Digital music owes its popularity to one key technology: compression. Without compression – a way to make files smaller, more portable, and easier to transfer over a network – there would not yet have been a digital music revolution. Sure, you can transfer uncompressed music files, but their large size virtually requires broadband Internet access.

One simple example: an uncompressed one-hour CD takes up about 616 MB of disk space. Even with most broadband connections, it would take more than 2 hours to download. Encoded with “typical” iPod-friendly compression – as explained below – the same CD requires about 54 MB, less than one-tenth the size and one-tenth the time.

image Select a song in iTunes, then select File > Get Info. The Summary tab shows the encoding format (Kind) and the Bit Rate.

But what is compression? And what are bit rates? What do these terms mean? Read on to find out how to choose the best way to import music to listen to on your iPod.

Part One: Choose Your Compression Format

Everyone has heard of MP3, the most ubiquitous compression format and the one that gets the most press. In fact, to people unfamiliar with digital music, all such files are “MP3” files. Savvy iPod and iTunes users know that there is another compression format called AAC, “advanced audio coding.” This format, which is part of the MPEG-4 standard, sounds better than MP3 at the same file size. iTunes uses this superior format by default, as does the iTunes Music Store.

 

image Choose the compression format and bit rate from iTunes’ Importing preferences.

However, the iPod and iTunes can use a few other file formats: they can play both WAV and AIFF files – raw audio files from CDs in computer-readable formats – as well as Apple Lossless, a compression format that preserves a CD’s original quality while cutting its size in half. (iPod shuffle fans, note: of these large formats, the iPod shuffle only supports WAV, and won’t play back AIFF or Apple Lossless.)

Because they sound very good and achieve better compression than these other formats, most iPod users will want to choose between AAC or MP3. There are pros and cons to using each of these formats.

AAC: This gives better quality sound at the same bit rate (compared to MP3), hence smaller files for equivalent quality, but you can only play these files on an iPod or with iTunes. If you don’t plan to use your digital music files on other players, then AAC is the choice to make.

MP3: While MP3 files don’t sound as good as equivalently sized AAC files, you can play them on just about every digital music player, as well as home DVD players, car stereos, cellphones, and other devices. However, to get the same quality as AAC, you’ll want to use a higher bit rate, which takes up more space.

Audiophiles, however, will want to consider either WAV, AIFF or Apple Lossless. Again, there are constraints: only iTunes and the iPod will be able to play Apple Lossless files – other players won’t support it – but just about every other piece of music playing software will be able to play WAV files. AIFF files are specific to Macs, so if you are planning to listen to your music on a PC, WAV is better.

There are other lossless formats, and fans of jambands (see our article here) will be familiar with these two acronyms: FLAC and SHN. Like Apple Lossless, these formats provide the same quality as uncompressed files, yet take up about 50% of the space of the original music files. These formats are used for (legally) trading live recordings, but if you want to listen to them with iTunes or on your iPod, you?ll need to use some extra software: etree.org gives you all the information and links to programs (PC/Mac/Linux) you can use to convert these files into WAV or AIFF files, which iTunes then converts to your favorite format (AAC or MP3).

Part Two: Choose Your Bit Rate

When choosing the best way to import your music, the “bit rate” is the second variable. As mentioned earlier, iTunes uses the AAC format by default, and the bit rate it uses (unless you manually change it) is 128 kbps. Kbps means “kilobits per second,” and the higher the number, the more space a music file will consume on your computer. Files are most often compressed to 128kbps, 160kbps, or 192kbps.

Compression at 128kbps is the bit rate the iTunes Music Store uses. Since AAC files sound better than MP3, you can assume that a 128 kbps AAC file is about the same quality as a 160 kbps MP3 file, which is what iTunes calls “high quality.” (For those keeping count, an uncompressed WAV or AIFF file is 1411 kbps – a bit more than 10 times larger.)

 

image iTunes lets you choose a few standard bit rates, or lets you select Custom, where you can access other bit rates.

You may find that your music sounds fine at this default bit rate, or you may discover that a slightly higher bit rate goes a long way. We’ve tried different formats and bit rates, and prefer to rip our music as AAC files at 160 kbps, or MP3 files at 192kbps. On good headphones or speakers, files compressed these ways sound better than 128 kbps files, but ratcheting them up another notch to 192 kbps AACs or 224kbs MP3s doesn’t make much of a difference.

But there is a trade-off between bit rate and file size; a 4-minute song requires about 3.7 MB of iPod/hard disk space at 128 kbps and about 4.7 MB at 160 kbps. If you go to 192 kbps, you’d have a 5.6 MB file, and file sizes increase accordingly each time you move to a higher bit rate.

 

image Some examples of file sizes for 4-minute songs. Your file sizes may differ, especially if your songs contain album art.

For many users, this size difference poses no problem, but if you have lots of music and want to fit it all on your iPod, you’ll have to make a choice. A good step before you convert your library: do a blind listening test. Find a few songs that best exemplify the music you listen to – try and choose a songs of different styles, and, above all, ones that use different combinations of instruments. Rip the songs at several different bit rates, and, if you want, in different formats (at least in AAC and MP3) and set up a playlist containing all these songs. (You’ll have to change the names of the songs, such as SongName 128 AAC, SongName 160 MP3, etc., so you can identify and easily compare them.)

Next, lie down in a comfortable position in a quiet room with the best headphones or earbuds you have. Listen to the songs, without looking at which is which, and make notes: write down how you think each song sounds. Don’t try and guess which song is encoded at which bit rate, but simply try and hear the difference and make notes about what you hear (or don’t hear).

Sync your iPod to your computer, then check the playlist; sort by Last Played, so you’ll be able to see the order in which you listened to them, then compare the songs and their bit rates to your notes. You may be surprised. Depending on your results, you may want to change the way you encode your music. Now that you know all about compression and bit rates, you’ll be able to make the best choice.