Science of making CD's

[KA4HJH]

>Thanks for the reply.  Here's another question for you.  A regular CD-ROM has
>about 600 meg of memory.  I take it this is the same for an audio CD.

CD's were originally designed for audio so the capacity is more properly
referred to by time, rather than bytes. A standard CD/CD-R has 333,000
sectors. On a CD-DA (digital audio) each sector is 2352 bytes, which is
about 747MB/74 minutes. A CD-ROM's sectors are 2048 (2kB) because there is
an error correction header. This leaves about 650MB for data.

There is a bizarre twist here: because of the space used up by the error
correction, a CD-ROM can only hold about 87% as much audio (stereo, 16 bit,
44.1kHz) as a CD-DA! This limits the capacity to about 64 minutes. On the
other hand, CD-ROM files can be read at full drive speed by any computer,
while reading CD-DA's faster than 1X usually requires a special program and
is more prone to error at higher speeds.

The data rate for a CD-DA (at 1X) is 150kB/s. This works out to 75 sectors
a second, so the fractional part of any time measurement is in 75ths of a
second. The rotational speed of the disc varies from about 500 rpm to 200
rpm from the inside to the outside to maintain this data rate. This means
that a 32X CD reader is whizzing around at up to 16,000 rpm! (and over
4MB/s--faster than a lot of hard drives can sustain). No wonder they tell
you not to put shaped discs in them!

The price of CD-R drives or "burners" is dropping fast, and the price of
blank media is about $1 apiece in hundreds. If you have a computer and are
making music, they are very, very nice.

>What
>about the new DVD disks?  How much memory do they have?  Is it possible to use
>the DVD format (because of it's greater memory size) to save other kinds of
>data besides video?  If they can, then why don't all CD-ROMS go to this
>greater
>storage format?

I don't have the numbers at hand, but DVD has
  shorter wavelength laser = smaller pits = more pits = greater capacity
  TWO lasers at different wavelengths = double capacity
  Double-sided discs = double the capacity again

Since this is the second time around for digital optical disc standards,
they've tried to correct the mistakes and omissions they made the first
time, and anticipate the needs of the future. Don't be fooled by the name,
this is just another optical disc and can be used to store anything. DVD
drives are being shipped in a lot of new computers and DVD-RAM (erasable)
drives are already available.

I can't remember exactly but off the top of my head I think a single-sided
DVD can store something like 5 or 10 hours of audio. Something paltry like
that. The video is, of course, well compressed.


I'm not a genius. I had to go look this stuff up to get the numbers
straight. Actually, I'm a SubGenius(tm).

Terry Bowman, KA4HJH
"The Mac Doctor"