Audio CDs

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CD Audio CD de audio

CD Format

A Compact Disc (also known as a CD) is an optical disc used to store digital data. It was originally developed to store sound recordings exclusively, but later it also allowed the preservation of other types of data. Audio CDs have been commercially available since October 1982. In 2009, they remain the standard physical storage medium for audio. Standard CDs have a diameter of 120 mm and can hold up to 80 minutes of uncompressed audio (700 MB of data). The Mini CD has various diameters ranging from 60 to 80 mm; they are sometimes used for CD singles or device drivers, storing up to 24 minutes of audio.


The vast majority of music CDs are encoded according to the Red Book Standard. The Red Book standard is a 16-bit, 44100 Hz PCM stereo audio signal. This is very similar (though not directly compatible with) WAV and AIFF files encoded at 16-bit, 44100 Hz. The sound quality is identical across those formats, but the data is organized differently, that is a data stream not a file which is why most operating systems can't open it.


The basic Red Book specifications state that:

  1. Maximum playing time is 79.8 minutes.
  2. Minimum duration for a track is 4 seconds (including 2-second pause).
  3. Maximum number of tracks is 99.
  4. Maximum number of index points (subdivisions of a track) is 99 with no maximum time limit.

CD-audio and Audacity

Audacity cannot read CD-audio directly. In order to edit audio from a CD using Audacity, the audio data must first be extracted from the CD and converted to a format that Audacity can read. This process is known as ripping. Audacity cannot rip CD-audio, other software must be used. See Importing data from CDs, below.

CD creation

Just as Audacity cannot read CD-audio, it also cannot be used to create audio CDs directly. Software that can is called CD-burning software.

Preparing Audio Files

The best way to prepare your audio for the CD-burning process is to export to the WAV or AIFF format at 16-bit, 44100 Hz. It may be possible to use other formats if your CD-burning software supports them: but they may not give the best quality, or they may not be playable on all CD players.

Once you have these files prepared, your CD-burning software can use them to burn a disc.

Common Pitfalls

  • Wrong sample rate. Most CD-burning software expects all files to be at 44100 Hz.
  • 2-second gap added to the end of each track. Your CD-burning software may provide an option to remove this gap (if you need to produce "gapless" CDs).
  • Unsupported formats. Some CD-burning software does not support compressed formats such as MP3, Ogg, or M4A/AAC.
* 2-second gap added to the end of each track. This is produced by the CD-burning software and is usually the default setting. Most CD burning software provides the option for "gap-less" tracks provided that it is supported by the CDRW drive.

Gapless burning requires that the CD writer supports DAO.
I think that the 2 second gap may be a recommendation of red book standard, though I'm not sure about that. Steve the Fiddle
  • Peter 18Nov9: The basic specifications state that
  1. Maximum playing time is 79.8 minutes
  2. Minimum duration for a track is 4 seconds (including 2-second pause)
  3. Maximum number of tracks is 99
  4. Maximum number of index points (subdivisions of a track) is 99 with no maximum time limit

Importing data from CDs

Audacity does not contain any function for importing (ripping) audio from CDs. This can, however, be achieved by using other programs to extract the data into a file format Audacity does support.

Introduction

Users new to audio editing are often surprised to find that they cannot import the audio from CDs into Audacity with the File > Import > Audio command. In fact, most operating systems don't actually allow the import of data from the CD tracks into applications, because audio CDs don't have files or a file system like computer media, but consist essentially of a stream of bits on the disk. That is why when you look at an audio CD in a file manager like Windows Explorer, each CD track will appear only as a small .cda "file" 44 bytes in size, which is merely header information for the stream.

So in order to import tracks from an audio CD, you must first usually extract (or "rip") the tracks to a WAV or AIFF audio file using CD extraction software. Then you can import that WAV or AIFF into Audacity with the usual File > Import > Audio command. You can also extract CDs to the much smaller MP3 format, but this is not recommended if you want to edit the audio in Audacity, because every time you encode to MP3 you lose audio data. On the other hand WAV and AIFF are lossless. You can always export your audio from Audacity to MP3 after you've edited it, but to save un-necessary losses, import the audio in the first place as WAV or AIFF.

Windows

For users on Windows, Audacity recommends CDex as a fully featured CD extraction program which can extract to the WAV format you need for editing the audio in Audacity.

In the CDex window, simply select the CD tracks you want to extract to WAV and press F8 or Convert > Extract CD track(s) to WAV. Normally, every CD track will be extracted to its own audio file, but CDex also has a nice feature that lets you extract any range of audio (including all of it) to a single file. So if you want to extract a sample of two CD tracks that starts in the middle of one track and ends in the middle of another, you can. To do this, right-click on any of the tracks > Extract partial CD Track, or press F10.

Make sure you know where to look for the exported WAV files when you import them into Audacity. By default CDex saves the WAV to one of your Documents and Settings folders for whatever account you are logged into at the time. If in CDex you click Options > Settings > Filenames tab and look in the second text box from the top (marked ".WAV --> MP3"), you will see the location where it saves its output files from CD extraction or file conversion.

You can also extract audio CDs to WAV with Windows Media Player 11 (click Tools > Options > Rip Music and choose "WAV (Lossless)" in the Format drop-down in "Rip Settings"), or to WAV or AIFF with the Windows version of iTunes. Earlier versions of Windows Media Player are not recommended for extracting CD audio for editing in Audacity, because they are unable to extract to WAV.

OS X

OS X users have a quick way to import CDs, because when a CD is put in the drive, the CDA tracks are mounted as AIFF files in the Finder. It's thus possible to either drag the AIFF files from the Finder into Audacity, or use the File > Import > Audio command, instead of extracting the audio. Note however that if you import CD tracks into Audacity from Finder and save them as a Project, the CD must be present next time you open the Project, unless you set Audacity to make a copy of the data. To do this, go to the Import / Export tab of Preferences and where it says "when importing audio files", check the radio button "make a copy of uncompressed audio files before editing (safer)".

Another possibility is to use Max, a free software CD-ripping and encoding application. It has full support for encoding into the FLAC loss-less audio format, which is rather tricky to set up in iTunes.

Linux

On Linux or other Unix-like systems you can use K3b for the KDE desktop or RipperX or Sound Juicer for GNOME. Or use any built-in CD extraction utility that comes with the distribution.

Burning CDs

Audacity does not enable you to burn CDs directly, so you need to export an audio file (usually a .WAV or .AIFF) from Audacity, then burn that file to a CD with burning software such as Windows Media Player (built into Windows) or iTunes (built into OS X). Either of these applications will require you to add the files for burning to a playlist before you can burn them. You can do this by simply dragging the files from the location you exported them to, into the playlist. Real Player is different and requires you to add files to "My Library" with its File > Add Files to My Library command before you can burn them to CD. See the Real Player help pages Import Media Files and Burn my own CD.

You can also use a standalone burning program like CDBurnerXP for Windows, Nero or Toast to burn your exported files. If you do this, it's always best to open the files from within that software, not drag and drop them from your file manager.

Hint: Windows Media Player 11 will not accept .WAV files for burning if they are dragged straight into a burn list, but have not previously been played in Media Player. You will see a red symbol and a "length of file cannot be determined" error. To solve this problem with a .WAV that has never been played in Media Player, drag the file into the Media Player Library or into a playlist, then from there into a burn list. Also, this version of Windows Media Player does not accept .AIFF files for burning, so you must export as .WAV. Neither of these problems affect previous versions of Media Player.

Burning different types of CD

There are two main types of CD - an "audio CD" and a "data CD". An audio CD (sometimes called a "music CD") will play on any standalone CD player, as well as as in your computer and in DVD players. A data CD (sometimes called an MP3 CD where the data it contains are MP3 files) will not normally play on standalone CD players. It will play on computers, most DVD players and in MP3 CD players. So if you want to play your CD on a standalone player, or give it to others and be sure they'll be able to play it, you need to tell your burning software to burn as an audio CD.

Audio CDs

Audio CDs always contain high quality uncompressed PCM stereo data at 44100 Hz sample rate, 16 bit resolution. So if you want to burn an audio CD, you should always export the file you want to burn as a 44100 Hz 16 bit stereo WAV or AIFF file. To configure Audacity to do this:

  1. At the bottom left of the Audacity window, set the Project Rate to 44100 Hz.
  2. If your Project does not already contain a stereo track, click Tracks > Add New > Stereo Track. It does not matter that this track is empty, its purpose is just to make sure Audacity exports your recording as a stereo file. This step is not needed if you are burning to CD with iTunes.
  3. Click File > Export and choose "WAV (Microsoft) signed 16 bit PCM" or "AIFF (Apple) signed 16 bit PCM" in the "Save as type" box.

Be sure to tell your burning software to burn an "audio CD" or "music CD" (not a "data CD" or "MP3 CD"). Always use a high quality CD-R disc, as some standalone CD players may refuse to play CD-RW discs properly.

Because audio CDs must always contain uncompressed 44100 Hz 16 bit stereo audio, they are necessarily limited on a 650 MB ("Red Book Standard") or 700 MB audio CD to 74 - 80 minutes playing time respectively. If you need more playing time (for example to try and accommodate a C90 cassette or two LPs onto one CD), some CD burners will let you "overburn" into the blank CD space so as to extend the playing time by a further few minutes, so giving you the possibility of up to 80 minutes' playing time on a 650 MB disc or up to 86 minutes on a 700 MB disc. Overburning (if your burning software and burner supports it) is always done using Disc at Once (DAO) mode in which the tracks are burnt continuously without turning the laser off. It is also theoretically possible to overburn using "90 minute" (790 MB) or "99 minute" (870 MB) CD-R discs. However there is no guarantee whatsoever that your CD burner will accept such CD-R discs, or that your CD player will play anything other than a Red Book Standard 650 MB disc burned with 74 minutes of audio.

Data CDs/data DVDs

For burning really long files to optical media, you must burn either a "data CD" or a "data DVD". For example if you burn MP3 files to a 700 MB "data CD" (which your burning software may call an "MP3 CD"), using Audacity's default 128 kbps MP3 export bitrate, this gives you over 11.5 hours' playing time on the CD. You can reduce the MP3 bitrate during the export step by clicking on the "Options" button in the export dialog box and reducing the MP3 bitrate to 64 kbps. You would reduce the quality of the exported audio but could then fit about 23 hours of audio on the CD. If your DVD player can read data DVDs containing MP3 files, then you could get for example nearly 80 hours of 128 kbps quality MP3 audio on a 4.7 GB data DVD.

Burning separate CD tracks from a long file or recording

If you have a file or recording from an LP or cassette and want to burn separate CD tracks from it corresponding to each track on the LP or cassette, see Splitting a recording into separate tracks.

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