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Why do 700 MB blank discs only fit what iTunes says is about 150 MB of songs?

I've always assumed that it writes the songs in a different way than they're stored on the computer's hard drive.

Is this why, or is there another reason?

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It sounds like your trying to burn a compact disk as an audio disk which is limited to 90 minutes. – Ramhound Mar 12 '12 at 12:03
up vote 6 down vote accepted

If you are burning the music using iTunes, or with some other software to make an "Audio CD", they are in a radically different format.

CD Audio is a format that stores uncompressed, 16-bit, 44.1 kHz audio, which is about 1411 kbps.

The iTunes Music Store sells compressed audio - 256 kbit streams, and other sources often use lower bitrates - as low as 64 kbit, but frequently 128 kbit or 192 kbit streams.

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Do most audio programs really respect the kilo/kibi distinction? I always assumed that 128kbps referred to 128Kib/s. – Lèse majesté Mar 11 '12 at 21:43
I am too old and set in my ways to remember that distinction exists out of the box; I meant traditional-and-wrong power-of-2 numbers everywhere. – Daniel Pittman Mar 11 '12 at 21:45
Well, your usage is correct regardless. Before the kilo/kibi distinction was made, kilo was used to refer to both. But with the (still rarely used) IEC system, kibi is officially 1024 while kilo is 1000, which is how the 1411 kbps figure is arrived at. – Lèse majesté Mar 11 '12 at 21:51

Audio CDs are a different format (as @kotekzot mentions) to mp3 or flac (or what ever format you are using). It's uncompressed and so takes up more space on the disk than any compressed format.

Wikipedia has more information on the format and the history of why it was chosen. The format of an audio CD is officially called Compact Disc Digital Audio or CD-DA and all CD's have to adhere to this format to be able to include the "CD Audio" logo on them.

The 44.1 kHz sample rate was chosen primarily on the need to reproduce the audible frequency range of 20 Hz – 20 kHz - approximately that of the average human

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Because Audio CDs have a bitrate of 1411kbps (uncompressed 2-channel 16-bit 44.1KHz audio), while regular lossy-compressed music is generally about 192-256kbps. If your devices can handle compressed formats (MP3, FLAC, OGG, etc), you would be better served by burning the files to a disk directly, rather than burning them as CDDA disks.

More info

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