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CD’s. 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 main difference is in the medium is which the data is stored and how that data is accessed.
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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 main 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.
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.
It’s done with mirrors • 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. • After bouncing off a mirror, the light passes through a lens which focuses it onto the designated region on the disk.
Electromagnetic spectrum ←Wavelength getting smaller IR (infrared) is light, we just can’t see it.
Upon further reflection, • 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. • The light is collected (more lenses and mirrors) and sent to a photo-detector.
Light Voltage or Current • 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.” • Furthermore, the photodetector does not have to move at all, just the lenses and mirrors. • Comparison • Floppies: the head is in contact with the medium • Hard disks: the head must be incredibly close to the medium • CD’s: the “head” remains a fair distance from the medium.
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 (specularly) 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.
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).
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 ratewas important. When the head is positioned at smaller radii, the disk spins faster to ensure a constant data rate.
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.
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.
CD Speed The word “Max” refers to this change from CLV to CAV.
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).
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.
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.”
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).
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. • Going toward SATA? • Jumpers (different for ATAPI and SCSI) • Audio connector: 3- or 4-wire connector goes to the sound card so one can play audio CDs.
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.
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.
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.
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.
Digitizing • Consider for example an analog voltage signal. It can be continuous in two senses: • the voltage varies continuously in time • 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.
Analogy: Digitizing an image Discretize space Discretize color
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
(pseudo)-Analog wave Continuous values Continuous in time
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.
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.
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.
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
Human-based sampling rate • Humans hear sound in the range 20 to 20,000 Hz. • Double the highest frequency (a la Nyquist) giving 40,000 (the actual number used is 44,100). • Use two bytes per sample. • Record in stereo. • Result: 44,100 2 2 = 176400 bytes/sec.
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
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.
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.
