Here’s something I’ve been curious about.
I’m going to write a music CD. Windows Explorer tells me that the folder containing the .wav files is 770 MB. CD-Rs hold 700 MB of data. Why, then, if I set up to make a CD-ROM of these .wav files, does it tell me that the project is 771.4 MB and requires two discs, but in Music CD creation mode, it all fits on one disc with a running time of 76:19, leaving 3:39 blank at the end? How are the other 71.4 MB still fitting on the disc with room left over?
Because .wav files =! CD Audio.
In Music CD Creation mode the .wav files will be converted to CD Audio which doesn’t take up as much space. Exactly why they take up less space I can’t help you with, I’m afraid. (Although some of the links from the wikipedia page may explain it)
Watch out for different definitions of MB when comparing file sizes too, as the definition is flexed a bit to suit marketing needs. Sometimes it means 1,000,000 and sometimes it means 2^20=1,048,576. There’s a move to call the latter a Mebi (symbol: Mi) to distinguish it from a Mega (M), but it’s also a move that few people can be arsed to implement.
It’s because CD-ROMs use error correction, but audio CDs don’t.
Audio CDs still use error correction, and would sound horrible if they didn’t as all CDs are full of errors to a greater or lesser degree. The system used is Cross-Interleaved Reed-Solomon coding, and there’s a sizable chunk of redundancy in the data of about 25%. And not all of the remaining data is audio - there’s all sorts of addressing and miscellany.
Thank you for the information! It almost makes sense to me now, but it brings up another question. If the .cda format can reduce 770 MB to less than 700 MB, how does the music file remain lossless?
It’s not that the .cda format compresses the actual data, fishbicycle, it’s that audio CDs in the original “Compact Disc” format use a smaller amount of error-correction than data CDs (as mentioned by previous posters), thus leaving space for more music.
When the original audio CD specs were being developed, a compromise had to be made between error-correction overhead and the total playtime of the disc. A small amount of playback error was felt to be acceptable, since most single-bit errors will be inaudible anyway.
However, when the time came to develop the CD-ROM (i.e. data) standard, there could be no such tradeoff. If a Windows installation disk (for example) has even one invalid bit, it may cease to operate. This would be unacceptable, so a greater amount of error-correction overhead (compared to audio CDs) was built into the standard. This greater overhead reduces the space available for the data.
The file / track sizes that you see in MB only refer to the data, and don’t include the error-correction overhead. A blank CD holds a certain number of “data + error-correction” bytes, which corresponds to about 700MB of data when the CD-ROM format is used (i.e. .WAV files), but about 800MB of music “data” in the CD-Audio format.