Why Does Joining AC3 with DOS Copy Command Damage Stream?

[LisaB]

It seems that someone suggested using this method, but I find that it is no good.

I joined 3 AC3 streams using the DOS Copy Command, then ran AC3Fix on the joined stream...AC3Fix found an error at the first join.

I then ran AC3Fix on each of the 3 component streams and it found no error!

I verified that the concatenated AC3 file had exactly the same byte size as the 3 source files.

I am confused as to where the error is. Can anyone recommend some program for editing AC3 w/o re-encoding?

Here is a copy of my DOS session:

C:\Documents and Settings\Owner>copy /b n:\start.ac3+n:\clip.ac3+n:\end.ac3 k:\
o.ac3
n:\start.ac3
n:\clip.ac3
n:\end.ac3
1 file(s) copied.

C:\Documents and Settings\Owner>ac3fix k:\o.ac3 n:\ofix.ac3
Starting. Input file is k:\o.ac3, output file is n:\ofix.ac3
Frame length is 1792
Checking frame 101138: Wrong frame length, expected 181236832, got 1792
Writing frame
Writing frame
Current wrongness is 672 bytes
Checking frame 172606: OK
Finished. Found bad frames. Final wrongness is 672 bytes

C:\Documents and Settings\Owner>ac3fix n:\start.ac3 k:\s.ac3
Starting. Input file is n:\start.ac3, output file is k:\sfix.ac3
Frame length is 1792
Checking frame 101138: OK
Finished. No bad frames encountered

C:\Documents and Settings\Owner>ac3fix n:\clip.ac3 k:\c.ac3
Starting. Input file is n:\clip2.ac3, output file is k:\c2.ac3
Frame length is 1792
Checking frame 30: OK
Finished. No bad frames encountered

C:\Documents and Settings\Owner>ac3fix n:\end.ac3 k:\e.ac3
Starting. Input file is n:\end.ac3, output file is k:\e.ac3
Frame length is 1792
Checking frame 71440: OK
Finished. No bad frames encountered

[chicola]

Do the ac3fix BEFORE joining!

Then join using the DOS command as you're doing.

That should solve it.

[LisaB]

well, as I mentioned in the post, I checked the 3 pieces, and they had no bad frames.....thus there was nothing that needed fixing....thus after I join them together, there should still be nothing that needs fixing....thus the mystery.....

are you trying to say that ac3fix will alter an ac3 file even in the case that there are no bad frames?....well, maybe...I guess I'll look into that next time.

[chicola]

well, as I mentioned in the post, I checked the 3 pieces, and they had no bad frames.....thus there was nothing that needed fixing....thus after I join them together, there should still be nothing that needs fixing....thus the mystery.....

are you trying to say that ac3fix will alter an ac3 file even in the case that there are no bad frames?....well, maybe...I guess I'll look into that next time.

Yep, I think it deletes extra things that don't need to be there.

[LisaB]

Ah, well, you're right...and you're not...

To test, I ran ac3fix on a file, and it reported no bad frames. However, I noticed that the fixed file was actually smaller than the original! The fixed file had 1 less frame, too!

This is probably a bug in ac3fix, but if the last frame has been clipped, ac3fix will delete it, and not tell you about it.

That is why, when I ran the fix on the 3 joined clips, it found a bad frame where the first join was...

It doesn't make any difference whether you run ac3fix before or after. Personally, I'd rather join once and fix once, rather than fix 3 times and join once...

Mystery solved.

[iantri]

Isn't there a header at the beginning of each file? Concatenating them together would stick a header in the middle of the file which ac3fix might see as an error.

Just a theory.

[BJ_M]

a lot of ac3 files found on - lets say - some split up 2 cd xvid type files have a bad frame in the point where it is split ..

running ac3 fix doesnt report anything - but as you noticed , it fixes it ...


if such a unfixed file is imported into maestro -- it also reports the last frame is broken and also deletes the last frame .. but tells you about it ..

[LisaB]

iantri, ac3 is a headerless audio format, as far as I know.

Now, if only VDubMOD was smart enough to delete the clipped frame when joing avi's, I'd be a happy camper.

As far as I can tell, you have to go through all the following steps to join avi with ac3:

1. load clip1 in vdubmod, demux ac3, fix with ac3fix
2. load clip2 in vdubmod, demux ac3
3. join fixed ac3-1 with ac3-2 using dos copy command
4. load clip1 in vdubmod, append clip2
5. disable current stream and add the stream you made with dos copy
6. save out the new avi

Now, there is a shortcut in some cases....after doing step 1, you can compare the byte size of the fixed clip1 with the original clip1, if it is different, then you have to do all the steps as outlined above....but, if it is the same, then you just have to do the following:

2. load clip1 in vdubmod, append clip2
3. save out the new avi