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DVD Audio And The Apple OS X Platform

Author: Roupen Mouradian, Chief Technical Officer

Audio on NTSC DVDs can be written in two different formats: Linear PCM and AC3.

When using the Apple platform to create your final DVDs, you are most likely using a common set of development resources: Final Cut Pro, Compressor, A.Pack, and DVD Studio Pro. When you are not experiencing space constraints on your DVD, you are likely using Linear PCM, which is uncompressed, lossless audio. Audio on Linear PCM can be up to 8 channels and have a bit rate of up to 6.144 MB/s. Clearly, Linear PCM is the best option for quality, but file sizes for this type of audio can make it an unreasonable choice. From Final Cut Pro or Compressor you can export directly into Linear PCM. When space becomes an issue, a lossy, but high quality encoding format known as Dolby Digital AC3 can be used. Similar to how MP3 works, AC3 retains much of the original quality of the audio at a much lower bit rate. Typically for 2-channel stereo audio, you can compress the audio at 192 kbps with no recognizable difference to the human ear.

With uncompressed Linear PCM files you get from Final Cut or Compressor (usually with the .aiff extension), you still need an application to convert your audio into Dolby Digital AC3. To the rescue comes Apple with A.Pack. A.Pack is a simple to use graphical interface allowing you to convert your audio from supported Quicktime audio formats (including Linear PCM) to AC3.

Many of the options on A.Pack you will likely never use for your standard DVD authoring. Most likely you simply wish to compress your 2-channel stereo audio into a smaller bitrate. To accomplish this, with A.Pack open, choose your Audio Coding Mode as "2/0 (L, R)". Set your data rate to 192 kbps. Set your Dialog Normalization to -31 dBFS. This ensures your audio levels are not altered. Lastly, click on the Preprocessing tab and set the Film Compression drop down to "None". With these settings all in place, drag your audio into the left and right channels and then you can encode.

My A.Pack Keeps Giving OS Errors 5000 and 1

All to commonly, A.Pack gives notoriously annoying 5000 OS Error and 1 OS Error from OS X. Apple has acknowledged that these OS Errors are present, however has offered no fix yet for them. The reason for the OS Errors are actually quite simple in 95% of the cases; you are using a Dual Processor machine.

Once again, to the rescue, comes Apple with a pseudo-fix. Part of a greater set of hardware and software debugging tools which Apple offers is C.H.U.D ( Computer Hardware Understanding Developer) tools. You can find C.H.U.D. at Version Tracker. At time of this document, the latest version was 4.0.1. After installing C.H.U.D., you can open up your System Preferences and on the bottom in the Other section you will be able to find a Processor icon. With this tool, you can enable and disable your 2nd processor. At this point all you need to do when you start to A.Pack is to simply disable your 2nd processor, start all your audio encoding, and go have lunch.
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