The digital-music revolution has sparked debate over the quality of music at different compression rates. Some complain that the standard iTunes bitrate offers poor quality, others are more than happy to listen to lower quality MP3s claiming they can't hear the difference.

It seems that sound quality doesn't matter to most Macworld Online readers. Over 28 per cent of those who voted in this weeks poll think that the 128kbps iTunes default is good enough for them, while another 20 per cent are happy enough listening to MP3s at just 160kbps of higher.

But not everyone is satisfied with the iTunes bitrate standard. For one-in-five (20 per cent) of voters, music has to be at 192kbps or higher to keep them smiling.

Other options included Apple Lossless, which the company claims reduces the size of CD audio tracks by about 60 to 70 per cent without any corresponding loss in sound quality. Lossless received just 10 per cent of the vote, MP3s at below 160kbps got 5 per cent, AIFF 4 per cent, and WAV just 1 per cent.

Quality v quantity

Does sound quality really matter, or is it all in the ears? Some Macworld online readers claim that they cannot tell the difference between low and high bitrates.

"I honestly can't tell the difference between a CD and my iPod mini," said one.

Another said: "All I can say is, I've taken the Pepsi challenge, and I swear blind that I cannot tell the difference. Perhaps my hearing is defective, perhaps my equipment is defective, or perhaps those people who say they can tell a difference, actually can't."

"Personally, I think it probably has something to do with the type of music you play and the volume at which you play it. I think quality issues become far more apparent when you're playing music at high volume, which I never do, so that might explain it," he adds.

Is our judgement of sound quality dependent on the equipment we are using? This looks likely. As one reader explains: "At the end of the day you need really good speakers to notice the difference."

Another adds: "The headphones supplied with most music players are awful anyway, but when you start listening to your digital music collection on decent speakers or through decent headphones the problem couldn't be more obvious."

Listen up

Some iPod users insist that sound quality doesn't matter to them because in environment in which they are using the player they wouldn't expect or appreciate very high quality music.

One explains: "I'm quite happy to use AAC 128 (iTunes default) as most places I listen to my iPod are when I'm out and about walking down the street, where other outside noises will compromise the listening experience."

Another notes: "If I want to really listen to something I'll play the original CD/vinyl, but for the convenience of having every track I own a scroll and a click away I'll put up with a bit of compression."

Another compares Apple's service with Sony's Walkman. He said: "The original Sony Walkman was never as good as a vinyl LP, but users were happy to trade a certain loss of quality for the convenience of the Walkman. With MiniDisk players, there was less of a quality loss compared to CD, in particular hiss was largely absent, but other unwanted artifacts were still quite obvious. With AAC, the quality gets closer to the original CD and I think that the default 128K rate is a very sensible compromise for most users."

Space, man

For many readers size is everything, and smaller files will always win out.

One explains: "In an ideal world, with a massive amounts of disk space, I wouldn't bother with any compression but go with 44.1Khz 16-bit from CDs and 24-bit from those sources that I'd digitised myself. However my current music collection would require 500GB of disk space uncompressed and another 10-20GB or so extra each year."

Another reader observes: "If the music wasn't compressed, the iPod wouldn't have taken off at all. We need to accept that a subtle loss of quality is part of the deal. If you run an iPod with music that is encoded at rates greater than AAC 128K, you will get fewer tracks on your hard drive and less battery life as the drive gets accessed more frequently."

The science bit

For those who do notice the difference there is good reason. Digital-music is compressed to make it a small file size, this means downloads are typically 1/12th of the size of the original track, but you can store more on your music player.

However, what you gain in portability, you lose in sound quality. To make the files smaller, some of the information is to be lost. Some readers explain the acoustic science behind this.

"At 128kbps there's noticeable swooshing in the high frequencies, and if you like your music hard and heavy, you'll discover that walls of distorted guitar sound positively weedy. It's a problem with dance tracks, too: with downloads, the all-important low end thump is conspicuous by its absence," says one.

Another says: "At 128kbps, MP3 loses a lot of the treble (high-end), and also sometimes makes for muddy bass, although if you only listen to MP3s for a long time (rather than also listening to CDs) this becomes less apparent."

Fortunately iTunes and the iPod support alternative rates and standards: AIFF, WAV, MP3, as well as AAC, so it is possible to override the default setting.