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AC3 with 6 full frequency audio tracks

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Alex Fahl
AC3 with 6 full frequency audio tracks
on Apr 15, 2010 at 10:23:20 am

Hello,

I have a rather special question:
I am trying to find out if there is a way to create a dolby digital AC3 file with 6 full fledged audio tracks instead of 5 plus 1 LFE track?
I have an AV Setup here that would theoretically be able to play back such an AC3, but it seems that no encoder has the option to create a "6 equal channels AC3".
I know that AC3s probably were never meant to be for 6 equal channels, but I wonder if anybody knows of a way to sneak in another full frequency track instead of the LFE?

Or maybe I am just missing out on something completely?

Thanks a lot in advance


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Jean-Christophe Boulay
Re: AC3 with 6 full frequency audio tracks
on Apr 15, 2010 at 2:34:57 pm

What channel format were those tracks mixed to? If you have 6 full-freq tracks, I imagine that would be 6.0. You have a center surround track on top of the usual L,C,R,Ls,Rs? That isn't really standard at all. The best move then would be to create an audio track, with the same length, bit depth and sample rate as your 6 others, that contains the sound of silence (not the song!). Import that as your LFE channel to encode, which will give you a Dolby Digital EX-compatible 6.1 output. The LFE will be dead, but you'll sidestep a few potential headaches at playback. Compressor does this perfectly.

Be aware that the LFE channel is there for a reason; surround speakers are usually not full-range boxes and will produce little or no LF. The LF will come only from the LCR trio and, for the bass image to be grounded in space, the mix should reflect that, with more bass sent to the center than the LR pair.

So it's pretty easy to output the AC3, but your mileage may vary depending on the mix. Even simpler would be to feed the discrete tracks directly, if that's possible on your installation.

IHTH


JC Boulay
Technical Director
Audio Z
Montreal, Canada
http://www.audioz.com


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Alex Fahl
Re: AC3 with 6 full frequency audio tracks
on Apr 15, 2010 at 2:46:39 pm

Thanks for your feedback Jean-Christophe. I am working on a video art installation and have 6 full-freq tracks, and an AV-Player device which can only output stereo or 5.1 AC3. So the trick with the 6.1 output unfortunately won't work.
I have a video track and 6 synchronized Audio tracks. Life would be so easy if there was an easy way to create an 5.1 AC3 with simply 6 full-freq audio tracks, but it seems really hard...


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Ty Ford
Re: AC3 with 6 full frequency audio tracks
on Apr 15, 2010 at 2:55:25 pm

Hello Alex and welcome to the Cow Audio Forum.

BTW, JC, brilliant solution. Love the way you came up with that!

Alex, maybe go 7.1, (don't use the .1) and get a player that'll play it.

http://www.dolby.com/consumer/setup/speaker-setup-guide/index.html

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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Alex Fahl
Re: AC3 with 6 full frequency audio tracks
on Apr 15, 2010 at 3:00:30 pm

Thanks for your tip Ty but I am kind of dependent on the Player I am using. And before I change the hardware system I would prefer to find out if there is any way to stuff 6 full-freq audio tracks in one 5.1 AC3. So far nobody told me that it's absolutely impossible...


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Ty Ford
Re: AC3 with 6 full frequency audio tracks
on Apr 15, 2010 at 3:26:09 pm

Oh, OK. It's absolutely impossible.

Regards,

Ty Ford


Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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Rob Neidig
Re: AC3 with 6 full frequency audio tracks
on Apr 15, 2010 at 3:27:39 pm

It doesn't seem like there should be any problem in creating 6 full frequency tracks. You can put anything you want into the .1 track. The problem will be how your system plays it. If it's sent to a sub, that speaker will have difficulty playing any higher frequencies. And many systems automatically have a crossover that sends all info below say 125Hz to the sub. So I would says the issue is not in the production, but in the playback.

Have fun!

Rob


Rob Neidig
R&R Media Productions
Eugene, Oregon


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Alex Fahl
Re: AC3 with 6 full frequency audio tracks
on Apr 20, 2010 at 1:46:43 pm

Hi Rob,

a late thank you for your reply. Based on your tip I tried to create a 5.1 AC3 in Apple Compressor and unfortunately it actually cuts the frequencies of the .1 channel so that I end up with a low freq LFE channel. I have the possibility to uncheck "enable low freq effects" which sounded promising, but then it just gets rid of my file I threw into the .1 Channel. So it really seems to be an unavoidable thing that if creating ac3 files the sixth channel's high frequencies get cut...?
If I understand it right though, the AC3 only "does" this in order to save space. As you said the job could also be done easily by the receiver (cutting the frequencies). So I still wonder if this is just the way my software (apple compressor) deals with the sixth file or if AC3s generally can't take 6 full freq audio files?

Best
Alex


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Rob Neidig
Re: AC3 with 6 full frequency audio tracks
on Apr 20, 2010 at 3:16:02 pm

Hi Alex,

You know, I could swear when I was at Sony we authored a project for a Japanese museum where they created 6 discreet channels of full spectrum frequency info and routed them to the appropriate speakers for their listening environment. This was not just a standard "surround" system, but speakers placed in specific places so that the sounds of birds came out of specific speakers, to simulate placement of birds around the room. But I just took a look at dolby's web site, and they definitely state that the ".1" channel is frequency limited (to below 120Hz). They state, in a paper on production guidelines for mixing for 5.1:

"5.1-channel audio typically consists of five discrete, full range main channels (Left, Center, Right, Left Surround, and Right Surround) plus an optional band-limited Low Frequency Effects (LFE) channel for added bass (the .1). Dolby Digital bitstreams deliver full frequency bandwidth main channels, from 3 Hz to 20 kHz, and a limited frequency bandwidth LFE channel, from 3 Hz to 120 Hz."

and also...

"In the Dolby Digital encoding process, the encoder will brickwall filter the LFE signal at 120 Hz."

That info and a bunch more is on dolby's web site at:

http://www.dolby.com/DocLibTechLanding.aspx?taxid=186

So I guess I must have confused myself about full range in the .1 channel. That museum must have had low frequency only info in the LFE channel and kept the birds in the the other 5. What I said about playback systems is true, though. Even if you haven't created a separate LFE channel, some systems will automatically route everything below a set threshold to the sub. That doesn't help you here, though.

So sorry about the misinformation. Keep plugging away and having fun!

Rob


Rob Neidig
R&R Media Productions
Eugene, Oregon


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Alex Fahl
Re: AC3 with 6 full frequency audio tracks
on Apr 20, 2010 at 3:23:14 pm

Thanks a lot Rob for your generous response. My usage for the AC3 is not too far away from what you did for this museum. I wrote to DOLBY now and see if they get back to me about this.

Cheers
Alex


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cowcowcowcowcow
John Phillips
Re: AC3 with 6 full frequency audio tracks
on Apr 22, 2010 at 11:48:55 am

Hi

Thought I'd jump in with some limited experience... I make multi-chl audio DVDs for my sound art installations in galleries, etc. Since most of my pieces are under 30 minutes, I don't use AC3, but make my 6 tracks full-range PCM files. My DVD player(s) have 6 RCA outputs on the back, these I attach to 6 powered speakers and it works fine. I have avoided home A/V receivers as amps because of the LFE routing.

hth, John

John J.H. Phillips: intermedia arts: http://terragizmo.net


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Alex Fahl
Re: AC3 with 6 full frequency audio tracks
on Apr 23, 2010 at 9:27:08 am

thanks John, unfortunately I am kind of obliged to use this particualar AV Player (Adtec Signedje) which can only output stereo or 5.1 sound. So DVD Players are nor really an option for me...


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