Encoding audio files for DVD by Matt Ottewill

Audio quality (sample rate & bit depth)

Digital audio files come in many different qualities. Audio quality is usually defined by sample rate and bit depth. For example 44.1 kHz 16-bit etc.

Computer file extensions

When computer files sit on a hard drive or CD-ROM they usually have extensions which identify the program that recorded (or created) them. For example ...

.sdII ... DigiDesign Sound Designer II, 2-track audio editor software

.wav ... PC interchange file format

.aiff ... audio interchange file format for all computers (used by Apple)

.mov ... QuickTime audio file

These extensions are independent of sample rate and bit depth, They tell you nothing about audio quality. They are simply "envelopes" that contain data. Most .sd2, .wav and .aiff files contain mono or stereo 44.1kHz 16-bit audio data.

Compressed & uncompressed formats

Audio files can be in compressed or uncompressed formats.

Uncompressed formats (sometimes called PCM formats)

CD quality sound has a sample rate of 44,100 (44.1kHz) and a bit depth of 16-bit.
DV (2-track mode) sound has a sample rate of 48,000 (48kHz) and a bit depth of 16-bit.
DVD can use uncompressed 48kHz 16-bit (1.5Mbps) and 96kHz 16-bit files (3Mbps).

Compressed formats

MiniDisc
MP3 audio can have variable sample and bit depths
AC3 (Dolby Digital)

Audio compression is employed for 3 primary reasons ..

to reduce file size so that more audio may be stored on a given media format (MP3 players, DVD-video discs, MiniDiscs etc)
to reduce file size so that files will download from a web site faster
to reduce data rate so that files will stream (broadcast) over a network such as the internet

DVD-video audio

DVD-video allows for uncompressed PCM audio in 48kHz or 96kHz 16-bit format and compressed AC3 (Dolby Digital) format. DVD-video players are required to be able to replay PCM (uncompressed) and AC3 (compressed) audio files. This is why all DVD-video players can play audio CDs.

Uncompressed stereo 48kHz 16-bit audio files will take 1.5Mbps of the available DVD bandwidth (about 15%) and 96kHz 16-bit files will take 3Mbps (about 30%).

Audio left in uncompressed format will take more bandwidth than if its compressed. AC3 compression can reduce the size of audio files by a factor of 12:1 with very little audible side effects.

AC3 (Dolby Digital) audio compression

Encoding (compressing) audio files to AC3 has 2 advantages ...

Reduced file size means more video, audio, menus etc will fit on a disc.
Reduced data rate means more is available for video streams.

An AC3 file can contain between 1 and 6 channels of audio, allowing 5.1 surround sound.

Preparing files for encoding to AC3

Before you encode to AC3 you should ensure your audio files are ...

Mono or stereo
16 or 24 bit resolution
48 or 96 kHz sample rate (if you are using A.Pack, use 48kHz)
AIFF, WAV or SDII formats

What do I need to encode audio to AC3?

Video files (such as DV) can be encoded with several programs ...

Apple's A.Pack encoder (included with DVD Studio Pro)

Apple's A.Pack encoder does a good job of creating multi-channel AC3 files. Comes with DVD Studio Pro.

Sony / Sonic Foundry 5.1 Surround Plug-In pack

Sony recently brought Sonic Foundry who developed an AC3 encoder for Vegas and Acid.

DVD-video authoring programs

Many DVD-video disc authoring programs come with their own AC3 encoder programs.

Choosing AC3 bit rates

Among other options, AC3 encoding programs allow you to set the bit rate.

The bit rate you choose will determine ...

Audio quality
File size
Percentage of overall DVD-video bandwidth
How much strain will be put on a computer's CPU when it is using a "soft MPEG codec" to play DVD-video files on a desktop computer

What bit rate settings should I use?

You can experiment and test, but here are some good guidelines ...

5.1 surround sound ... between 224 and 448 kbps (kilo bits per second)
Stereo ... between 192 and 224 kbps
Mono ... between 64 and 128 kbps