The amount of preparation required will be different for NTSC and PAL, due to different framerate differences. We'll cover each one in detail, but first let's talk a little about the processes involved and tools you'll be using. Taking audio from source files to DVD assets is simpler than creating video assets. That's because our options are more limited. The ideal audio for most titles on DVD will be Dolby Digital with however many channels the source has. Bitrates will generally be 96kbps for mono, 192kbps or 224kbps for stereo, and either 384kbps or 448kbps for 5 channel surround.
DVD-Video ComplianceDVD specs mandate either LPCM (uncompressed) or AC-3 (Dolby Digital) audio to be present on all DVDs. That's why these formats are referred to as mandatory. MPA (MPEG-1 Layer 2) is a mandatory format for PAL discs but not ntsc. The vast majority of players can handle MPA audio for both PAL and NTSC, but due to inoconsistencies in decoding between different chips it's generally recommended to avoid it. In order to keep everything the way I feel it should be, we'll be using nothing but AC-3 for our audio compression.
SamplingDVD-Video also requires that audio be sampled at a different frequency than is used by CDs and many older video formats like VCD. We'll need to make sure all of our audio conforms to this, re-sampling where necessary. The process of changing the playback speed of audio also involves reducing the samplerate to a non-standard number. The last part of this step will also be re-sampling.
AC-3 EncodingSome of our source MPEG-2 files have Dolby Digital (AC-3) audio already. Others have MPEG-1 Layer II audio which is also known as simply MPEG Audio (MPA). The MPEG-4 Files have MPEG-4 Part 4 (AAC) audio. I'm going to encode all the audio to AC-3. I'm shooting for 100% standards compliance, so it doesn't matter if players will be able to handle MPEG Audio. Since this process will require decoding to uncompressed audio first, we'll be able to do any other processing we want before it gets encoded to AC-3.
Commercial AC-3 EncodersUnfortunately, since Dolby Digital itself isn't open source, there are no open source encoders that are as reliable as one certified by Dolby Labs. There also aren't a lot of AC-3 encoders priced for the consumer market. The primary tool featured in this guide is TMPGEnc's AC-3 Plugin, which doesn't have a trial version, but is reasonably priced. It's designed to integrate with other software, and can be run by DVD-Lab Pro, which I'll be using for authoring. Some people are put off because it has to occasionally access the internet to validate your license. If this bothers you, this probably isn't the software for you.
There are a few other encoders on the market, but as a rule they're intended for the high end professional market. For example, Sonic Scenarist, a high end DVD authoring package, includes an AC3 encoder that can encode up to 5.1 channels. The TMPGEnc encoder can only encode mono and stereo files. Unfortunately Scenarist costs several thousand dollars. Sony's Vegas Studio also includes a good AC-3 encoder, and sells for a few hundred dollars.