File Formats and Bit Rates

MP3, AAC, AIFF, WAV…is the computer industry incapable of speaking plain English!?

It may seem so, given the plethora of acronyms floating through modern-day Technotopia. But the lingo and the basics behind it aren't terribly difficult to understand.

MP3, AAC, AIFF, and WAV are audio file formats. The acronyms stand, respectively, for MPEG-1, audio layer 3 (if you must know, MPEG is an acronym for Moving Picture Experts Group); Advanced Audio Coding; Audio Interchange File Format; and Windows Audio Volume.

The compression methods used to create MP3 and AAC files are termed lossy because the encoder removes information from the original sound file to create these smaller files.

Fortunately, these encoders are designed to remove the information you're least likely to missaudio frequencies that you can't hear easily, for example.

AIFF and WAV files are uncompressed, containing all the data from the original. When a Macintosh pulls audio from an audio CD, it does so in AIFF format, which is the native uncompressed audio format used by Apple's QuickTime technology. WAV is a variant of AIFF and is used extensively with the Windows operating system.

Early versions of iTunes supported MP3, AIFF, and WAV encoding. iTunes 3 allowed you to play back AAC files but not encode them. The first two generations of iPods were incapable of playing AAC files, but with the introduction of iTunes 4 and the latest iPod software, all iPods can play them.

iTunes 4 supports both the unencrypted .m4a AAC files created by iTunes and the encrypted .m4p AAC files sold at the iTunes Music Store.

iTunes 4.5 introduced the Apple Lossless Encoder. This is termed a "lossless" encoder because the encoder doesn't shrink the file by removing portions of the audio spectrum; rather, it removes redundant data. This scheme allows you to retain all the audio quality of the original file while producing a copy just over half the size of that original file.

Now that you're familiar with these file formats, let's touch on resolution.

You probably know that the more pixels per inch a digital photograph has, the crisper the image (and the larger the file). The number of pixels per inch helps describe the resolution of the image. The higher the resolution (the more pixels per inch), the better the quality.

Resolution applies to audio as well. But rather than using a standard of pixels per inch, audio defines resolution by the number of kilobytes per second (Kbps) contained in an audio file. With files encoded similarly, the higher the kilobyte count, the better-sounding the file (and the larger the file).

I emphasize "with files encoded similarly" because the quality of the file depends a great deal on the encoder used to compress it. Specifically, many people claim that if you encode a file at 128 Kbps in both the MP3 and AAC formats, the AAC file will sound better.

Before version 4, iTunes imported audio CDs as MP3 files encoded at 160 Kbps by default. This setting nicely balanced file size with sound quality and was a good choice for tunes downloaded to an iPod.

With iTunes 4, Apple changed the default import settings to AAC at 128 Kbps. The company claims that its AAC encoder produces better-sounding files at a lower resolution (and, thus, smaller files) than those created by its MP3 encoder.

The presence of this AAC encoder helps explain why Apple now claims that the current 20 GB iPod holds up to 5,000 four-minute songs, whereas, on release, the second-generation 20 GB models held 4,000 four-minute songs. Apple reduced the default resolution from 160 Kbps (for MP3 files) to 128 Kbps (for AAC files).

When you understand that a four-minute song encoded with the MP3 encoder at 160 Kbps consumes 4.7 MB of hard drive space and that the same file encoded with the AAC encoder at 128 Kbps weighs in at only 3.8 MB, you see how Apple can make this claim. By default, iTunes 4 creates smaller sound files, allowing you to pack more tunes onto your iPod.

Although iTunes' default setting is perfectly swell, you have the option to change it. To do so, choose Preferences (located in the Apple menu in OS X and in the Edit menu in Windows and Mac OS 9.2 and earlier), and click the Importing tab (or the Importing button, in the case of iTunes 3 and 4) in the resulting iTunes Preferences dialog box.

The Import Using pop-up menu lets you choose to import files in MP3, AIFF, or WAV format in all versions of iTunes. iTunes 4 adds AAC encoding, and iTunes 4.5 adds the Apple Lossless Encoder.

All iPods can play files encoded in the AAC, MP3, AIFF, and WAV formats. Only those iPods with the bottom Dock connector (third- and fourth-generation iPods and the iPod mini) can play songs formatted with the Apple Lossless Encoder.

The Configuration pop-up menu is where you choose the resolution of the AAC and MP3 files encoded by iTunes. In iTunes 4, the default setting is High Quality (128 Kbps).

To change this setting, choose Custom from the Setting pop-up menu, and in the resulting AAC Encoder window, choose a different settingin a range from 16 to 320 Kbpsfrom the Stereo Bit Rate pop-up menu.

Files encoded at a high bit rate sound better than those encoded at a low bit rate (such as 96 Kbps). But files encoded at higher bit rates also take up more space on your hard drive and iPod.

There are few good reasons to alter other settings in the AAC Encoder window. You have the option to change the sample rate to 48.000 kHz, for example, and the channel from stereo to mono. But a higher sample rate is useful only for professional audio gear (such as Digital Audio Tape decks and high-end audio editing applications), and converting stereo to mono is helpful only if you're Brian Wilson.

The default settings for MP3 importing include Good Quality (128 Kbps), High Quality (160 Kbps), and Higher Quality (192 Kbps). If you don't care to use one of these settings, choose Custom from this same pop-up menu. In the MP3 Encoder dialog box that appears, you have the option to choose a bit rate ranging from 8 to 320 Kbps.

You can change other settings in the MP3 Encoder dialog box. By choosing the Use Variable Bit Rate Encoding (VBR) option, for example, you might produce better-sounding MP3 files (though they may be larger than if you hadn't chosen this option).

You can also change the number of channels (mono or stereo) and stereo mode. For most people, the default settings work perfectly well.

It's up to you to balance quantity and quality. If you want to cram as many songs as possible onto your iPod and have what less-sensitive souls disparagingly term a "tin ear," feel free to use a low bit rate, such as 96 Kbps for AAC files and 128 Kbps for MP3 files.

But unless you're listening to anything but narration (a "books on tape" kind of recording, for example), don't venture below these resolutions unless you really don't care about quality.

And marching much above 192 Kbpsparticularly if you're listening over headphones or computer speakersis mostly a waste of time for all but those who have the most discerning ears.