MP4 will not play after I edit metadata (internal checksum?)

[vodpod]

I used a python script to edit the metadata of a mp4 file binary data:

data = data.replace(b'someword',b'newword')

...the video would NOT play, but if do just (replace a word with the same word):

data.replace(b'someword',b'someword')

...the video DID play, which leads me to believe there is some king of checksum thing going on. I don't know anything about video encoding so I hope somebody could explain what's going on and maybe how to fix it. Please note, I have to use python so plase don't recommend any installed software.

Thanks.

[poisondeathray]

what words did you replace? maybe you inadvertently replaced critical data ?

[vodpod]

what words did you replace? maybe you inadvertently replaced critical data ?

I know it is not critical data, it's in the cmt (comment right?) and it's in a sentence about the video.

also, I'm testing with replacing just one word, such as: "wonderful"

One thing I should note is the this comment does not show up in the metadata section when I check the video using several methods: including right click->properties (win7) ,MP4Explorer, and MP4Info.exe

[poisondeathray]

One thing I should note is the this comment does not show up in the metadata section when I check the video using several methods: including right click->properties (win7) ,MP4Explorer, and MP4Info.exe

Does the metadata comment show up with mediainfo (view=>text) ?

It should show up somewhere with a hex editor

I know it is not critical data, it's in the cmt (comment right?) and it's in a sentence about the video.

I don't know enough about mp4 structure, but if the same text string was replaced in other sections that could cause it not to play


If you provided information on what "someword" was, that might be a better clue

[vodpod]

@poisondeathray it does show up in MediaInfo and I wanted to replace the word "language" or all of the comment. Why would this cause the whole video to be corrupted?:

General
Complete name                            : E:\Videos\sep48106.mp4
Format                                   : MPEG-4
Format profile                           : Base Media / Version 2
Codec ID                                 : mp42
File size                                : 73.7 MiB
Duration                                 : 4mn 38s
Overall bit rate mode                    : Variable
Overall bit rate                         : 2 223 Kbps
Encoded date                             : UTC 2011-12-14 19:34:49
Tagged date                              : UTC 2011-12-14 19:34:49
Writing library                          : Apple QuickTime
Copyright                                : ©2011 All Rights Reserved
Comment                                  : on language and thought

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : Main@L3.1
Format settings, CABAC                   : No
Format settings, ReFrames                : 6 frames
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 4mn 38s
Source duration                          : 4mn 38s
Bit rate mode                            : Variable
Bit rate                                 : 1 998 Kbps
Maximum bit rate                         : 2 180 Kbps
Width                                    : 1 280 pixels
Height                                   : 720 pixels
Display aspect ratio                     : 16:9
Original display aspect ratio            : 16:9
Frame rate mode                          : Constant
Frame rate                               : 23.976 fps
Standard                                 : Component
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.090
Stream size                              : 66.3 MiB (90%)
Source stream size                       : 69.3 MiB
Language                                 : English
Encoded date                             : UTC 2011-12-14 19:34:49
Tagged date                              : UTC 2011-12-14 19:34:49

Audio
ID                                       : 2
Format                                   : AAC
Format/Info                              : Advanced Audio Codec
Format profile                           : LC
Codec ID                                 : 40
Duration                                 : 4mn 38s
Bit rate mode                            : Variable
Bit rate                                 : 128 Kbps
Maximum bit rate                         : 135 Kbps
Channel(s)                               : 2 channels
Channel positions                        : Front: L R
Sampling rate                            : 44.1 KHz
Compression mode                         : Lossy
Stream size                              : 4.24 MiB (6%)
Language                                 : English
Encoded date                             : UTC 2011-12-14 19:34:49
Tagged date                              : UTC 2011-12-14 19:34:49

[poisondeathray]

You can rule out if that is the cause (or not) by searching the string you used earlier with a hex editor (a search or find & replace function)

e.g. if you replaced "apple" and it actually shows up in 2 places... chances are that's what caused the problem

But If your script only replaced 1 entry, it shouldn't cause video to be corrupted. To prove this you can manually edit the video with a hex editor, and I bet it will still play

[vodpod]

@poisondeathray, I just tried that and clearly something is preventing any changes to the metadata (probably somekind of copyright protection feature?)
I used a hex editor to change the word "language" to "lang" saved it, tried to play the video, would not play. I changed the "lang" back to "language", saved, and it played. Also tried changing "Apple", won't play

[poisondeathray]

Could these ones be DRM protected?

Did you use the same #spaces/characters ?

I just tried, and it worked ok on some random mp4 from Apple movie trailer

I replaced "Warner" with "PDR "

Format                           : MPEG-4
Format profile                   : Base Media / Version 2
Codec ID                         : mp42
File size                        : 3.87 MiB
Duration                         : 9s 468ms
Overall bit rate                 : 3 426 Kbps
Encoded date                     : UTC 2012-01-28 01:59:24
Tagged date                      : UTC 2012-01-28 01:59:24
Copyright                        : © 2008 PDR    Bros. Pictures. All Rights Reserved

[poisondeathray]

Apple used to have an old style DRM that you could get rid of be re-muxing it into a new container e.g. mp4box or mkvmerge . It's worth a shot . Sometimes even remuxing it discards the metadata

[vodpod]

Is there a tool to check for DRM?

[poisondeathray]

You can access those metadata comments with quicktime pro, they are called "annotations"

The comment line is actually stripped when I remux with yamb (mp4box) in the examples I have (but they were originally MOV from apple, and not DRM protected - just movie trailers)

Sorry - I don't know enough about DRM.. except that it sucks :0

[teodz1984]

Is there a tool to check for DRM?

Not any that I am aware of...
Ad I'm sure it doesn't involve just changing the headers of videos..

And why do insist doing a python hack for this? lol

Perhaps doing more research on the mp4 data structures might help you...
http://developer.apple.com/library/mac/#documentation/QuickTime/QTFF/QTFFChap3/qtff3.h...0939-CH205-SW1

Well if you are comfortable working with 'atoms' and objects stored in network byte order (big-endian format), more power to you.

[Gavino]

Did you use the same #spaces/characters ?

That's the key point which I think teodz1984 has missed.
If just doing a dumb binary replacement, the offsets of everything else in the file will be wrong if you change the number of characters. To maintain internal consistency if changing the length, you would need to use software that understood the file structure.

I just tried, and it worked ok on some random mp4 from Apple movie trailer

I replaced "Warner" with "PDR "

Copyright                        : © 2008 PDR    Bros. Pictures. All Rights Reserved

Note that PDR added 3 spaces after "PDR" to match "Warner".

[vodpod]

Note that PDR added 3 spaces after "PDR" to match "Warner".

oh, snap! that was it! I completely missed the part PDR mentioned #spaces/characters. I just passed that part thinking "well of course you think i'm an idiot?" without actually understanding what he meant.

Thanks PDR! & thanks Gavino for making me re-cognize