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CD-ROM

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The medium has changed, but the geometry is the same (almost)

• CD-ROMs are random access devices. • CD, compact discs, are geometrically

similar to hard disks. • The difference is in the medium is which

the data is stored and how that data is accessed. – Where hard disks use magnetism, CDs use

light.

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Spiraling out of control

• Actually CDs are somewhat different geometrically. A CD consists of one continuous spiral rather than the concentric tracks that hard disks have.

• Nevertheless, one still talks of tracks and sectors. A CD sector contains 3234 bytes.

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It’s done with mirrors

1. A laser provides a beam of light (infrared, not visible). The beam is bounced off of a mirror. The mirror serves as the “head”, the main moving part that directs the beam of light to the data of interest.

2. After bouncing off a mirror, the light passes through a lens which focuses it onto the designated region on the disk.

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Electromagnetic spectrum

IR (infrared) is light, we just can’t see it.

←Wavelength getting smaller

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Wavelengths getting smaller →

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Upon further reflection,

3. The light is then reflected from the CD surface. The amount of light that gets reflected depends upon whether or not the surface has a pit. The binary information, 1’s and 0’s, are encoded using pits which can be detected in the amount of light reflected.

4. The light is collected (more lenses and mirrors) and sent to a photo-detector.

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Light Voltage or Current

5. The photodetector takes a light signal and converts it into a voltage or current signal which is compatible with what the rest of the computer “understands.”

• Compared to floppies in which the heads are in contact with the medium and the hard disks in which they must be incredibly close, the CD “head” remains a fair distance from the medium.

• Furthermore, the photodetector does not have to move at all, just the lenses and mirrors.

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The pits

• The CD starts off flat and then the data is written by creating the pits. – Parts of the disc that are not pitted are called “lands.”

• The lands reflect light cleanly while the pits diffuse (spread out the light).

• Thus there is a difference in the amount of light collected when the laser reflects from a land versus when it reflects from a pit.

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The same but different

• A conventional CD-ROM drive is like the hard-drive in that a spindle motor rotates the disk and the “head” is positioned radially. So data is located by finding the correct radius and waiting for the right angle (sector) to swing around. The CD even has servo information like the hard drive.

• What is different is that the hard disk rotates at a constant angular velocity, CAV, while the CD rotates at a constant linear velocity, CLV, (and thus a variable angular speed).

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Keeping the beat• Recall that with hard drives either we wasted storage

capacity (density) at the larger radii or we used zoned-bit recording to store more data there. Modern hard drives opt for the latter and thus have uneven data access rates. Data is accessed more quickly at the larger radii since more data is stored there.

• The CD technology grew out of the music industry, and there a constant data rate was important. When the head is positioned at smaller radii, the disk spins faster to ensure a constant data rate.

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Speed• A standard audio CD spins from anywhere

between 210 to 539 revolutions per minute (RPM) – depending on the head’s radial position.

• There was not much motivation to change this speed for audio CDs but when CDs started to be used for data storage, there was.

• The speeds were increased in multiplicative factors of the standard audio CD speed (2X, 3X, 4X, etc.

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CLV CAV

• As speeds increased for data reading, the technology switched from constant linear velocity to constant angular velocity. – It is too difficult to vary the speed when it is spinning

so quickly. • Halving your speed when you’re going at 10 mph is one thing,

halving your speed when you’re going 100 mph is another thing entirely

– The speeds are still reported as multiplicative factors of the standard audio CD speed.

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CD Speed

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CLV vs. CAV

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CD

• An audio CD holds about 783 MB of data. • Basically a CD is a piece of plastic. • The plastic has small pits (or bumps)

organized in a long spiral. • The plastic is sprayed (sputtered) with

aluminum to provide a reflective surface. • Then the aluminum surface is covered with

more plastic for protection. (the label side).

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What is a pit when viewed from the label side is a bump viewed form the other side, which is what is actually done.

The data is read through the bottom but is stored closer to the top.

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Writing

• CDs which the user can write to are made differently.

• A CD which can be written by the user once is called CD-R. The “R” is for “recordable.” A.k.a. “write once.”

• A CD which can be written many times by the user is called CD-RW. The “RW” is for “rewritable.”

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Sizes• A track is about half micron (millionth of a meter)

wide. • There is 1.6 microns between tracks. • A pit/bump is 0.5 microns wide (the width of the

track), as short as 0.83 microns long and 125 nanometers high.– The length varies depending on the data. – A nanometer is a billionth of a meter.

• The spiral would be 3.5 miles if it were stretched out (unrolled).

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CD Drive Parts

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Connectors and Jumpers

• CD-ROM connectors and jumpers are fairly standardized. – A four-pin power connector. – 40-pin data connector for IDE/ATAPI or 50-pin

connector for SCSI. – Jumpers (different for ATAPI and SCSI)– Audio connector: 3- or 4-wire connector goes

to the sound card so one can play audio CDs.

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CD Drive Form Factor

• CD-ROM drives fit into a standard half-height bay (5.25 inches wide and 1.75 inches high).

• Tray-loading CD-ROM drives, the standard kind, must be mounted horizontally.

• Caddy-based drives can be mounted vertically but typically are mounted horizontally.

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CD Formats

• Basically, all CDs are the same, pits and lands are used to store binary information. However, CDs have different formats, i.e. different ways of organizing and encoding the information.

• A CD’s format is somewhat like the idea of the file system of a hard disk.

• A given CD drive may not understand all of the formats.

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Coloring Books

• When one is discussing the specifications of various CD formats, one talks about the color of the book.

• For example, the specifications for standard audio CDs (CD-DA, digital audio) are kept in the red book.

• The specs for CD-ROM EA (extended architecture) are in the yellow book.

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Books and Formats

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CD-DA

• The first CDs were audio CDs. • The standards for this format were set in 1980 by

Philips and Sony. They constitute the “red book.”• Since this was the first set of standards, it includes

both the physical standard as well as the logical standards.

• The physical standards include the size and shape of the disk as well as how the data is read.

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Digitizing

• Consider for example an analog voltage signal. It can be continuous in two senses: 1. the voltage varies continuously in time

2. At a given instance, the voltage can take on any value from a continuum

• To digitize the signal, the time continuum and the voltage continuum have to be converted into discrete sets of values.

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Analogy: Digitizing an image

Discretize spaceDiscretize color

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Sampling

• Breaking up the time continuum is known as “sampling.”

• Motion pictures are an example of sampling: a rapid succession of snapshots (still pictures) are taken, if the sampling frequency (the number of pictures (frames) per second) is sufficiently high, the brain perceives the playback as continuous motion.

• Muybridge demo

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(pseudo)-Analog wave

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-1

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Continuous in time

Con

tin

uou

s va

lues

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Sampled Wave

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One of Nyquist’s Theorems

• Signals can be thought of as being comprised of sine waves of various frequencies (Fourier).– Demo

• Nyquist says that to accurately represent a signal, one’s sampling frequency must be at least double its highest constituent frequency. – For example, in the phone system the choice was made

to sample at a frequency of 8000 Hz.

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Nyquist Sampling Example• In the following sequence of graphs, a sine wave

is sampled. • The frequency of the sine wave is doubled each

time, while the sampling frequency is kept fixed.– Case E does not resemble a sine wave but alternates up

and down with the correct frequency– Case F oscillates very quickly (alternating up and

down), but its amplitude seems to vary at a much lower frequency. This was not a feature of the actual wave being sampled.

– Case G only has the slowly varying feature when the actual wave sampled varying quite rapidly.

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A: sf=10, f=0.159sf: sampling freq. F: freq.

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B : sf=10, f=0.318

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C : sf=10, f=0.637

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D : sf=10, f=1.273

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E : sf=10, f=2.546

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F : sf=10, f=5.093

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G : sf=10, f=10.186

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The other half of the problem

• At the instance one is sampling, the signal can still take on an infinite number of values.

• Digitizing requires one to choose a discrete set of allowed values. – For example, to digitize an image one can choose two

values (black and white) or allow for shades of gray or allow for combinations of red, blue and green, etc.

• For example, in the phone system, it was decided that 256 values would be allowed. – 256 values can be represented by 8 bits.

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Sine: 5 values

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Sine: 9 values

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Sine: 17 values

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Sine: 33 values

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CD-DA Sampling• The phone system uses a sampling frequency of

8000 Hz and uses 1 byte (256 levels) to represent the possible values of each sample.

• A higher quality sound is expected from CDs, the red book specifies a sampling frequency of 44,100 Hz and use 2 bytes of data (65536 levels) to represent the possible levels of each sample. And the sampling is done in stereo.

• This corresponds to 176,400 bytes /second. 176,400 = 44,100 2 2

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CD-DA (Cont.)• In CD-DA, the disk is broken into blocks or

sectors.

• A sector has 3234 bytes. In CD-DA, 2352 of those bytes are actual data. The rest is – Data for timing and location (the CD analog of

the hard disk’s servo information) – 98 bytes – Error-Correction-Code (ECC) and Error

Detection Code (EDC) – two sets of 392 bytes

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Capacity and Rate• The CD-DA sampling specs require

– 176400 bytes/second – 10584000 bytes/minute– 10336 kilobytes/minute– 10 megabytes/minute (actual data)

• That is, one minute of CD audio (uncompressed) corresponds to 10 MB.

• CD-DA specifies a capacity of 74-minutes of digital audio or approximately 747 MB of actual audio data.

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Fixing Mistakes

• CDs have a lot of ECC so that errors can be fixed. • If the data for a given sample cannot be recovered

using ECC, one can interpolate. Assume the bad sample is halfway between the previous and the next sample.

• Interpolation is not available to CDs used to store data, so they have even more space devoted to ECC.

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CD-DA Recap

• The CD-DA standard as set out in the “Red Book,” defines a sector or block of 3234 bytes.

• Of those 3234 bytes, – 2352 are actual audio data– 98 are control (analog of servo information,

synchronization and location)– 784 are EDC/ECC

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CD-DA Percentages• Each CD-DA block or sector is

– 72.7% actual audio data– 24.2% EDC/ECC– 3% control data

• For every 8 bits of data, there are 3 more bits of control/error data.

• If a CD-DA error cannot be corrected, one can interpolate.

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CD-ROM

• The red-book CD-DA standards were adapted by the “Yellow Book” standards to handle non-audio data, i.e. regular data files, programs, etc.

• These standards are for CD-ROMs, compact disc – read only memory –information written by the manufacturer and not changed by the user.

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By any other name• The original CD-ROM standard did not completely specify

the logical structure (i.e. they did not nail down the analog of the hard disk’s file-system).

• This was leading toward proprietary solutions and incompatibility.

• The leading manufacturer’s met at the High Sierra Hotel and Casino in Lake Tahoe, NV, in 1985, to standardize CD-ROMs more fully.

• The outcome is known as the High Sierra Format.

• A slight modification was adopted by ISO as their standard (#9660).

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CD-ROM Mode 1• CD-ROM Mode 1 starts with the basic CD-DA

sector division 3234 = 98 (control) + 784 (error)

+ 2352 (data)and devotes some of the data portion to additional error code and control yielding 3234 = 98 (control) + 784 (error)

+ 304 (more error/control) + 2048 (data).

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CD-ROM Mode 1 Percentages

• Each CD-ROM block or sector is – 63.3% actual data– 33.6% EDC/ECC– 3% control data

• More than one-third error detection and error correction code

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Less Data/Fewer Errors/More Control

• The CD-ROM standards impose more control because one must be able to locate data with more precision.

• The CD-ROM standard imposes more error detection and error correction because by the nature of the data it stores it does not have available the interpolation approach to dealing with errors that CD-DA does.

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CD-DA Review

• SPEED: 44,100 Hz sampling frequency, 2 bytes to represent the levels and another factor of two for recording in stereo– 176400 bytes/second. – 75 blocks/second using 2353 bytes/block

• CAPACITY: 74 minutes of music – 333000 blocks on a CD– 783549000 bytes or 765184 KB or 747 MB.

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Mode 1 Speed and Capacity

• SPEED: – 75 blocks/second – 153600 byes/second (using 2048 bytes/block) =

150 KB/s

• CAPACITY: – 333000 blocks – 681984000 bytes or 666000 KB or 650 MB.

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Speeds• The 150 KB/s is the base data transfer rate for CD-ROM.

• Higher speeds are reported as multiplicative factors of this base: 2, 3, etc.

• Recall that as disc speeds got higher, they switched from CLV (constant data rate) to CAV (variable data rate).

• The latter data rates may be reported with the term “Max” to indicate that the reported data rate is the maximum of the range (i.e. When one is on the outer edge of the CD).

• Such drives must still support CLV at lower speeds so that they can play audio CDs.

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The Other Speed Factor

• The data transfer rate is the rate for reading consecutive data. Another speed factor is average access time which is the typical time required to locate a “random” address.

• As with hard disks, this involves the radial positioning of the head (seek time) and the angular spinning of the disk (latency).

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CD-ROM Mode 2

• The CD-Rom standards define a second mode that contains less error correction code. Mode 2 has

3234 = 98 (control) + 784 (error)

+ 16 (more error/control)

+ 2336 (data).

(which is 72.2% actual data)

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Going Backward?

• Mode 2 almost returns to the the CD-DA standard and was meant for data such as compressed audio or video which didn’t require as much data integrity as regular files (again because interpolation becomes available).

• The original Mode 2 standard was not used.

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References

• www.pcguide.com

• PC Hardware in a Nutshell, Thompson and Thompson

• http://entertainment.howstuffworks.com/cd1.htm