Sound Advice: Compression affects audio quality

  • Article by: DON LINDICH , Special to the Star Tribune
  • Updated: June 24, 2011 - 1:00 PM

Answers to your multimedia questions.

Q Would you explain the difference between downloading a CD to a computer using lossless vs. lossy compression formats? Would I be able to hear the difference using headphones? Is there a recommended program that is easy to use and universal for doing this?

A Lossless compression reduces the size of the digital file without affecting the sound quality. You get the same sound quality as the original source, but it takes up less storage space on a CD, DVD, Blu-ray, hard drive or portable device. Examples of lossless compression include Dolby TrueHD and DTS Master Audio, found on Blu-ray, and Apple Lossless compression, used to convert CDs to compressed digital files.

Lossy compression reduces the file size with some loss of sound quality compared with the original. Lossy compression schemes reduce file size more drastically than lossless compression, but you might pay a price in sound quality depending on how much compression you use as well as the encoding (compression) scheme. The amount of compression is usually referred to as bitrate, in megabits per second (Mbps.) Some examples of lossy compression are Dolby Digital Surround and DTS Surround, found on DVDs, and the MP3 and AAC formats, used with computers. This encoding and the small resulting files are the key to fitting thousands of songs on an iPod.

Whether you can hear the difference depends on a few factors. First is your hearing acuity. Second is the quality of the playback system. A high-quality, high-resolution system is more likely to be able to resolve the difference between a lossy file and the original or a lossless file. The third is the quality of the digital file.

Lossy files recorded at higher bitrates (say, over 256 Mbps) sound closer to the original source than one recorded at 128 Mbps. The encoding scheme affects this, as well. For example, the ACC encoding used on iTunes is regarded to have better sound quality at 128 Mbps bitrates than MP3 encoding at the same bitrate. Music purchased from the iTunes music store uses AAC at either 128 or 256 Mbps.

My favorite program for working with audio is iTunes, which is a free download for PC or Mac from Apple.com. You can use it to encode using AAC, MP3 and Apple Lossless Compression as well as organize your music and burn CDs.

You should choose the type and amount of compression based on how you will use the material. If you are downloading to your computer and have lots of hard-drive space available, import the CDs with Apple Lossless Compression to preserve sound quality. If you are transferring the music to an iPod, then AAC at 128 Mbps to 256 Mbps will provide a nice balance of small file sizes and good sound quality. If you have good headphones or will use your iPod with a sound system, going above 256 Mbps might be called for. If you import the music using Apple Lossless Compression and convert the files to AAC or MP3 for use with an iPod, you will get the best of both worlds.

Submit questions and read past columns at www.soundadviceblog.com.

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