Digital Audio 101
You could easily write several guides on the background, development and technical intricacies of digital audio and all the audio formats available. In this section however my aim is simply to provide you with a basic, non-technical introduction to the terminology used by the programs covered in this guide.
Any form of audio can be captured and stored in a variety of digital formats on your computer, such as WAV, MP3 and OGG. The first question I hear you ask is "why isn't there just one digital format...why are there so many?" Well the main reason is compression, whereby the file size taken up by audio is dramatically reduced with varying levels of audio quality loss. This is exactly the same as with digital images and the quality and size of the various formats like BMP, JPEG and GIF. This choice of quality and compression gives you (and game developers) several options as to which to use for different purposes. Of course some of the formats are proprietary, which means they can't be used without a license - this explains why certain formats are preferred by game developers.
The basic attributes for an audio file are typically presented like this:
WAV PCM 44.1Khz 16 Bit Stereo
This information is interpreted respectively as Audio format, Compression format, Sample Rate, Bit Rate and Channels. Each of these are described further below:
Sample Rate - Also referred to as Sampling Frequency or just Frequency. To create a digital equivalent of a sound, a computer has to take samples of the audio source every second. The more samples it takes of the audio, the more faithful the digital sound is to the original source. Sample Rate is shown in thousands of cycles per second (KHz). As a benchmark, CD Audio has a sample rate of 44.1 KHz.
Bit Rate - Just like every other piece of information on a computer, digital audio is made up of Binary Digits (or Bits for short), where 8 bits makes one Byte, and 1024 Bytes are known as a Kilobyte (Kb), and so forth. The bit rate may be represented as either bits per cycle per channel per second (e.g. 16 bit) or as an overall data transfer rate in Kilobits per second (e.g. 128kbps). The higher the bitrate of the audio, the better the quality. As a benchmark, 128kbps is extremely close to CD Audio.
Channels - Channels are the distinctly separate sources of sound which form the final audio when played together. Stereo is two distinct channels played together, Mono is one channel and Joint Stereo is a compromise between the two. Most audio is Stereo because human ears usually need two distinct sources of sound to correctly perceive "3D" audio.
Compression - To work out how large an audio file would be without compression (e.g. when in WAV PCM format) multiply the length of the audio in seconds by the Sample Rate, Bit Rate and Number of Channels. For example 44.1Khz 16 Bit Stereo audio when uncompressed is 44100 x 16 x 2 = 1411200 bits per second. Divide by 8 to get bytes per second, and by 1024 again to get Kilobytes (not kilobits) per second and you have 172Kb for each second of audio. Based on this, a 3 minute track in WAV format will be 180s x 172Kb/s = approximately 30MB. If however this file had the same attributes but was only 3MB, we could determine that the compression used to encode it was 10:1 (e.g. MP3 compression).
Codec - This term is often confused with audio format. A CODEC (COmpressor DECompressor) is a program which allows audio to be compressed and decompressed to/from the audio format for which it is designed. Audio files achieve compression by using special algorithms, and it's the codec which can encode/decode these algorithms. If you can play or record in a particular audio format, you have a codec for that format already installed on your system (see Essentials section for details on how to view/remove codecs).
As mentioned previously, the proliferation of audio formats is primarily due to the need to reduce the size of audio files so that it's practical to fit large amounts of sound/music onto a typical data CD or hard drive for use in games and the like. The most common digital formats in use today are:
WAV - The Wave file format developed by Microsoft is the industry standard on PCs. It is a high quality format, particularly when uncompressed, but consequently it is also quite large in size. This is the format ultimately used by CD burning software for burning onto Audio CDs. Compression for WAV files is available, such as MS ADPCM, but it only provides around 4:1 compression at best.
MP3 - (MPEG Level 1, Audio Layer 3) This format is extremely popular because it provides a good balance of size, quality and compatibility. An MP3 file is typically 1/10th of the size of the equivalent WAV file with minimal noticeable quality loss (as long as the overall bitrate is 128kbps or higher). It achieves this compression level by cutting out many frequencies the human ear can't detect. Most every software audio player can play MP3.
WMA - Microsoft being Microsoft, they developed a format which has excellent quality and compression levels even better than MP3 (WMA version 8 and above). Technically speaking, WMA audio is great, but it has much less compatibility: Windows Media Player is the main tool used to play WMA. There are also protection aspects to WMA, as MS tries to enforce digital copyright by making WMA protected files difficult to copy and burn.
OGG - This format is non-proprietary and hence can be used by anyone without a license required. Much of recent game music is in OGG Vorbis format because of this factor combined with the fact that it provides great compression and quality which is even better than MP3. Compatibility is still not across the board for OGG, however many audio players have plugins (Codecs which are supported by the player) you can download to use OGG, and others are now supporting OGG "out of the box" - such as Winamp 3.
CDA - This is the file type which shows up when you explore the contents of an audio CD in Windows. Most players can play it back, but CDA files are not actual audio files themselves. CD Audio is actually in WAV file format, and the CDA files are more like "shortcuts" to these WAV files. Use an audio CD extraction program like Easy CD-DA Extractor to copy ("rip") the WAV tracks from an Audio CD.
RM - Real Media format, which is usually only playable on RealPlayer or RealOne Player (see Tools section below). Not a particularly great format due to the high compression it uses to allow internet streaming of audio. Commonly found in video files, much more rare for audio.
ASF - This is the Microsoft Advanced Streaming Format, and as the name implies, it's a proprietary format for streaming internet audio/video (similar to RM). Compatibility is usually fine through players such as Windows Media Player, however since this format is often used by Electronic Arts to protect the commercial music they include as part of their games' soundtracks, playback may be unsupported even on WMP. In that case a special decoder such as Game Audio Player (with the right plugin) may be able to play back and convert these modified ASF files.
BIK - The Bink format is commonly used for the introductory and in-game movies in several popular games. It is, strictly speaking, a video format. However, since this guide includes at least one game with BIK format audio, I'll include it here. You can play BIK files only with the appropriate codec which comes with RAD Tools (see Tools section).
As you can see there are many formats available, with only a few of them listed above. There are many articles comparing the quality and compression of the popular formats, such as this one. In terms of game music, you'll only really run into MP3, WAV and OGG most commonly. The ASF and BIK audio formats are also used, but mainly in EA Games.
The important thing to remember is that while there are software audio players which play the above file formats - even though you may need to download the appropriate codec as a plugin - the bottom line is if you want to burn any audio to CD, you'll need to convert it to WAV. Some burning programs such as Nero can convert MP3 to WAV automatically before burning, however usually your best bet is to manually convert a file to WAV before attempting to burn.
The next section covers the tools I'll be using in this guide to extract, convert and play the above file formats from games.