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Accessing corrupt m4a audio file

Posted on 2012-12-29
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Last Modified: 2013-01-29
Hi - I was recording an interview with my iPhone 5 when after about 75 minutes into it someone called my phone. Although an m4a audio file was created (it's about 29MB in size) and I am able to copy it to my computer, my usual audio editor (AVS Audio Convertor) is telling me that the file is corrupt and it won't play in Windows Media Player when double clicked (although other m4a files do). Any ideas how I can get at the contents of this file? Cheers Ed
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Question by:TedMorey
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by:Insignificant Volunteer
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You are aware that the M4A file type is just a "container" or "wrapper" and that the audio content inside it could be of any number of encodings.  Not knowing this, we can't really be certain whether an interrupted recording is missing header data or perhaps significant "markers" that need to be read by software.  It is possible that the file is still in a "temp" stage, never having been properly saved.

The first application I would normally install to see if it plays the file would be VLC Player.  It supports a wide variety of formats natively using its own codecs, and might just play the file.  If so, you can then play it and use the streaming option to "record" it out to a new file, or else set the input source of any other audio recording software to your sound card and record the audio as VLC Player plays it back.

Just be careful after installing VLC Player not to associate it with any of your commonly used file types or else some files that formerly opened in eg. Windows Media Player will then open in VLC Player when double-clicked.  It normally shows a file association preferences dialog during setup, but if not you can do so post-setup:
Tools > Preferences > Interface > (scroll down) > Click "Set Up Associations" button.
Untick those you don't want associated > Click "Apply" > click "Save" back in main Preferences window.

Even if you have no file associations ticked, you can open the audio file from the Media menu > Open File option.

If you don't intend keeping the application, then you may also wish to de-select the following options in the Preferences:
Interface > Deselect "updates" and set album art to "manual download"
Audio > "submit last played tracks to last.fm

Let us know how you get on.
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by:TedMorey
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Hi BillDL - thanks for the comprehensive response, much appreciated. Unfortunately VLC player wouldn't play the play. It looked like it was going to for a split second but the start / end times went blank. I agree with your supposition that the file is missing header / marker info - is there a way to get at the raw audio data inside the file??? Cheers Ed
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by:Insignificant Volunteer
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Pity about that Ed, I had my fingers crossed.
I've successfully transposed good header data from a good JPG image file to a corrupt JPG image using a hex editor a few times, but it was time consuming trial and error stuff using some notes from an expert in the field, and I have a feeling that the audio file might be considerably more complicated given that the actual (probably) AAC audio content is effectively wrapped in an M4A blanket.

I have to confess that I'm not at all familiar with iPhones and how deeply you can delve into the filing structure using interface software, nor exactly how the recording process works, but here's a shot in the dark.  Could it be possible that, like many PC processes, an empty shell file is created as the destination one and it is then populated in stages from a "temp" file that remains when the process is terminated suddenly?  I'm just wondering if it might be possible to monitor the file creation process my starting a recording then pausing it and looking at the files created, then transplanting the current temp file with important temp file and renaming it accordingly.  Just thinking out loud here.  This would probably be something that only a Forensic Data Expert would know about though.

Do you think you could make a very short recording and finish it normally, then start another and have somebody phone you to interrupt it?  Maybe if the complete and incomplete M4A audio files are small enough you can upload them here (changing the extensions to *.txt) and we can have a look at them?
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by:akahan
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The only software that I know of which even CLAIMS to be able to fix corrupt m4a's is this:

http://www.realconvert.com/MediaFixer/index.htm

(The "what's new" page says that it can fix m4a format starting at version 7.9...)

I can't personally vouch for it, but depending how desperate your situation is, it might be worth a try.
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by:TedMorey
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Hello BillDL - sorry to have gone quiet, holidays got in the way! I have spent a few hours this evening playing with my (new!) Hex editor and taken a look at the insides of both the good and bad files to see what's going on, here's what I found...

1. The good file had a different header - so I copied and pasted that into the bad file. The bad file still would not play.

2. The good file had a different footer - so I copied and pasted that into the bad file. The bad file still would not play.

3. I then wondered if the MD5 / CRC32 checksums would make any difference so I found a utility to calculate those for me and pasted the new values into the bad file. Still the bad file would not play.

From your previous response I agree that I need the help of an expert in the M4A file format to help get at all the data - I can see it all it just won't play!! Got any ideas what to do next??

Cheers
Ed
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Assisted Solution

by:AeroQ_Tech
AeroQ_Tech earned 100 total points
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Hello Ed,

I think, as you already noticed, that you should get the help from a professional service in order to get back your damaged m4a file.

Maybe you could take a look at following post in order to get more details about how to proceed. Corrupt Audio Files: How to Repair

Kind Regards,
Javier

I may receive some form of compensation, financial or otherwise, from my recommendation or link.
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Insignificant Volunteer earned 400 total points
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I think you should have said "will receive" rather than "may receive", Javier.  You are the company's support technician and you should be asking for an increase in salary for being inventive enough in signing up here to help market the company's services ;-)

Ed

I'm sorry I didn't return here earlier.  For some reason I missed the email notification of your last comment.

As Javier's linked page describes, your *.M4A file could potentially contain audio in a few entirely different encodings.  There are a number of free tools around that provide very full details about how the audio and video streams are encoded in multimedia files but, when you consider the complexity of the content, I really don't think repairing them is something that you stand much chance of achieving unless you have a good and complete sample file and a lot of inside knowledge and experience of the different formats.

Attempting to transplant corrupt or missing header data from a good file in a hex editor is just a very small part of what could potentially be the problem, and you really have to know what bits and bytes belong/don't belong, and where each data section begins and ends.  I think you made a valiant attempt with the hex editor though.  Companies or individuals who have spent quite a number of years building up databases of commonly occurring data and from closely inspecting files are, I suppose, entitled to create a little bit of magic and mystery about their repair techniques even if the repair is a relatively simple one to them.

We could mess about for weeks and weeks doing trial and error copying and pasting in hex editors using dozens and dozens of backup files and possibly still have a completely unplayable M4A file, but on the other hand could just get incredibly lucky and accidentally paste the right data in the right place.  I would guess that the former would probably be the most likely.

I am not in the position to personally recommend Javier's company software / web service over any other company's simply because I have never had to use such a service, but the company sites are well documented and I have seen that the founder has posted useful advice (along with the company link of course) in a number of help and multimedia-related forums.

Just so you are fully informed:

http://aeroquartet.com/company

http://www.simplemoviex.com/company.html
http://mp4repair.org/pwt6/faq.html#guys

No harm in making some enquiries.

Bill
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by:TedMorey
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Hi again Bill

An update on how I've got on.

I contacted Aeroquartet and uploaded my file to their system for a free of charge analysis of the chances it could be recovered. It scored 50%. I then requested that they have a go at recovering it - which they offered on a 'no win-no fee' basis. The outcome is that they were able to recover and the cost was $79 (which is based upon length of the clip - just over 1.5 hours - and the device it came from, in my case of course an iPhone). I also swapped emails with Javier a few times and I found him to be helpful and friendly. So all-in-all I'm very happy with the whole experience!

Cheers
Ed
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Author Closing Comment

by:TedMorey
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Check out http://aeroquartet.com/ - it works!
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by:Insignificant Volunteer
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That's great news Ed.  50 UK pounds isn't bad for an important multimedia file, and the fact that you were offered a "no win no fee" was excellent.  Thanks for updating.
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