# Thread: Capacity of a cd

1. basically im doing a coursework on cds and i come to this equation
Data capacity in Mb for an audio-CD
74 min
= 333,000 sectors * 2352 bytes / sector
= 783216000 bytes
= 746.9 Mb

and i dont get how from 783216000 bytes they get 746.9 Mb

i kno theres 8 bits in a byte but when u times 783216000 by 8 u dont get 746.9

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3. Assume 1024 bytes = 1KB, 1024KB = 1MB

783216000 / (1024x1024) = 746.932

Shouldn't this be in hardware world?

4. Indeed.

They only call it KB as a matter of convenience.

Are CDs considered hardware.

5. this was the only place i could think of to get a reply, but thanks for the answer was exactly what i was looking for, also could you give me any ideas about whats physicsy about cds so far ive got info on pits and grooves of a cd and how much storage capacity it can have but im a bit too out of ideas

6. have you got all the cd trivia stuff like:
the pits were close to the smallest man made thing when first done
the length of the track (spiral) is xxx metres long
cds are partially made out of polycarbonate which is same material as bullet proof glass

Other than that you could talk about manufacture, ie master copy and stamping the others off it. You could talk about the structure of them ie tiny metal layer between plastic sheets. Or you could talk about how cd-r / rw work. You could talk about how the reader works with lasers tracking one reads it by destructive interference etc.
Or you could jsut copy a website like:
http://electronics.howstuffworks.com/cd.htm

edit: arsebiscuits jpaul beat me to the link

7. yea ive pretty much got all the trivia stuff. i think im going to talk about angular velocity.

speed of cd surface = 2*pi*r/t
t is time and r radius

but i dont know how many revolutions a cd makes per second, it makes 500 in the center per minute and 200 on the outsides per minute, i want to also talk about

at these high speeds, friction with the surrounding air particles is high. This results in the generation of thermal energy which consequently heats up the cd

and how the material of the cd has to have a higher melting point then the amount the cd heats up but i cant find out how to find out how much it heats up

EDIT: the destructive interference can you elaborate on that a bit more

8. "it makes 500 in the center per minute and 200 on the outsides per minute, i want to also talk about "

Woah there tiger, every bit of the CD makes the same number of revolutions per minute. Otherwise it would break up. The outside travels faster than the inside, because it must cover a greater distance in the same amount of time.

9. Constant Linear Velocity (CLV) is the principle by which data is read from a CD-ROM. This principal states that the read head must interact with the data track at a constant rate, whether it is accessing data from the inner or outermost portions of the disc. This is affected by varying the rotation speed of the disc, from 500 rpm at the center, to 200 rpm at the outside. In a music CD, data is read sequentially, so rotation speed is not an issue. The CD-ROM, on the other hand, must read in random patterns, which necessitates constantly shifting rotation speeds. Pauses in the read function are audible, and some of the faster drives can be quite

That is where i got that information from, if its the same speed Jpaul then what would that speed be?

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