Can I add the Dolby Surround flag to an existing AC3 file?

[Neon Kitten]

Hi all,

I have a DVD project I'm putting together where the two-channel audio track is surround-encoded. Problem is, the source I am working from is already in AC3 encoded format, and I don't have an uncompressed version of it, so I have to use the existing AC3 file.

What I want to do is set the Dolby Surround flag in the AC3 file so that Pro-Logic decoding will happen automatically on playback. But I can't seem to find a way of "editing" the file's headers to add this flag. I don't want to decode and re-encode the AC3 file for quality reasons.

Does anyone know of either a tool I can use to do this, or some other way around the problem? It's got me completely stumped.

[Cornucopia]

Here's something to try (assuming you have access to an AC3 encoder)...

Make a short 10 sec soundfile-sine tone, white noise, silence, music, whatever.

encode this file to an AC3, carefully taking note of all the settings. Just make sure that you set the surround flag to OFF/NO SURROUND/NORMAL STEREO. name it something like "***sample_OFF.ac3"

Do it again with the same file, but this time change the surround flag to INDETERMINATE/UNKNOWN. name it something like "***sample_UNK.ac3"

Do it again with the same file, but this time change the surround flag to ON/DOLBY SURROUND. name it something like "***sample_ON.ac3"

Do a binary file compare on all the files (pairwise--OFF vs. UNK, OFF vs. ON, UNK vs. ON--best to use an app that can show you the HEX codes). In each of these instances, you should come up with only a single discrepancy (or it may be repeated). THIS will be where the flag would reside. Make note of the HEX codes (and location?) for each of these flag states.

Then, using a HEX editor and a COPY of your desired file, do a SEARCH AND REPLACE. Save and you should be done. Test it out and tell us how it went.

Sorry it's so involved, but it's one of the least invasive ways to do it. I wish I knew of a more straightforward app, but haven't heard of any yet.

HTH,
Scott