# CD writing: on filesizes and play times **URL:** \ **Category:** Factual Questions\ **Created:** [April 14, 2006, 11:57am UTC](https://boards.straightdope.com/t/cd-writing-on-filesizes-and-play-times/352647 "2006-04-14T11:57:49Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![fishbicycle](https://avatars.discourse-cdn.com/v4/letter/f/aeb1de/32.png) [@fishbicycle](https://boards.straightdope.com/u/fishbicycle)\ **Post date:** [April 14, 2006, 11:57am UTC](https://boards.straightdope.com/t/cd-writing-on-filesizes-and-play-times/352647/1 "2006-04-14T11:57:49Z")
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? ---
**Author:** ![shefDave](https://avatars.discourse-cdn.com/v4/letter/s/77aa72/32.png) [@shefDave](https://boards.straightdope.com/u/shefDave)\ **Post date:** [April 14, 2006, 12:58pm UTC](https://boards.straightdope.com/t/cd-writing-on-filesizes-and-play-times/352647/2 "2006-04-14T12:58:52Z")
Because [.wav files =! CD Audio](http://en.wikipedia.org/wiki/Wav#Audio_CDs). 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) ---
**Author:** ![Fridgemagnet](https://avatars.discourse-cdn.com/v4/letter/f/22d042/32.png) [@Fridgemagnet](https://boards.straightdope.com/u/Fridgemagnet)\ **Post date:** [April 14, 2006, 1:10pm UTC](https://boards.straightdope.com/t/cd-writing-on-filesizes-and-play-times/352647/3 "2006-04-14T13:10:43Z")
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](http://en.wikipedia.org/wiki/Binary_prefix) (symbol: Mi) to distinguish it from a Mega (M), but it’s also a move that few people can be arsed to implement. ---
**Author:** ![KeithT](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@KeithT](https://boards.straightdope.com/u/KeithT)\ **Post date:** [April 14, 2006, 1:41pm UTC](https://boards.straightdope.com/t/cd-writing-on-filesizes-and-play-times/352647/4 "2006-04-14T13:41:44Z")
It’s because CD-ROMs use error correction, but audio CDs don’t. > [@Wikipedia](#): > > [CD-ROM Specifications](http://en.wikipedia.org/wiki/ISO_9660#Specifications) > > The smallest entity in the CD format is called a frame, and holds 24 bytes. Data in a CD-ROM are organized in both frames and sectors. A CD-ROM sector contains 98 frames, and holds 2352 bytes. > > CD-ROM Mode 1, usually used for computer data, divides the 2352 byte data area defined by the Red Book standards into 12 bytes of synchronisation, 4 bytes of header, 2048 bytes of user data and 288 bytes of error correction and detection codes. > > CD-ROM Mode 2, usually used for audio/video data, divides the 2352 byte into 12 bytes of synchronisation, 4 bytes of header and 2336 bytes of user data. > > **The main advantage of Mode 2 is that it provides an additional 14 per cent of the user data space per sector. The reason is that Mode 2 does not have the additional EDC and ECC error correction data of Mode 1.** ---
**Author:** ![Fridgemagnet](https://avatars.discourse-cdn.com/v4/letter/f/22d042/32.png) [@Fridgemagnet](https://boards.straightdope.com/u/Fridgemagnet)\ **Post date:** [April 14, 2006, 3:18pm UTC](https://boards.straightdope.com/t/cd-writing-on-filesizes-and-play-times/352647/5 "2006-04-14T15:18:03Z")
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](http://en.wikipedia.org/wiki/Cross-Interleaved_Reed-Solomon_Coding) 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. ---
**Author:** ![fishbicycle](https://avatars.discourse-cdn.com/v4/letter/f/aeb1de/32.png) [@fishbicycle](https://boards.straightdope.com/u/fishbicycle)\ **Post date:** [April 14, 2006, 6:36pm UTC](https://boards.straightdope.com/t/cd-writing-on-filesizes-and-play-times/352647/6 "2006-04-14T18:36:38Z")
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? ---
**Author:** ![Antonius\_Block](https://avatars.discourse-cdn.com/v4/letter/a/51bf81/32.png) [@Antonius\_Block](https://boards.straightdope.com/u/Antonius_Block)\ **Post date:** [April 14, 2006, 10:48pm UTC](https://boards.straightdope.com/t/cd-writing-on-filesizes-and-play-times/352647/7 "2006-04-14T22:48:45Z")
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.