How to recover audio from an incomplete or corrupted AAC/m4a file

Question

I recorded an 85-minute AAC/m4a file using Notability on my iPad. This app has always worked well before but for some reason there was a problem this time. The relevant file shows up as having 21MB (about the right size) and appeared to be recording normally, but when I tried to play it (using Notability or any other player - tried Quicktime, Real Player, Audacity, VLC and WondersharePlayer) it shows up as having zero duration. I am using a Mac with Mavericks installed.

In desperation I ran the file through: http://mp4repair.org/pwt6/preview.html It came up straight away as a 1 hour 25 minute sound file with a 95% "repair score". There is also a preview that sounds right. The only problem is that they want about $90 for what looks like a pretty simple fix. Can anyone suggest a way that I might be able to get access to this file without paying lots of money? It seems like it should not be difficult but I just don't seem to be able to find the key to the puzzle.

Many thanks in advance for any thoughts/suggestions you might have,

James

Answer 1

This guide worked 100% for me - it takes you through each step. Afterwards, the m4a file plays! Note: this is completely free. http://sysfrontier.com/en/2014/12/31/hello-world/

Also pasting instructions in case the link goes down in future:

How to fix corrupted voice memo (m4a) files.

Do you have broken voice memo files? You can fix those files by yourself. I will show you the steps in this article. Voice memo files have the extension “m4a”. It is audio data encoded in the AAC format and encapsulated in MPEG4 file container. You may fix files recorded with other programs like QuickVoice or on Android. File extensions may be mp4 or 3gp.

The m4a files get broken when some parts of header lost correct information. For example, when the bytes indicating the length of the audio data become 0, the file can not be replayed any more on iPhone, iPad, Mac or Windows PC.

But, in some cases you can fix the broken header by yourself by extracting the audio data, encoding it, and putting it into the new file container.

Let’s do it together.

STEPS for Windows PC

1. Download faad.exe

Open http://rarewares.org/aac-decoders.php Click “Download (225kB)” of “FAAD2v20100614 CVS snapshot for Win32“. faad

Extract faad.exe from the downloaded zip file. Place it in the “Downloads” folder, for example.

2. Download faac.exe

Open http://rarewares.org/aac-encoders.php Click “Download (191kB)” for “FAACv1.28 Binary for Win32“.faac

Extract faac.exe from the downloaded zip file. Place it in the same folder as above.

For example, you see faad.exe and faac.exe in the Downloads folder as below. downloads

3. Download binary editor

You can choose any favorite binary editor. If you don’t know well, I will recommend HxD. Download a zip file from http://download.cnet.com/HxD-Hex-Editor/3000-2352_4-10891068.html or http://mh-nexus.de/en/downloads.php?product=HxD Extract setup.exe and click it to install.

4. Place the file to the same directory.

Move your voice memo file from your iPhone to PC via iTunes. You can use other tools like iFunbox.

Please place the copied file to the same directory as where you placed the above faad.exe and faac.exe. We call the file “20140615 163625.m4a”, for example.

5. Open the file with binary editor.

Please copy the broken m4a file to some folder for backup. Start HxD or your binary editor program and open the broken m4a file.

Press F key with control key to open the Find box. Type “mdat” and press the Find button.

Select from the beginning of the file to the “t” of “mdat” as shown below.

find_mdat

Press Delete key to delete the selected area. Press S key with control key to save.

6. Decode by faad.exe

Press R key with Windows key to open the dialog. Type “cmd” and press OK button to open the Command Prompt.

In the command prompt window that was opened, type “cd Downloads” to move the Downloads folder. Type faad.exe “20140615 163625.m4a” and press the Enter key. In seconds or tens of seconds, you will find the decoded wave file named “20140615 163625.wav“.

Check if the command histories are like below. cmd1

7. Encode by faac.exe

In the command prompt window, type faac.exe -b 160 -o repaired.m4a “20140615 163625.wav” and press the Enter key.

In seconds or minutes, you will find the encoded file named repaired.m4a. Please replay the repaired.m4a to see the audio content is what you want.

Check if the command histories are like below. cmd2

Company Information

SysFrontier Inc. 3-23-16, Ekoda, Aoba Ward, Yokohama, 225-0005, Japan.

Answer 2

You could try rewriting the file using ffmpeg with the following command:

ffmpeg -i damagedfile.mp4 -c copy fixedfile.aac

Alternatively, you may use FAAD and a hex editor (e.g. Hexplorer). Find out where the actual data starts in a hex editor. Not always after the mdat entry - there may be a lot of zeros in the beginning of the file.

Simply copy everything afterwards to a new binary file, e.g. raw.aac. For the AAC format, every sample seems to begin with 0x21, which is a hint that you found the right position.

You then use FAAD to convert the corrected file using this command:

faad.exe -a out.aac raw.aac

Answer 3

Reaper is a quite successful DAW in opening corrupt and unsupported audio files and may help with this problem. It is not free but has a 60-day unlimited and never expiring trial which will be more than enough for your case.