MP3 Versus CD

Not so many years ago, music CDs were all the rage. And then along came MP3, offering much smaller file sizes while mostly preserving the sound quality of the original recording. Here is a glimpse at the two technologies to help you better understand the pros and cons of each.

MP3

MP3 is an abbreviation for MPEG-1 Audio Layer III, a digital audio compression format. It is also used to refer to MP3 formatted audio files which typically have a file extension of ".mp3". MP3 compression is designed to shrink the size of an audio file with minimal loss of sound quality. It works by selectively eliminating sounds that are considered to be beyond the auditory resolution of most people using a technique known as perceptual coding or perceptual noise shaping and compressing the rest of the song using various compression algorithms.

The size and sound quality of an MP3 file is determined by the sampling or bit rate which generally ranges from 96 to 320 kilobytes per second (Kbps). As the bit rate increases, the sound quality increases but so does the file size. A commonly used bit rate for MP3 files is 128 Kbps. This produces a file that is 11 times smaller (11:1) than the original file with a sound quality that is similar to what one would hear on the radio.

Bit rates of 160 Kbps (9:1) and higher are generally used to create MP3 files with sound quality comparable to a CD, although selecting a specific bit rate is a subjective process. Some audiophiles consider 256 Kbps (5:1) to be the minimum bit rate for preserving CD-quality sound. Others consider 320 Kbps (4.4:1) to be the minimum acceptable bit rate.



CD

An audio CD (Compact Disc Digital Audio) contains uncompressed sound data with a sampling rate of 44,100 samples per second, each sample consisting of a 16-bit (2-byte) value for each of the right and left channels. This translates to a bit rate of 1,411 Kbps (2 channels x 44,100 samples per second per channel × 16 bits per sample = 1,411,200 bits/second). In other words, a CD file is 11 times larger than a 128 Kbps MP3 file.

Audio CDs adhere to the Red Book audio specification. This standard addresses the encoding algorithm and sampling frequency, as well as parameters such as maximum playing time (79.8 minutes), minimum duration for a track (4 seconds), and maximum number of tracks (99). An audio CD can hold at most 80 minutes of music which is typically 15 to 20 songs. In comparison, a data CD can hold 100-200 songs in MP3 format (depending on the bit rate), representing several hours worth of music.

Sound Quality

The relative sound quality of CD audio versus MP3 audio is largely subjective. Some argue that MP3 files sound inferior to CD files regardless of the bit rate. Others argue that it's impossible for the human ear to detect differences between an uncompressed CD file and an MP3 encoded at a high bit rate such as 320 Kbps.

Some audiophiles prefer CD audio to MP3 audio. Whether they can truly discern a difference in sound quality is a topic for another day but certainly there is something to be said for having a "hard" copy archive of a music collection with the music at its full uncompressed resolution.

The playback device also matters. If music is played through mid-grade computer speakers or a mini stereo system, it is difficult to discern a difference between CD and 128 Kbps MP3.

Sound quality aside, an MP3 file is usually substantially smaller than the uncompressed CD version of the file, making MP3 files much more convenient for sharing, storing, and transferring.

Playback

Most CD players can only play audio CDs and not data CDs containing MP3 files. In order to play MP3 files, the data has to be burned or ripped onto an audio CD. This process decompresses the data so no more than 15-20 songs will fit on the audio disc.