player is one to solve most of the problems because it was designed for AC3 playback right from the start, others can solve the probs with BSplayer or Zoomplayer, but we cant talk about a good compatibility now. It all comes down to the fact that there are no GPL AC3 DirectShowFilters ( Dolby lab would mind ) available, so its in fact pain in the ass.

Compatibility shouldn't be an issue with non-5.1 equipment since the player should downmix no problem if you are using a 5.1 track. In your conclusion, you said you are using AC3 2.0 tracks, so I don't quite understand the compatibility problem, since any player should be able to play that with the ac3 codec installed.

If it works it will do even with downmixing. But in many cases there are huge probs like missing sync, etc.

Multi-channel MP3s I believe is unlikely to come to light, but Multi-channel Ogg Vorbis is coming soon. Can't wait :)

Read my latest news ! DarkAvenger modified vorbis.dll and will have vorbis support in HeadAC3he soon, DSPguru is looking into this also, and we have the possibility to mux DivX4 and/or XviD with Vorbis audio to Ogg format right now. Check here http://www.powerdivx.com/phpbb2/viewtopic.php?t=108 .

[tangent]

Sorry, my bad, i always mix the two. I dont mess around with the noise shaping and leave this at default ( q2 ? ), am talking about the VBR quality ( V 1 ..... V 3 = VBR quality ). They never give me bitrate above 170 kbps in an average using old routine on my movies ! Honestly, i cant remember getting a 192 kbps MP3 from any of my movie MP3 encodings. Strange ! For explanation : i set a very low minimum bitrate, my parameters are always -b56 or -b64 , plus a -B320 for maximum, so i let the codec decide what bitrates to use. A a result of that, in silent scenes etc. the lower bitrates are used a lot, which makes sense to me.

Doh, silly me, I assumed you were using a "sensible" VBR preset such as --r3mix, or --alt-preset standard/extreme. Sorry. Anyway, even with --r3mix, it is suspected that ABR may still give better or equal quality at the same bitrate. This is the subject for an upcoming listening test you may see details of on the HA/r3mix forums.

Azid can make downmix to plain Stereo from a 5.1 track, the downmix matrix is very well described in the docs, so there is no information lost. Both recommended AC3 transcoding tools, BeSweet and HeadAc3he, are basing on azid.dll and will support the necessary switches.

Sorry, my mistake again. It should only be as simple as L' = L + 0.707 * (SL + LFE). When you first described the process I was thinking it simply threw out everything else.

VCD norm dictates MPEG 1 layer II , this is MP2.

This one was a typo :)

Unfortunately this is not correct. The issues with AC3 sound are uncounted, PowerDivX player is one to solve most of the problems because it was designed for AC3 playback right from the start, others can solve the probs with BSplayer or Zoomplayer, but we cant talk about a good compatibility now. It all comes down to the fact that there are no GPL AC3 DirectShowFilters ( Dolby lab would mind ) available, so its in fact pain in the ass.

So AC3 really only plays back on these 3 players? Admittedly, I use BSplayer all the time and have not tried with WMP yet...

[Acaila]

WMP can playback AC3 (and I have yet to find any problems with that myself), but it wasn't build to truly support it.
It's like trying to hit a baseball with a stick instead of a bat. It works, but only crudely. :D

[Rhaegar Targaryen]

The main problem that people have with using AC3 in their rips is that it's not "legitimately" possible to play "for free" an AC3 stream, through a DirectShow filter. AFAIK all the DS filters for playing AC3 are commercial so you would have to "buy" a copy of WinDVD or PowerDVD. And I have no idea what the alternative-OS people would do.

[Taric25]

What is Ogg's compatibitily in Mac and Linux? Another reason I make VCDs is because since they are so compatible, I can play them in the Macs at school with very little effort. I even played a MC-MP2 file and it was downmixed correctly. :)

Also, I would like to bring up the issue of SACDs and Audio DVDs (if it will ever be possible to transcode them). I would like to play 48kHz multichannel MP3s that are made from SACDs or Audio DVDs in a platform of my choosing. I just think that would be too cool. :)

[tangent]

Ogg Vorbis is opensourced. Of course you can play it on just about any platform you want :)

[ChristianHJW]

.... but right now Tobias' DirectShowFilter is only compiled for Windows ( and based on DirectX SDK ), also the muxing filter is based on Graphedit. Anybody aware of a Vorbis DSF for Linux ?

ingoralfblum was doing major work for his MediaXW Vorbis DSF to get rid of the Windows libraries, so this one could be compiled to any OS .... but i dont know what the status is here, Linux fans may ask at http://mediaxw.sourceforge.net/ ...