AC3 file created w/Compressor had wrong byte order

[John McDaniel]

I've recently encoded an AC3 file with my MacBook Pro using Apple's Compressor application. The file is to be MUX'ed with an MPEG-2 stream for playback from a dedicated server in an on site installation. A co-vendor's MUXing application is unable to recognize my .ac3 file. I have a file inspection application from Manzanita Systems that also fails to recognize the .ac3 file created by Compressor.

I found this thread in the Apple forums:

Apple Compressor Forum Thread

Then I found this bit of info online:

another user had a problem with an AC3 recently as well. In that case the file was created with a wrong (well, it's not actually wrong but it's unusual for Intel PCs) byte order which makes it impossible for ProEx to handle the file. DVD Studio Pro sees it just fine.

If you've got a hex viewer you could find out by yourself. The first two bytes of an AC3 must be 0B 77. If they are shown the other way around then the byte order has to be changed.

I looked at my .ac3 file with a hex editor and it starts with 77 0B.

Then I looked at an .ac3 file that I created with Compressor on a G5 in back in October '06. It began with 0B 77. It was also recognized by Manzanita's MPEG-ID application.

I can't find anything on Apple's website regarding this issue. No Knowledge Base article; no update; nothing.

WTF?

Anyone using the latest version of Compressor on an Intel based Mac to create AC3 files? Having problems sending them out to people who are NOT using DVD Studio Pro to handle the files further?

Right now, I'm looking for a utility app that can perform a byte reversal on an entire file. In a month, I've got to be on the road with the ability to make AC3 files on my laptop.

Thanks for any insight.

j mcd

[pneyrinck]

There are three types of AC3 files: normal, normal+timecode, and PC. The first two use a big-endian byte order and "normal" is the most standard format and that is what Compressor is creating. It does not matter if you use a PPC or Mac Intel computer. Compressor is programmed to create the same kind of file no matter what kind of computer you use. The PC file type uses little-endian byte order and is required by some applications. Those applications should be improved to accept any type of AC3 file. An alternative to Compressor is to use the SoundCode For Dolby Digital plug-in. It can create any file type and can be used with Pro Tools LE/M-Powered on a laptop on a Mac or a PC. Even though Pro Tools LE/M-Powered is not a surround app, you can place each channel of a 5.1 surround mix on a separate track, highlight all six tracks, and encode it. The decoder will decode a 5.1 AC3 file into six mono tracks for confidence testing.

Paul
www.neyrinck.com

[John McDaniel]

The first two use a big-endian byte order and "normal" is the most standard format and that is what Compressor is creating. It does not matter if you use a PPC or Mac Intel computer. Compressor is programmed to create the same kind of file no matter what kind of computer you use.

I think that it is demonstrably true that it DOES make a difference whether you use a PPC or Intel based Mac. Did you read the Apple Discussion Forum thread I referenced? It could be considered a failing that Compressor does not offer a "reverse byte order" checkbox like some applications.

The PC file type uses little-endian byte order and is required by some applications. Those applications should be improved to accept any type of AC3 file.

I think most of us have been in the position, at one time or another, of telling a PC user that the reason the file created on a Mac doesn't work for them is because their "application should be improved." You know where THAT line gets you.

It is appropriate that two versions (big-endian and little-endian) of a single file format exists using the same extension? Do "aware" apps just look at the first two bytes, or is there info in a header?

There's also the issue that, although DVD Studio Pro can handle both types, it can't use them both in the same project.

I will have to fire up a G5 with the latest version of Compressor to run some tests of my own.

j mcd

[pneyrinck]

I think that it is demonstrably true that it DOES make a difference whether you use a PPC or Intel based Mac. Did you read the Apple Discussion Forum thread I referenced? It could be considered a failing that Compressor does not offer a "reverse byte order" checkbox like some applications.

I apologize John. My brain was working in the wrong byte order when I read your post and I understood everything backwards. It sounds like Compressor on Mac Intel has a bug because it should create the same type of file on both PPC and Mac Intel. The programmers and testers probably did not notice because the little-endian file type is a valid type and DVD SP can read it. And it sounds like those other apps recognize the "normal" type so my comment that they should be improved was wrong.

It is appropriate that two versions (big-endian and little-endian) of a single file format exists using the same extension? Do "aware" apps just look at the first two bytes, or is there info in a header?

Unfortunately, AC3 files contain raw stream data and have no header. And so apps must look at the bytes like you say.

I Googled "byte swap utility" and there is some stuff out there but I could not find an appropriate one for OS X. You need to find something that byte swaps 16-bit words.

Paul