Question: Today I am standing in front of a shelf with various 74 minute CD-Rs. Should I purchase the CD-Rs with 780 MB or the CD-Rs with 650 MB? Actually, I would like to pack as much information onto the CD as possible, but provided that 780 MB CD-Rs are at least as good as 650 MB CD-Rs. Can I buy the 780 MB CD-Rs with confidence?
Answer: You have fallen for a marketing ploy. 74 minutes are 74 minutes, that cannot be disputed. Both CD-Rs, however, store about the same amount of data. You will, of course, ask why the MB statements do not agree. This is quite simple: 330,000 blocks fir on a 74 minute CD-R. If you write 330,000 blocks to the CD-R, you end up with 330,000*2.048 bytes, i.e. 681,984,000 bytes (650 MB). Some firms state the capacity as 680 MB for marketing reasons (where 'M' no longer stands for "mega", but "million"). If you then fill the 330,00 blocks of the CD-R with music data, you end up with 330,000*2.352 bytes, i.e. 783,216,000 bytes (746 MB), since music data does not require bytes for the sector header, ECCs or EDCs. To work the figures as much as possible, some firms then state the capacity as 780 MB or 783 MB.
In conclusion, one can say that 650 MB, 680 MB, 750 MB and 780 MB CD-Rs all have the same storage capacity!
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