Gordian Knot and AC3 sound

[Zoltrix]

I have searched the guides and forums, but can't seem to find an answer to my question. I am using Gordian Knot, up until now everything I have encoded has had 2ch AC3 sound, which I assume is simple stereo/Dolby Pro Logic audio? Using the Gordian Knot guide on this site I have been able to encode the audio using 1x vbr-mp3 and it has worked fine.

Now the latest thing I am trying to encode has a 6ch AC3 sound track. Which I assume is 5.1 Dolby Digital? I would like to encode this in AC3 sound, but the guide on doom9 doesn't seem to cover this step, and I can't seem to find precise instructions anywhere. When I try and use robot4rip that comes with Gordian Knot to encode the audio, it now stops and tells me it cannot find an audio stream, previously on all DVD's with 2ch AC3 it didn't have a problem.

Now that the audio source is 6ch, do I need to do the audio step separately, and not use robot4rip? What options should I select in Gordian Knot to get 6ch AC3 sound? Any help would be greatly appreciated.

Cheers,
Zoltrix

[Zoltrix]

I just finished encoding my first attempt at something with AC3 sound. The final product has sound and seems to be working fine, but I have no idea if it's proper 5.1 AC3 sound.

Now because robot4rip wouldn't encode the audio, I used DVD2AVI to get the audio track from the VOB files. In DVD2AVI I selected: Audio > Dolby Digital > Demux All Tracks.

Then in Gordian Knot, under "Interleaving & AVI-Overhead" I selected the option "1x ac3/cbr mp3". Then under "Audio A" I pointed it to the ac3 file DVD2AVI created.

Can someone confirm if what I have done is correct? Also are there any tools or anything I can use to determine the exact audio format my video is using?

Cheers,
Zoltrix

[Nel]

Yeah what you did is correct as long as you selected Just Mux instead of one of the MP3 options on the audio 1/A tab.

What I usually do is in dvd decrypter, I go to the stream processing tab and select the video and 1 audio track I want and deselect the rest. Usually that's the 6ch AC3. Then proceed to select mux at the final step. I think you did it right. Only way I can tell is from outputting it through my realmagic x-card to the receiver and the dolby digital indicator comes on. Otherwise it just says PCM for MP3 sound. You shouldn't need to use robot4rip.

[Zoltrix]

Thanks for the feedback Nel. I was selecting "Just Mux" in the final steps before encoding, so hopefully everything is alright. What audio codec does Gordian Knot use for the 6ch AC3 sound? I've read people mentioning ogg, aac, etc, so it's all a little confusing.

The first thing I encoded the AC3 file was 68MB, now the next one I am starting to do, the AC3 file is 138MB, both are the same length in time, why has the second video got such a large sound file compared to the first? I am talking TV episodes here, so having such a large AC3 file takes away a lot of MB's that could be used for video. I think I've read something about downsampling the AC3 sound, should this be done if the AC3 files are so large?

Cheers,
Zoltrix

[Nel]

Not sure about the codec. I always thought it just added the file directly in there uncompressed. You can also mux .mp3 or .wav files and they are unchanged on the file size. My ac3 files are usually around 200-300 MB on movies so my guess is it remains just like that. If I keep the DD I always use 2 CDs so the bitrate stays decent.

Dont know why you have such a big difference in file sizes on your episodes though. I've done the entire Star Trek TNG and Lexx series and never really paid attention to the sizes. I did them all as MP3 anyway because the DD wasn't exactly that great.

I just checked the latest movie I did, Cast Away, which was 2:23 and the AC3 file was 461MB so I'm pretty sure muxing just keeps the same file unchanged. I did that movie on 3 CDs. Maybe one of the mods can confirm this.

[jggimi]

AC3 = Dolby Digital. The codec packaged with Gknot is called AC3Filter.

[Zoltrix]

Thanks for the info fellas. I found out the episode that had the 68MB AC3 file was only 2ch, even though robot4rip said it was 6ch, that would explain the size difference. With the next episode which had the 6ch AC3 sound and a file size of 138MB, I used AC3Machine to lower the bitrate from 384 to 224, which cut the file size down to 80MB. This should still retain the 6ch's right? Is it a good idea to lower the bitrate of a 6ch sound track? Or I am better sticking with an MP3 sound track?

[nate klefsas]

I think selecting the ac3 in g-knots encode audio tab, have to select ac3, then mp3 custom perameters, and -L -3db -s surround2 -c normal for your azid, gives you surround, sound can be heard from rear speakers, its just not dolby digital, its a small price to pay cause I could put the entire cast away movie and the sound all on one cd and it would still look great when I play it on a tv. 300 MB. is alot of space to be useing for sound, why would you want 3 cd's it would suck, the only thing you could do is keep the movie on you computer and play it from their unless you wanna switch the cd twice for one movie. but if you like DDS sound, which is really nice then as far as I know 3 cds is the best your gonna get. and maybe their is something out their you can use, if so let me know about it!post here

[Nel]

No need to switch cds. I usually mux all the cds together to form one file then watch that. Or for 2 disc movies, I just put one in each drive and queue up both files then there's a slight pause but don't need to switch anything.

[nate klefsas]

its probly worth the sort pause cause the video quality is much better with 2 cds. I just think its more convienent to keep an entire movie on one cd, and it saves a little room on my hard drive. I should try 2 cds some time. any word on making a surround 5.1 audio file that is smaller than ac3? less than 200 to 300 mb's

[nate klefsas]

I DECIDED TO DO MY FIRST TWO CD MOVIE AND AM USEING AC3 AUDIO, I SELECTED A BITRATE OF 128 UNDER AUDIO A, THEN UNDER ENCODE I SELECTED THE AC3 FILE THEN MUX ALL. ALSO WHEN I RIPED THE FILE WITH DVD DECRYPTER I SELECTED THE 6 CHANNEL AC3 FILE AND UNSELECTED ALL OTHER AUDIO FILES, ALSO CHECKED 16:9 NTSC THING TOO, COULD BE 4:3 TOO IN THE SECOND TAB CALLED STREAM PROCESSING, HAVE TO CHECK A LITTLE BOX TO BE ABLE TO SELECT WHAT AUDIO FILES YOU WANT. IT SEEMS LIKE ITS THE EASYIEST WAY TO DO IT, FORGET ROBOT RIP, I THINK IT IS A WASTE OF TIME CONSIDERING IT USES DVD DECRYPT ANYWAYS FOR IT RIPPER

[jggimi]

Nate: If you're selecting 128kbps rather than the .ac3 file in the Bitrate tab, you are making a procedural error. The Audio A box should be used in one of two ways. a) Either as a bitrate estimator prior to transcoding to mp3, or b) as an actual file size value. When "just mux" is used, you should click the "size" option in Audio A, then select the file you plan to mux. By selecting 128kbps, you will throw off size calculations dramatically, as most 5.1ch AC3 is closer to 448kbps than 128kbps. This will impact your starting b/p*f value and any compressibility checks percentages, as they are based on audio size estimations. In addition, be sure to select the proper AVI overhead value.

Your last 3 posts have been all caps. Please see #9 (http://forum.doom9.org/forum-rules.htm). Thank you.

[nate klefsas]

I just made my first encode putting it on two cd's instead of one for the first time. the quality is astonishing, I am so happy with the results. I got near dvd resolution, 99% compression, and to top it off, surround sound useing ac3 audio instead of mp3, which now really sucks in my opinion unless you are pressed for file size. I just wanna recomend putting files on two cd's instead of just one for all of those of you out there who only use one cd, it's amazing how much better the video quality and sound can be useing 2 cd's. I doubt I will ever go back to one cd ever again now, of course unless I wanna put something on one cd. I only wish I would have tryed it sooner.

:confused:for ac3 audio I selected bitrate 128 in audio a tab, then ac3 file under encode tab, and mux all, also 2x ac3/cbr mp3 in interleaving & avi-overhead under bit rate tab. is that right or should i use vbr mp3+ac3 in interleaving & avi-overhead to get 6 channel, dolby 5.1 audio? can anyone help me

[jggimi]

Nate,

I've merged your new thread into this previous discussion, as it's apropos to this and I didn't want it spread across different threads....is that right...can anyone help meAs I stated above, not quite. Here's the procedure:[list=1] In the Audio A tab, press the Select button.
Select the .ac3 file you're going to mux.
When you return to the Bitrate tab, you'll see "Size" selected and the size of your .ac3 file in KB and MB.
Select "1x ac3/cbr mp3" radial in the interleaving and overhead box.[/list=1]

[nate klefsas]

thanks for the help, it was simple, to the point, and extreamly clear. I appreciate it very much and so will my ears