How It Works: Hardware MP3 Players

The digital music revolution has freed your favorite songs from the tyranny of a physical form. Virtually identical digital copies, in formats such as MP3 or WMA, allow you to listen to the same song over and over without fear of wearing out a cassette or scratching a CD.

But the idea of digital music isn't nearly as interesting if you can listen to it only on your computer. That's the beauty of MP3: Not only can you transfer these small files but you can store them easily on inexpensive portable playback devices. All you need is a player to transfer your music files to. You can now find a ton of portable players and components for your stereo that set the music free from your PC.

Here are the vital statistics:

• Many hardware MP3 players use a digital signal processor (or DSP) to handle the tasks of transferring and decoding MP3 files.
• In addition to the traditional portable players, manufacturers now produce MP3-capable stereo components, car stereos, and even digital cameras and mobile phones that can play MP3 files.
• You can expect the next generation of players to store more music, have faster processors, and support more music file formats.

A hardware MP3 player is a stripped-down, application-specific computing device, with enough power and the specific components needed to store, organize, and play digital music files, as well as display information about them. The player itself behaves as an application would, managing the digital music files you've created and downloaded to the player beforehand.

The heart of any MP3 playback device is its digital signal processor. The DSP handles data transfers, controls the device's interface, and decodes the file for playback. The DSP does just a few things but does them quickly, and uses little power in the process (an especially beneficial trait for portable players).

The process begins when you create or download a digital music file using your PC. When you create a file in the MP3 format (or in the competing WMA and AAC formats), the software that creates the file tosses out bits of data to make the file smaller, a process called lossy compression. (For more on this process, see our companion article, "How It Works: MP3.")

Size Is Everything

The main limitation of most hardware MP3 players is the amount of onboard storage space for holding digital music files. You can create smaller MP3 files by compressing them more. You can tell how much information has been thrown out by looking at the compression level, expressed as the number of kilobits of data that represent 1 second of music. The lower this number, the greater the compression. However, the playback quality suffers below a subjective threshold that varies from person to person but averages about 128 kbps. A song MP3-encoded at 160 kbps will sound much better than the same song MP3-encoded at 96 kbps, but the 160-kbps song will take up nearly twice the space, leaving less room for additional songs.

Once you have a file, you must transfer it to the device's memory. If the device uses only built-in memory, you will have to connect the player to a PC's USB, serial, or parallel port connection, which sends the data from the PC's hard drive to the unit's memory. If the device uses a removable storage medium, you may be able to copy the files directly to the storage medium more quickly than you can via the USB port. Some stereo-component MP3 players also have a built-in encoder that can read and compress digital music files directly from a CD and store it on the unit's memory, without requiring a PC.

The first generation of players used nonvolatile flash memory -- a type of data storage that is similar to your computer's RAM but doesn't lose its contents when you cut off the power -- to store the digital audio data. Newer players offer more choices, including internal hard drives or removable storage such as Iomega's PocketZip or IBM's Microdrive.

After you select which song you want to hear (using the player's built-in controls and, often, an LCD screen that gives song, artist, and track time information) from the files in the player's memory, the data moves to the DSP, which decompresses the file. The decompression software can be embedded on the processor or in the unit's memory. Next, the DSP passes the song data to a digital-to-analog decoder, which converts it from binary digital information into the analog audio signal that controls how your headphones or speakers create the music. Some players also have small preamplifier circuits that boost the strength of the audio signal as it emerges from the decoder, before it reaches the headphone jack.