Apart from bitrate, the sound quality of digital music is also affected by its format, which is determined by the software used to compress it, known as a codec. MP3 is one of the older techniques for compressing audio and is not widely used by online stores. Apple has chosen a newer format called Advanced Audio Coding (AAC), which plays on iPods and some other devices. Most other online stores use the similarly modern Windows Media Audio, or WMA, which does not play on iPods.

Image Credit... Frank Frisari

All three of these formats are “lossy,” meaning the encoding software surgically trims out audio information that is not easy to hear, because it is covered up by other sound or is situated at the highest and lowest ranges of human hearing. The Norah Jones track “Come Away With Me” is 33.4 megabytes when stored in an uncompressed format; the lossy compression methods bring that down to 6.1 megabytes at 256 kbps, or 3.1 megabytes at 128 kbps, regardless of the codec used. (When turning your CDs into song files on your PC, you can choose the bitrate you want in the settings of iTunes or Windows Media Player.)

Codecs do vary in quality. Mr. Wragg of EMI said that as a rule of thumb, an MP3 at 320 kbps is roughly the same as an AAC file at 256 kbps. “The difference between WMA and AAC is more difficult to say,” he added. “Each has a slightly different way of getting compression. But in double-blind tests they perform pretty similarly — bitrate for bitrate they sound similar, but some prefer one over the other.”

Until now, online retailers have dealt in 128 Kbps tracks — most retailers, that is. Two years ago, a group of audiophiles created MusicGiants, a digital download store that specializes in “lossless” files that are compressed in a way that does not discard any audio information, resulting in tracks that average 25 megabytes in size. MusicGiants now has more than 500,000 songs from most major labels.

Scott Bahneman, chief executive of MusicGiants, said that comparing lossless tracks and compressed tracks was like comparing photos taken with a high-end digital camera and those taken with a camera phone. “Every bit counts when you’re trying to get sound quality, resolution or anything else,” he said. The site’s core audience is the type of person who spends large sums of money on home theater equipment, and wants music stored as digital files rather than on CD.

Mr. Bahneman said his company planned to offer better-than-CD-quality music in files originally created for the DVD-Audio and Super Audio CD disc formats, which did not catch on with consumers. Each song will be 250 megabytes, about the same size as one episode of a sitcom on iTunes, but without the video. These “Super HD” files will have a bitrate of up to 11,000 kbps (that is, 11 megabits per second), and will be sold by the album rather than the track, at $20 each. Mr. Bahneman said that with the latest broadband services and huge hard drives, downloading and storing high-resolution audio files should not be a big hurdle.