Mono AAC Export Does Not Work

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Level 1

20 points

Mono AAC Export Does Not Work

I am currently using Soundtrack Pro 2.0.1

I am preparing audio assets in AAC format from mono WAV source files and would like the resulting AAC files to be mono as well, for file size reasons. It is imperative that I keep the file sizes down.

I found a bug in Soundtrack Pro that prevents me from outputting mono AAC files. No matter what I do, the resulting output file is always in stereo. I have experimented using the MP3 export option instead of AAC, selecting the "mono" setting from the "Channels" export dialogue, and it works fine.

MP3's will export in mono. AAC's will not.

Here is a visual example of what is happening:

In this first picture, I have created a new project and placed two mono audio files onto adjacent tracks. Their pan pots are set hard left and hard right. The audio files are recordings of a voice over actor reading from two different sections of a book. By panning the two tracks left and right it makes it very easy to hear that, currently, this mix is definitely in stereo. Two different voices are coming from each speaker:

Next, I go to File > Export and choose "AAC/Podcast File" from the "File Type" dialogue. From here, I click on the "Channel" dialogue which is currently set to "Stereo":

Next, I choose "Mono" from the "Channels" dialogue:

I then export the file and open it in Quicktime. If everything worked, I should no longer hear two different voices coming from each speaker. Instead, I should hear both voices coming equally from both speakers as the mix was summed to mono. Also, the properties for the newly exported file should clearly show that the AAC audio contained within it is single channel: Mono, not stereo:

As you can see, the file is stereo. And as I can hear, it is most definitely still stereo, with each voice panned hard left and right.

Is the AAC export function simply broken in this version of Soundtrack Pro?

Michael Ayotte
http://www.michaelayotte.com

Mac OS X (10.5.8)

Posted on Feb 6, 2011 4:16 PM

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Answer 1

NLEdit

Level 4

1,645 points

Feb 6, 2011 4:21 PM in response to Mike Ayotte

your images didn't work. You might want to look at the preview tab before posting, just to check.

Try using "center" instead of mono in your tracks. Mono really just means one channel. I'll have to look at the rest when you get the images up. Let me know if you need help with that part.

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Answer 2

Mike Ayotte Author

Level 1

20 points

Feb 6, 2011 7:04 PM in response to NLEdit

I can't seem to figure out how to use images on these forums, so I can't give you the visual walkthrough I wanted.

Regardless, choosing to export your project as AAC/Podcast and selecting the "Channels: Mono" option from the export settings window results in a stereo file without summing the left and right channels.

I need legit mono files, not simply stereo files with everything panned centre so that it technically "sounds" like mono. I need a single channel of audio, not two identical channels of audio, as would happen with a stereo mix where everything is panned up the middle.

Exporting a file with two identical channels of audio is redundant and only wastes disk space, a priority for me.

I need true single channel AAC files with Podcast markers. And right now, I do not have a version of Garageband that supports the Podcast markers feature, so I can't fall back on using that.

Message was edited by: Mike Ayotte

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Answer 3

NLEdit

Level 4

1,645 points

Feb 6, 2011 6:56 PM in response to Mike Ayotte

It's not stereo, it's two track mono.
It's doing what you're asking it to do. Right-click on the panners and select "center" on both tracks.

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Answer 4

Mike Ayotte Author

Level 1

20 points

Feb 6, 2011 7:21 PM in response to NLEdit

Sorry to disagree but it is "Stereo."

Yes, a stereo interleaved file is in fact two mono files playing simultaneously: One file plays from the left speaker, one file plays from the right speaker.

If I were to create a mix with everything panned up the middle, it is technically a "mono mix" as there is no stereo separation between the tracks. All tracks are equally coming out of the left speaker as the right speaker. No separation equals a "mono mix."

But...

A "Mono Mix" does not necessarily mean you have a "Mono Audio File." There is a difference.

A stereo audio file contains two channels of audio, two files essentially. Whether they contain the same information or not does not matter. A stereo file will always contain two channels of information.

Likewise, you could create a 5.1 multichannel audio file with all 6 channels playing the exact same thing. There would be zero separation among the speakers. The same signal would be coming back equally from all speakers around your room. This is technically a "mono mix" as there is zero separation among the speakers but the audio file itself is most definitely still a multichannel surround audio file as it contains 6 unique streams of data. It just happens to be carrying 6 copies of the same data.

You see, a multichannel audio file carrying six channels of the exact same recording will sound identical to a stereo audio file carrying two channels of the same recording, which will also sound identical to a mono audio file carrying only one channel of that same recording. They are all "Mono Mixes" but their files sizes will differ greatly.

Is it really necessary to send someone a 6 channel surround audio file in which all 6 streams of data are identical, or wouldn't it make more sense to only send someone a single stream of audio data if that is all that is required? A "Mono Audio File" as opposed to a "Multichannel" of "Stereo" audio file.

I need a "Mono Audio File," a file which contains only one channel of audio.

Soundtrack Pro is not summing the output to mono in way when using AAC to export the project.

Message was edited by: Mike Ayotte

Message was edited by: Mike Ayotte

Message was edited by: Mike Ayotte

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Answer 5

NLEdit

Level 4

1,645 points

Feb 6, 2011 7:14 PM in response to Mike Ayotte

You're really just digging yourself into a pointless hole. I can send you a 6 channel mono file. It's not stereo, it's mono, but it has six channels, all centered in it. You just want a smaller file. Just make it smaller and mono they way described.

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Answer 6

Feb 7, 2011 8:11 AM in response to Mike Ayotte

It's a bummer that STP isn't working, if you want mono then dual mono isn't going to cut it.

I would try doing a stereo bounce (dual mono) to wav or whatever format, open it again in STP, convert to mono and save, then try saving that file as mono AAC. If that doesn't work, I'd save the mono wav then do the conversion to AAC in iTunes or another app. Unfortunately STP is pretty buggy and you may have run into one of the many bugs.

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Answer 7

NLEdit

Level 4

1,645 points

Feb 7, 2011 9:35 AM in response to Mike Connelly

works fine here in 3. Stereo to mono out. No problem.

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Answer 8

Feb 7, 2011 10:09 AM in response to Mike Ayotte

Also seems OK here on 3.0.1, may have been a bug in 2 that was fixed in 3. Upgrading may fix it, or you can create a mono wave and convert that.

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Answer 9

Feb 7, 2011 4:08 PM in response to Mike Ayotte

I just tested this in 2.02, and I got a mono file every time. However, weird things happened with the export when I hard-panned the tracks. That is, I got one track but not the other. Worked fine when I panned center. Did you also try panning center?

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Answer 10

NLEdit

Level 4

1,645 points

Feb 7, 2011 4:45 PM in response to barreltone

I did it panned hard without a problem. But you'd want it centered. I also made the submix a single channel and that worked too. This was on 3
Two track mono is called, well, two track mono.

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