CSE 370 Mini Research Project: CDROM Tech

 

 

Group: Confused

Dan Thompson

Jason Dang

Kenny Risk

 

Table Of Contents:

·        Objective

·        Assumptions

·        Answer

·        Conclusions

·        References

 

Objective:

 

We were asked to go beyond 1s and 0s and research on how the string “CSE370” is represented at the physical level in a number of different media.  We had a choice of media to concentrate on:  CD-ROM, bar code, magnetic tape, 56KB modem, and fluorescent lights.  Our group chose CD-ROM as the media to concentrate our efforts on. 

 

Thus, our objective of this project is to discover how the string “CSE370” is physically encoded onto a CD-ROM.

 

 

Assumptions:

 

The core assumptions we made are as follows:

 

• Each character is encoded using the ASCII standard.
• There is no need for error detection of the data string “CSE370”, so the assignment of parity bits, which is a crucial step in the encoding of digital data, was ignored.

 

 

Answer:

Physical Media

Encoding Schemes

The String “CSE370”

 

 

Physical Media:

            At the very base level a CD is a collection of ones and zeros, or rather, pits and lands, engraved on a very thin sheet of aluminum. Manufactured under very specific specifications, the pits are a mere .15 microns deep, .5 microns wide, and between .83 to 3 microns long (1 micron = 1 millionth of a meter). There are only 1.6 microns between tracks, resulting in a massive track density of 16,000 tracks per inch. During the reading process, a laser of wavelength 780 nanometers is shined on the disc as it rotates, and is reflected back. The reflected beam is passed through a beam splitter, and then into a sensor. As the laser passes over the pits and lands, the wavelength is varied, as shown below. Whenever the signal crosses zero, this is interpreted as a one, whereas anything else is interpreted as a zero.

 

Encoding Schemes:

 

As our first step, we must encode each character into a series of bits.  The ASCII standard of encoding character data assigns one byte to each character.  Our string “CSE370” can be broken down as follows:

 

Character	Hex	Binary
C	43	0100 0011
S	53	0101 0011
E	45	0100 0101
3	33	0011 0011
7	37	0011 0111
0	30	0011 0000

 

So once we have encoded our string into a series of 1’s and 0’s, we’re done you say?  Nope.  We must go through another step of adjusting the data—namely, using EFM modulation.  EFM modulation is a technique used in the coding of data onto a CD-ROM.  What EFM does is take each 8-bit segment of data and change it to a 14-bit segment of data.  This way, the 0-1 and 1-0 transitions are minimized, thus reducing the appearance of small pits, which furthermore reduces the chances of errors.

 

Under the EFM encoding standard, our string then is expanded to:

 

Character	8-bit	14-bit after EFM
C	0100 0011	10001000100100
S	0101 0011	00100000100100
E	0100 0101	00000000100100
3	0011 0011	10000100010000
7	0011 0111	00100010001000
0	0011 0000	00000100000000

 

The String “CSE370”

 

“100010001001000010000010010000000000100100100001000100000010001000100000000100000000”

 

 

Conclusions:

 

            Based on the research conducted, we were able to derive both the digital and physical form of the string “CSE370” on a CD-ROM.  However, further research is recommended, specifically on the in depth error correction schemes applied in the CD-ROM standard. Many additional forms of error correction are employed, as noted in the assumptions, such as parity check and interleaving of data, which allows for incredible error detection and correction.

 

 

References:

 

http://www.pctechguide.com/08cdrom2.htm#Manufacturing

http://www.ee.washington.edu/conselec/CE/kuhn/cdmulti/cdhome.htm

K. Pohlman, The Compact Disk Handbook, A-R editions, 1992