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Understanding the MP3 Format

Balancing quality and quantity

Heads up!

Welcome to this article from the Crutchfield archives. Have fun reading it, but be aware that the information may be outdated and links may be broken.

The MP3 audio format has been the de facto standard for online digital music files since the early 1990's. Virtually every computer, smartphone, and tablet recognizes and plays MP3 files, even if its primary format is a proprietary one (such as AAC with iTunes®, or WMA with Windows® Media Player).

In order to get the most out of this ubiquitous format, it's important to understand what an MP3 file is, and what it does to your music. With that knowledge, you'll be better able to set up and manage your digital music collection to suit your needs.

How it works

MP3 is a digital audio codec; that is, it's a method of compressing and decompressing digitized sound. The digital information on a standard audio CD requires about 10 megabytes per minute of music. When a song is ripped from a CD to a computer and converted to an MP3, that same minute of music typically is reduced to about 1 megabyte — a tenth of the original file size.

The MP3 codec shrinks the source file by removing portions of the original signal considered to be essentially inaudible — a technique know as "perceptual coding." MP3 and other codecs are lossy formats of compression. That is, some frequencies are lost in the encoding process, and can't be restored by reconverting the file to its original format.

The trade-off between sound quality and storage space

Despite its lossy nature, MP3 can still deliver near-CD sound quality. Just as you can save your digital photos at higher or lower resolutions to save memory space (with a proportional loss in detail), so too can you adjust the "resolution" of an MP3.

The bitrate, or average amount of data required per second of music, determines the audio resolution of an MP3. The higher the number of kilobits per second (kbps), the closer in sound quality the MP3 is to the original source — and the larger the file size.

Most Internet download sites are primarily concerned with facilitating faster downloads, which means keeping MP3 files as small as possible. 128 kbps MP3s are generally considered to by the lowest acceptable level of sound quality.

Generally, most download services offer MP3s at 192 kbps or 256 kbps. These higher resolutions provide a balance between sound quality and convenience. For many, music at this resolution sounds "good enough," and the file sizes are small enough to fit hundeds of song onto a smartphone or tablet.

You can also compromise by using a variable bit rate, or VBR. Rather than encoding audio at a constant rate, VBR encoding ups the bit rate for complex passages, and uses a lower rate for simple ones. A VBR-encoded sound file usually has more sonic detail than one encoded at the same constant bit rate, yet it's a smaller file than a constant bit rate file of comparable resolution.

Hear more of the original detail with higher sound quality

Although the overall character of the music is preserved, the greater the compression, the more sonic details are lost. Extremely high and low frequencies usually get discarded with even slight compression. Although considered inaudible, they reinforce harmonic frequencies that "shade" the sound, giving it much of its fullness and presence. Further compression can diminish the differences between loud and soft passages, decreasing dramatic impact. Extreme compression — down to 64 kbps and lower — can completely flatten the sound, making it harsh and muddy.

By contrast, MP3 files of 192 kbps, 256 kbps or greater preserve most of the sonic information of the original WAV file. Acoustic instruments tend to keep their natural warmth at these resolutions, and electronic instruments sound fuller while retaining their punch.

So how can you make sure the MP3s you're importing into your music library are at the best sound quality for you? Depending on the source, you can have some control over the process.

  1. Importing music from CDs — Change the default settings of your PC's media player. Almost all of these programs let you adjust the MP3 resolution from the standard 128 kbps up to at least 320 kbps. Many let you customize the setting by typing in your own number.
  2. Purchasing music online — While most sites only offer songs at 192 kbps, some offer tracks at a higher resolution. Many of these higher-resolution tracks also come without copy protection, giving you the added advantage of enjoying your purchased downloads in more ways and on more players. Many of these tracks are offered at 256 kbps resolution.
  3. Downloading from other sources — Band websites, podcast directories, and other sources of Internet music usually offer MP3s at 128 kbps. Try downloading other formats (discussed below) if your player supports them. You might find one that sounds better to you than the others.

Other codecs, too

As mentioned above, sometimes you aren't offered a choice of MP3s with different levels of compression. Instead, you might be offered a choice between different formats.

Each codec has its own algorithms to determine what to discard, and so the same song saved in different formats can vary slightly in sound. Some people find they like the way a particular codec shapes the sound, and prefer that format over MP3.

There are also formats that retain all the information of the original files, and just store them more efficiently in a slightly smaller space. Because all of the information is retained, these are known as lossless codecs. See our guide to high-fidelity computer audio for more information.

In the end, it's up to you

Keep in mind that you can always compress a large file into a smaller one, but you can't restore the resulting file back to its original form. The information lost in the compression process is permanent. So it's best to choose the highest resolution when you first import a track. A 128 kbps file can't be "bumped up" to 256 kbps any more than a low-resolution photo can be blown up to a poster-sized print.

The most important thing to remember when you're changing settings to improve sound quality is this: your ears are the final judge. Some people can distinguish between a 256 kbps and a 192 kbps MP3. To others, there's no appreciable difference between a 128 kbps file and the original CD track.

Only you can determine the ideal MP3 resolution for your music. But it's best to do so only after you've given the higher resolutions a fair audition. You might be surprised at what you've been missing.

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