Compressed Audio vs. CDs: Can You Tell the Difference?

Many people complain that compressed audio doesn't sound as good as CD audio. To verify that claim, we subjected a group of typical listeners to blind tests to see whether they could tell the difference between music from CDs and compressed versions of the songs in a variety of file formats. We also asked the testers to rate the compressed versions on a quality scale.

Before you read the results of our tests, bear one thing in mind: The quality of compressed music is very much dependent on how you play it back. A compressed file will sound considerably different when played through a high-end sound card and headphones (such as those we used in our tests) than it will through a portable MP3 player with a pair of cheap earpieces. We did not test any portable players; our primary goal was to test the quality of the compression formats, rather than the quality of hardware.

How We Tested

From each of five pieces of music, we ripped a 30-second clip from a CD to a .wav file. The five music selections were an acoustic version of Daughter by Pearl Jam, Radioactivity by Kraftwerk, a cellos-only version of Wherever I May Roam by Apocalyptica, a live version of Time by Pink Floyd, and O Fortuna from Carl Orff's Carmina Burana. These presented a range of challenges to the compression programs, as they ranged from subtle acoustic sounds to full-on orchestral splendor.

We compressed each clip to a variety of bit rates using Windows Media Encoder for the WMA format, Real Jukebox Plus for the RealAudio format, MusicMatch Jukebox Plus for the MP3 format, and RealJukebox Plus with the Dolby AAC plug-in for Dolby's AAC format.

We then played back both the original files and the uncompressed versions to 30 testers (mainly PC World staff, but we also got representatives from Dolby laboratories, Microsoft, and Real Networks to participate). We asked them to identify the compressed versions. We used a double-blind testing methodology in which each tester was given three files to listen to, called A, B, and C: The A file was the .Wav file ripped directly from the CD. The other two were either the .Wav file or a compressed version.

We then asked the testers to rate what they thought was the compressed file on a quality scale of 1 to 5:

• 5 - No perceptible differences between compressed and uncompressed
  
  
• 4 - Perceptible differences, but not annoying
  
  
• 3 - Slightly annoying differences
  
  
• 2 - Annoying differences
  
  
• 1 - Very annoying differences
  
  

The files were played back on a Dell system with a Pentium III-600 CPU, 128MB of RAM, and a CreativeLabs Sound Blaster Live Platinum sound card and its optional Live Drive component using Cakewalk Sonar, which allowed the testers to listen to the files side by side and to switch between them at will. The testers listened to the sound clips through Sony MDR-7506 headphones connected to the Live Drive headphone socket.