howto convert ac3 to ogg?

[Sujao]

Well the topic says it already. I am looking for a guide or a HOWTO on how to convert ac3 into ogg. I need to retain the 5.1 sound. Afaik OGG is capable of 5.1, is it? I searched the forums but didnt find a single thread about it.

I looked at doom9.org and vorbis.com and didnt find a signe article, what I find strange. I hope what I am looking for is possible at all.

[GordSki]

Found this: http://www.divx-deux.com/oggyaac/Manual%20OGG%20y%20AAC.htm

It's not in english and the progs are windows based, but it's a start......


G.

[iarwain]

HeadAC3He is a very good tool to do that. Yes, I know it's for windows. But read this, taken from the official web page:

[Kraymer]

transcode should be able to do it:

[Sujao]

yesterday I spent about 5 hours playing around with mlpayer, oggenc and transcode and got stuck at the step where I encode either a 6-channel-pcm or 6-channel-raw-stream-ac3 to ogg.

1. alternative

• mplayer dumps the audiostream to a raw ac3 stream
  

  Code:

  mplayer -aid 128 -dumpaudio -dumpfile dumpfile /home/user/tmp/city/DVDVOLUME/VIDEO_TS/VTS_01_5.VOB

  
  
  
• i can play it just fine with:
  

  Code:

  mplayer -rawaudio on:format=0x2000 dumpfile

  
  
  
• then i use transcode to encode it to ogg. this way it works, but i only retain 2 channels
  

  Code:

  transcode -i dumpfile -x null,ac3 -y null,ogg="-q 4" -N 0xfffe -m transcoderesult.ogg

  
  
  
• i added "-C 6" to the ogg options hoping to retain all 6 channels....
  

  Code:

  transcode -i dumpfile -x null,ac3 -y null,ogg="-q 4 -C 6" -N 0xfffe -m transcoderesult.ogg

  
  

but here I get the problem I was unable to solve. The resulting ogg is played to fast, about 30% too fast.

2. alternative
Before this I tried another way to get my audio stream and dumpled it directly to PCM with mplayer with this command:

[Kraymer]

[Cintra]

ref http://www.un4seen.com/ it would seem multi-channel ogg is possible. Haven't played the sample yet myself tho'.

mvh

Edit: just played it but the outputs do not correspond to my speaker layout, and back left is missing, whereas DTS samples from http://www.sr.se/multikanal/english/e_index.stm do play from the correct speakers..
_________________
"I am not bound to please thee with my answers" W.S.

[GordSki]

The reason it sounds faster is because it is treating a 2 channel input as a 6 channel input giving you a thrid of the samples you should have. So the oggenc part of the scheme is doing what it should be, it the input that is wrong.

Do you have an mplayer output for playing the PCM file you created? I suspect that mplayer has down mixed it to 2 channels before dumping the PCM.....


G.

[GordSki]

After a quick bit of googling I can't seem to find any ac3 to 6-channel OGG or PCM linux progs......

Possibly your best bet is to try the CLI version of BeSweet under wine (http://dspguru.notrace.dk/).

Either that or get down and dirty with the mplayer code to remove the down mix

Good luck!

G.

[GordSki]

I think I've got it!

I've found this:

[GordSki]

Had a bit of a play my self and I got it working!

Here are the commands I used:

[Kraymer]

[GordSki]

[Kraymer]

[Sujao]

@Gordski: Yeah, I tried your solution too, but I thought it didn't work cause when I play the resulting file I hear an echo and the speech is very quiet. So either the encoding went wrong or the mplayer is not capable of donwmixing the ogg-stream to 2 channels. Do you have a 5.1 audiosystem?

And how did you get the raw ac3 file?

I used:

[GordSki]

I got the ac3 file from a website: http://www.kellyindustries.com/sounds.html. I do have a 5.1 amplifier but I can't anything other than AC3 out of my PC to it. I've played the resulting file through mplayer and it sounded like it down mixed it alright.....

Though with music it can be hard to spot the errors.....

What sort of content (movie, music, etc) is it your trying to encode?

G.

EDIT: I've just had a look at the mplayer man page and I spotted this:

[Sujao]

It's a movie. I suppose the ogg decoder of mplayer is not able to downmix the 5.1-ogg stream, cause when I play ac3 it downmixes the signals just fine. I have only two speakers. Do you have an idea where to get another ogg-decoder and make mplayer use it. Or first how to verify that its due to the decoder.

[Sujao]

I think I have tracked the problem now. As the decoder is responsible for the downmixing of the channels (6->2) it must be the decoder for raw pc and ogg used by mplayer. When I play the raw ac3 stream and chose to play only 2 channels everything is normal. Mplayer tells lib52 to decode the video and downmix to 2 channels. Everything is normal. When I do the same with raw pcm the decoder doesnt do that and I hear 2 of 6 channels. The data of the other 4 channels is lost. Same with OGG. The differnce between downmixed 2 channels and cut 2 channels is hearable. The cut 2 channels contain much more envoiromental noise. I suppose I hear the sound of the front speackers.

Here are all the commands I use:

Dump raw AC3 from VOBs:

[GordSki]

I managed to find a vob file with an ac3 test tone (ie a hiss and a voice saying which speaker the his is coming from) and ran that through the ogg encode process.

When I played the ogg file with no options, mplayer cut all but the left and right channels. With the '-channels 6' I still only got the left and right, however I think mplayer was giving all the channels to alsa but alsa was dropping the ones it couldn't play. I guess if you have a 5.1 capable (and configured) sound card you would get all the channels.

I think what needs to be done now is to figure out if there is a way to get a downmix filter into the audio chain.

More fun

G.

PS. I got the test file here: http://www.digital-digest.com/dvd/downloads/trailers.html

[PrakashP]

Vorbis 5.1 is of no practical use currently. In contrast to AC3 it offers no channel coupling (not even the 2 channel coupling used in stereo mode!) so good space savings while preserving quality is *not* possible. Monty wanted to do some 5.1 work into Vorbis, but till now nothing happened.

[GordSki]

Yet again 'man mplayer' might have the answer:

[Sujao]

[PrakashP]

coupling is a common technique used to reduce data. Basically some frequence bands of channels which contain more or less the same data, will be encoded as one band instead of individual bands. That's why ac3 isn't really discrete, but on the other hand offers great quality/bitrate.

[Sujao]

Ah I heard of that but didnt know thats the name for it.

[Kraymer]

From what you describe.. is it the same as 'Joint stereo'?