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Resolved Question

What size the pits on a cd?

Jan 6, 2015 5:08AM PST

I have just managed to squeeze 6 hours of mp3 onto a cd and I wondered how small and how close together the pits have to be for the cd to hold this much info. I assume the pits form one long spiral, and bearing in mind how fast the disc spins, would I be right in assuming this must be many metres, kilometers long?

Many thanks
S.

Discussion is locked

hawkijo has chosen the best answer to their question. View answer
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Clarification Request
That's on the web.
Jan 6, 2015 5:13AM PST

Rather than answer I suggest we learn how to use search engines like duckduckgo.com, bing.com, yahoo.com and such.

On top of that, MP3 is compressed too so hours can be held in the same CD that once held 74 minutes (AudioCD.)
Bob

Best Answer

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Re: size of pits on a cd.
Jan 6, 2015 5:51AM PST

A guess:
1. A cd holds 700 MB of data, each byte = 8 bits, so that's 55.6 Gb.
2. The writeable surface of a CD = pi * ((5.5)^2 - (3.5)^2) square cm = 56.5 sqare cm.
3. That's (nearly exactly) 1 Gb per square cm. That's sqrt (10^9) bits per cm. That's approximately 3*10^4 bits per cm, or 3*10^6 bits per meter. So that's approximately 0.3*10^-6 meter per bit.
4. So 56 Gb is .3*10^-6*56.10^9 meter, or 18 km long, give or take a factor 2.

If anybody finds an error in the calculation, please correct me.

Kees

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Answer
What, is that @ 32kbps? ;) (n/t)
Jan 6, 2015 5:54AM PST

n/t

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Answer
I thought the pits and bumps are constant, regardless...
Jan 7, 2015 1:38AM PST

whether it's 1 hour or 6 hours recording..or am I wrong. Assuming we are using the full disc in each case.