Subler 'corrupting' AC3 5.1 in existing files when editing/adding metadata

[Tim Tschirner]

My normal workflow for converting files into MP4/M4V for playing them on an Apple TV usually is this:

‌

1. Convert file (usually from MKV) using MP4tools (http://www.emmgunn.com/mp4tools-home/)
2. Add and edit Metadata using Subler
3. Play file on Apple TV 4K

‌

This combo has worked fine for a long time, but I noticed that recently files that have an AC3 Dolby Digital 5.1 audio track will get corrupted after editing metadata with Subler - specifically, the Apple TV will detect the audio as AC3 2.0 and only play it back as such. This is definitely an issue with the current Subler (1.5.15) and I think this might have gotten introduced a few versions back.

When converting files using Subler directly the AC3 track will work fine.

‌

Steps to reproduce:

1. Convert file into M4V file using MPtools

   1. MP4 tools can be downloaded from http://www.emmgunn.com/mp4tools-home/
   2. As an example file to convert, I used “Dolby Cinema - Element” in the MK4 version from [https://thedigitaltheater.com/dolby-trailers/](https://thedigitaltheater.com/dolby-trailers/)
   3. For the conversion of the example file, I selected “Pass Thru” and “AAC (2-Ch.)” at 160kbs so that two audio tracks are being generated

2. Add the output file to iTunes and play back on Apple TV - observe that DD 5.1 is playing properly

   1. I’m using an Apple TV 4K on the latest tvOS (12.3)

3. Open the output file in Subler, add metadata, save it.

4. Add the newly added file to itTunes and play back on Apple TV - observe that DD 5.1 is playing back as DD 2.0

‌

This might be an interplay between Apple TV and whatever the current version of Subler does to the header/atom. When opened in MediaInfo (https://mediaarea.net/en/MediaInfo/Download/Mac_OS) or VLC, the edited file will show that is has an AC3 5.1 track and AAC 2 channel track.

[Damiano Galassi]

Can you provide a short sample created by MP4tools?

[Tim Tschirner]

I’ve attached two files:

‌

1. Sample file converted with MP4tools (1 Sample …)
2. The sample file created by MP4tools, then opened in Subler, Metadata added, then saved.

‌

Original file was an MKV file from https://thedigitaltheater.com/dolby-trailers

[Damiano Galassi]

There is no difference in the ac3 track extradata between the two files. The only difference is that

1. your tracks are missing the fallback metadata, so the AppleTV can’t switch automatically to the ac3 track.
2. Because there is no fallback metadata, and the audio track languages is the same, Subler set the “auxiliary” metadata on the second track.

[Tim Tschirner]

Thanks for looking into this. I am not sure if I understand your reply properly.

The symptoms:

1. Sample File #1 plays fine with AC3 5.1 on the Apple TV
2. Sample File #2 does not (it plays as AC3 2.0, not AAC 2.0).

How do I edit Sample File #2 to play proper AC3 5.1? Set the “Fallback” in Subler to Track 2 (Stereo)?

[Damiano Galassi]

You can disable the automatic generation of media characteristics tags in the advanced preferences.

[Damiano Galassi]

Set the fallback and remove the “auxiliary” media characteristic tag.

[Tim Tschirner]

Thanks - I will try setting the fallback and removing the auxiliary media characteristic tag tonight and report back.

‌

For the automatic generation of the media characteristics tag, can you confirm that the option is called “Infer Media Characteristics”? If so then I think I understand what happened: During one of the recent Subler updates this option got turned on by default.

I just updated Subler on a different Mac where the version was 1.5.8 and had the “Infer media characteristics” explicitly turned off. I just updated that version to 1.5.15 via the automatic update and the “Infer media characteristics” option got enabled without me having done so. Didn’t see this called out in the Release History anywhere but it would definitely explain the behavior I was observing.

[Damiano Galassi]

Yes, that’s the one. I think I enabled it as default some versions ago.

[Tim Tschirner]

Confirmed that setting fallback and removing the “Auxiliary” tag fixes the issue.

‌

Curious what the “Infer media characteristics” option is supposed to do? It seems to wrongfully create the “Auxiliary” tag when it’s enabled - that’s probably not intended?