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Understanding Digital Audio Formats

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Digital audio is everywhere, from compact discs to portable music players and beyond. In this article, Michael Miller demystifies the types of digital audio and explains the different file formats you may encounter.
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Even if you've never downloaded any music from the Internet, chances are you've still heard of MP3 files. MP3 is a particular type of digital audio format that compresses music to fit within reasonably-sized computer files—while maintaining near-CD quality sound.

As popular as the MP3 format is, it's not the only digital audio format you'll encounter online. There are several other ways to compress music into relatively small audio files, many with far better sound than the MP3 format, that you can consider for use when you rip your own CDs to hard disk.

When you copy a digital audio file, you can either copy the file exactly (in non-compressed format) or you can use some sort of compression to reduce the otherwise-huge file sizes. If you choose a compressed format, you can opt for formats that use either lossy or lossless compression. Lossy compression loses some of the original audio information to create a smaller file, resulting in music that isn't quite as good-sounding as the original. Lossless compression doesn't affect the original sound quality, but results in larger files sizes—although not near as big as non-compressed files.

If you want to rip your music for playback on your computer or a portable music player, you'll probably use a lossy compression format, such as MP3, AAC, and WMA. If you want to archive your CD collection for playback on a home audio or home theater system, you'll probably want to use a lossless compression format, such as FLAC or WMA lossless, or copy your CDs in non-compressed format.

Read on to learn more.

How Digital Sampling Works

All digital recordings—starting in the recording studio—are made by creating digital samples of the original sound. The way it works is that special software "listens" to the music, and takes a digital snapshot of the music at a particular point in time. The length of that snapshot (measured in bits) and the number of snapshots per second (called the sampling rate) determine the quality of the reproduction. The more samples per second, the more accurate the resulting "picture" of the original music.

Compact discs sample music at a 44.1kHz rate—in other words, the music is sampled, digitally, 44,100 times per second. Each sample is 16 bits long. When you multiply the sampling rate by the sample size and the number of channels (two for stereo), you end up with a bit rate. For CDs, you multiply 44,100 X 16 X 2, and end up with 1,400,000 bits per second—or 1,400Kbps.

All these bits are converted into data that is then copied onto some sort of storage medium. In the case of CDs, the storage medium is the compact disc itself; you can also store this digital audio data on hard disk drives, or in computer memory.

The space taken up by these bits can add up quickly. If you take a typical three-minute song recorded at 44.1KHz, you end up using 32MB of disk space. While that song can easily fit on a 650MB CD, it's much too large to download over a standard Internet connection, or to store on a portable music player.

This is where audio compression comes in. By taking selected bits out of the original audio file, the file size is compressed. If the right bits are excised, you'll never miss them.

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