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Understanding Audio File Formats (Part 1)
Written by Scott R. Garrigus - 2006, Cengage Learning. Reprinted with Permission.
Just as different physical methods of storing audio were developed over time for different applications and reasons (vinyl records, tapes, compact discs, and now DVDs, etc.), different computerized methods for storing audio data have been developed as well. These methods come in the form of audio file formats. An audio file format is simply a specification stating the structure of how audio data in a file should be stored. For example, one audio file format may specify that the bits and bytes of audio data should be ordered in a certain manner, and another format may specify that the data be ordered in an entirely different manner. Of course, this is a very simplified explanation, but what it boils down to is that the same audio data can be stored in a variety of different ways.

Why do you need more than one audio file format? Because you may want to use your audio data for different tasks, such as playback on a CD, music or sound effects in a video game, a film or video soundtrack, or even for downloading over the Internet. Each task may require that your audio data be saved in a different way. For example, audio for a CD must be stored using a bit depth of 16 and a 44.1 KHz sampling rate. But for downloading over the Internet, you use adifferent bit depth and sampling rate because at 16 bit, 44.1 KHz, every minute of stereo audio consumes about 10 MB of disk space! That's a lot of data to push over a lowly phone line.

In addition to providing different bit depths and sampling rates, some audio file formats also offer data compression. This means that by saving to certain file formats, you can shrink the size of your audio files for use in low-bandwidth situations, as mentioned earlier with the Internet. Sometimes, the compression doesn't affect the quality of your audio, but most of the time it does. With compression, you have to find a good compromise between the quality of your audio data and the size of the file you want to end up with. There are many different compression schemes available, and I talk about those shortly.

Different audio file formats also exist because of the many different computer platforms that have been created over the years, such as the Amiga, Macintosh, NeXT, and the Windows PC. To provide you with as much flexibility as possible, Sound Forge allows you to open and save a large number of the existing audio file formats. Some of these, you might never use, but just in case, it's good to know that you can if the need arises.

Dialogic VOX (.VOX)
This is an optimized audio file format that is mainly used for telephony applications. The Dialogic VOX format saves 16-bit audio data and compresses it to 4-bit audio data, which gives you a 4:1 compression ratio. This means that you can save files that are very large in size and compress them significantly. Of course, the quality of the audio is affected, and the format only supports monophonic data. Dialogic VOX audio files have a .VOX file extension, and they use ADPCM as their compression method.

Intervoice (.IVC)
Like the Dialogic VOX format, the Intervoice format is for use in telephony applications. It only supports 8-bit, monophonic audio data, but it provides a number of different compression schemes. These schemes include both A-Law and u-Law. Intervoice files have a .IVC file extension.

Audio Interchange File Format AIFF (.AIF/.SND)
This is the standard file format for saving audio data on the Macintosh. If you ever need to transfer audio files between the PC and the Mac, this is the format you should use. The format supports 8-bit and 1 6-bit monophonic and stereo audio data. Files in this format may or may not also contain a Mac-Binary header. If a file of this type doesn't contain a Mac-Binary header, it probably has .AIF for a file extension. If a file of this type does contain a Mac-Binary header, Sound Forge opens it but identifies the file as a Macintosh Resource instead. In this case, the file probably has .SND for a file extension.

MP3 Audio (.MP3/.MPG/.MPEG)
More than likely, you've heard of the MP3 audio file format. It's all the rage (literally) these days with people on the Internet. News about the format has even made it into the mainstream media because the format is being used to post illegal copies of music all over the Web. Why is the format so popular? Because it compresses your audio data at about a ratio of 12:1, and the quality of the audio is very close to CD quality. Sound Forge provides support for opening MP3 files and saving to MP3.

For more information:
* Understanding Audio File Formats (Part 2)
* The Sony Sound Forge Power! Books Series


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