[ i thought acc was one of the surround sound formats on dvd's and the like? [ that allows for 5 channels (or more probably) whereas your standard mp3 [ format is a single stere channel [ [ i'm probably wrong tho Hi Shaun, You're thinking of AC-3, which is different than AAC. The acronyms do sound a lot alike! After digital audio was added to LaserDisc, Dolby designed AC-3 to fit on one of the analog audio tracks. It's highly compressed, and currently capable of anything from mono to 5.1 or 7.1 surround. I'm not sure what formats were initially introduced, since LaserDisc already had uncompressed digital that could handle mono or stereo, but they certainly did not go beyond 5.1 in the early days. Once DVD was introduced, the AC-3 format was dubbed Dolby Digital, and mono and stereo were certainly available by then. Since DVD was introduced, they've taken it beyond 5.1 in a way that is compatible. AAC (Advanced Audio Coding/Codec) is also highly compressed, and not lossless, but it does sound better than previous lossy codecs. At least they fit higher quality in the same bits, but not full quality. Apple offers a lossless encoder which puts CD quality audio on your drive (or iPod) at about half the space of WAVE or AIFF. But this is about five times the size of the lossy MP3 or AAC files. The Lossless files are named .m4a, and I'm not sure if they are a kind of AAC file, or a kind of MPEG-4 Audio file (maybe those are the same things anyway). P.S. Part of the distinction between AC-3 and Dolby Digital is that AC-3 typically is delivered over an analog channel with Frequency Modulation encoding, while DD is just a file on the DVD. AC-3 inputs must decode the incoming FM (no tuning involved, just demodulation) before the AC-3 data is available. DD inputs are already digital format, but they are simply compressed rather than full digital audio. P.P.S. After AC-3 came DTS (Digital Theatre Systems). Instead of compressing 5.1 down to a single analog channel (very inefficient), DTS takes over the two digital audio channels. DTS still covers the same range of options as AC-3, from mono to 5.1 and beyond, but DTS has about 4 times the data, and likewise is much less compressed. Just like higher bit-rate MP3s sound better, DTS should sound better than AC-3. Lately, however, movie studios have cut the bit-rate of DTS in half so they can fit more features on DVD. DTS is also available on CD format, but listening to it would sound like noise without a DTS decoder. Many bands have 5.1 surround audio discs, like Santana, Steely Dan, the Police, (and Supreme Beings of Leisure, although that's AC-3 DVD) which are pressed on CD (easier to manufacture than DVD) and still offer high-quality surround. It would *really* be nice if Yello material were available in a surround mix on DTS CD! Brian Willoughby