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Old 19th March 2011, 01:13   #1  |  Link
rik1138
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Editing AC3 header information...

I found a couple of threads about this, but they all ended in discussions about why the question was asked, not if there was a way to do it...

Is there a tool out there that can edit the header of an AC3 file? I need to change the Dolby Prologic surround flags in a bunch of AC3 files, and was looking for a tool to allow me to do that (without re-encoding, just changing the header info...)

Being able to change all the settings would be nice (Bitstream mode, dial norm, input filtering, etc...), but the surround flag is an important one right now...

Any help?

I tried encoding the same file with and without Prologic too look at what has changed, but the flag that is changed is different on every 'frame' of audio apparently... Simple search&replace won't work.
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Old 19th March 2011, 02:53   #2  |  Link
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- Bitstream mode, for what?
- input filtering, to late the signal was already filtered.
- Dial Norm, you can use eac3to (or DN31-Ac3.exe attached).
- Set the DPL flag, you can use DplAc3.exe attached.
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File Type: 7z Ac3_tools.7z (30.5 KB, 311 views)
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Old 19th March 2011, 04:06   #3  |  Link
rik1138
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Quote:
Originally Posted by tebasuna51 View Post
- Bitstream mode, for what?
- input filtering, to late the signal was already filtered.
- Dial Norm, you can use eac3to (or DN31-Ac3.exe attached).
- Set the DPL flag, you can use DplAc3.exe attached.
Wasn't sure which of those settings actually do something to the audio file itself, or are just flags for the decoder to do stuff... (I.e., input filtering..)

Bitstream Mode, options are:
Complete Main
Music & Effects
Visually Impared
... Etc.

Not sure what they do actually, just one of the options in the encoder. Never know when you might need to change something.

RF overmodulation, is that a 'flag', or does it actually change the audio during encoding? Something I'd like to be able to turn off without re-encoding, if that's an option.

Being able to change the prologic flags will be a huge help, thanks a lot for that! Will this allow changing between all 3 settings, or does it just turn PL on?

Thanks again, Rik
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Old 19th March 2011, 04:35   #4  |  Link
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To know about data en AC3 header read:
Digital Audio Compression Standard (AC-3, E-AC-3) Revision B

DplAc3.exe turn PL on.
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Old 19th March 2011, 10:02   #5  |  Link
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Is there utility that can change DN to any value? Not only to -31dB.

And may be utility for change "Center mix level" and "Surround mix level"
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Old 19th March 2011, 10:47   #6  |  Link
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I don't know.

But you have the code, isn't difficult make the changes:
1) Read 1 frame
2) Change the value
3) Reconstruct crc1
4) Goto 1 until end of file
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Old 16th April 2011, 07:01   #7  |  Link
nickintheforest
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Quote:
Originally Posted by TDiTP_ View Post
Is there utility that can change DN to any value? Not only to -31dB.
ChangeDN
You may use
Code:
change_dn.exe input.ac3 DN
where DN is integer of [0..31]
For testing this tool you may use test.exe
Code:
test.exe 1.ac3 27
FileSize : 315820288 bytes
SamplCod : 0 (2=32, 1=44.1, 0=48 KHz.)
BitRate  : 448 Kb/s
FrameSize: 1792 bytes
DialNorm: -31db
0xXX
0xXX
First hex-value is byte (bytes) where is the information of DialNorm in first audio frame.
Second - other frames.
Of course, they must coincide.
If ac3 is mono then hex-values will be for example
Code:
3b 80
3b 80
DN bits are bits beginning 13 bit of BSI.
3B 80 = 00011101 10000000
11011 = 27

If 3/2 (L, C, R, SL, SR) или 3/1 (L, C, R, S)
Code:
DC
DC
DN bits are bits beginning 17 bit of BSI
0xDC = 11011100 (bin)
11011 = 27

Else DN bits are bits beginning 15 bit of BSI
Code:
с3 60
с3 60
с3 60 = 11000011 01100000
11011 = 27
--------
Russian forum

tebasuna51, thanks for source.
Attached Files
File Type: 7z ChangeDN.7z (9.1 KB, 110 views)

Last edited by tebasuna51; 14th January 2013 at 13:22. Reason: Add attachment to avoid dead links
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Old 13th January 2012, 17:55   #8  |  Link
Abradoks
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Both links are dead now. Here is mirror: ChangeDN.7z

[Also dead]

Last edited by tebasuna51; 16th January 2021 at 10:48.
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Old 13th January 2012, 18:16   #9  |  Link
Midzuki
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Thanks!
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Old 14th January 2021, 12:20   #10  |  Link
junh1024
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Hi, is it possible to modify ChangeDN to work on E-AC3 files too?

E-AC3 Specification


Sample E-AC3 file


Currently, the tool gives a crc1 error.
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Old 16th January 2021, 19:46   #11  |  Link
tebasuna51
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@junh1024
This one work with your sample:
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File Type: 7z DNx_eac3.7z (36.0 KB, 15 views)
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Old 17th January 2021, 11:05   #12  |  Link
junh1024
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Thank you tebasuna51, it seems to work on EC3 files of other bitrates . Can I upload the sources & binaries from this thread to Github as a backup?

PS: The EC3 is actually EC3 Atmos, so hopefully they still work on AVR.
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Old 17th January 2021, 19:23   #13  |  Link
tebasuna51
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Quote:
Originally Posted by junh1024 View Post
Can I upload the sources & binaries from this thread to Github as a backup?
All are simple free soft.
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