Editing AC3 header information...

[rik1138]

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.

[tebasuna51]

- 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.

[rik1138]

Originally Posted by tebasuna51

- 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

[tebasuna51]

To know about data en AC3 header read:
Digital Audio Compression Standard (AC-3, E-AC-3) Revision B

DplAc3.exe turn PL on.

[TDiTP_]

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"

[tebasuna51]

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

[nickintheforest]

Originally Posted by TDiTP_

Is there utility that can change DN to any value? Not only to -31dB.

ChangeDN
You may use

change_dn.exe input.ac3 DN

where DN is integer of [0..31]
For testing this tool you may use test.exe

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

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)

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

с3 60
с3 60

с3 60 = 11000011 01100000
11011 = 27
--------
Russian forum

tebasuna51, thanks for source.

[Abradoks]

Both links are dead now. Here is mirror: ChangeDN.7z

[Also dead]

[Midzuki]

Thanks!

[junh1024]

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.

[tebasuna51]

@junh1024
This one work with your sample:

[junh1024]

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.

[tebasuna51]

Originally Posted by junh1024

Can I upload the sources & binaries from this thread to Github as a backup?

All are simple free soft.