CD-ROM, DVD, and Flash Memory Storage Technologies
CD-ROM
Emergence and Evolution of the CD-ROM
The CD technology was a creation of Philips and Sony. In the middle of the decade, the first computer CD drive units were introduced. These units used the same physical format, the CD, but they could contain not only recorded music but also data and computer programs.
- CD-R: This is a recordable CD; however, there is no possibility to delete and start again to record information.
- CD-RW: This CD can erase and rewrite many times as desired.
The Operation of CD Technology
Disc: A disc has a diameter of 12 cm and a thickness of 1.2 mm. The main material that makes up a CD is a circular piece of plastic polycarbonate. It has millions of tiny holes printed on it that encode the information stored on the disc. These holes are called pits, while areas that have not been drilled during the manufacturing process are called lands. The polycarbonate plastic piece is covered with a thin layer of aluminum, only 0.125 microns thick. Finally, a layer of transparent acrylic material is deposited on the aluminum layer, serving as protection. As for the capacity of the CD, initially, it was 650 MB and later increased to 700 MB.
Reading a CD: The reading process translates the series of lands and pits into digital data (zeros and ones). The CD has a single spiral track covering the entire surface of the disc, starting from the innermost part. The width of the track is only 0.5 microns, and the distance between two successive turns of the spiral is 1.6 microns.
The reader head consists of a laser and a photoelectric cell that captures how the laser beam is reflected on the track of the CD. The laser is reflected differently depending on whether it is incident on a land or a pit, and this difference is what the photocell captures.
Rotational Speed: The speed of rotation of the CD determines the data transmission speed of the unit. The reading speed of CD players for music is 150 KB/s. If we have a 32X reader and want to know its reading speed, we just multiply the 150 KB/s of CD-DA readers by 32.
The Redefinition of the CD: CD-R
In 1990, Philips set the standard for recordable CDs, CD-R. The CD-R had different physical characteristics from the medium used for CDs up to that point. Instead of drilling, the CD-R has tinted areas that reflect less light.
A Further Redefinition: the CD-RW
In 1997, the new format of recordable and rewritable CDs, CD-RW, was defined. In the case of CD-RW, the layer that stores the information by simulating the lands and pits is formed by a special material that can change from a crystalline to a non-crystalline state reversibly.
The Units Recorders/Rewritable CD
The new recorders could now also record in CD-RW format; these are called rewritable. For example, a CD recording drive with a speed of 52X32X52:
- Writes CD-R at 52X
- Writes CD-RW at 32X
- Reads all types of CDs at 52X
DVD Units
DVDs appeared in 1996. They were created by the DVD Forum. The advantage of DVD over CD is due to the increased capacity of the former, which goes from 4.7 GB to 17.1 GB.
DVD Technology
DVD Innovations:
- Shorter laser wavelength, which means that information can be more compressed.
- The separation between turns and the minimum length of the pit is smaller than that of the CD.
- It is possible to have 2 layers.
- DVDs can be double-sided, side A and side B.
A DVD with one layer and one side has a capacity of 4.7 GB. If it is dual-layer, the capacity is doubled, and if it is double-sided, the capacity is doubled again.
- DVD-5: One side and one layer, 4.7 GB capacity
- DVD-9: One side and two layers, 8.5 GB
- DVD-10: Two sides and one layer, 9.4 GB
- DVD-14: Two sides, one layer on one side and two layers on the other, 13.2 GB
- DVD-18: Two sides of two layers, 17.1 GB
The Speed of DVD Drives: The speed of a 1X DVD is 1.32 MB/s, about 9 times that of a 1X CD. From there, units appeared that reached speeds up to 16X, i.e., 21.13 MB/s, a limit very difficult to overcome for physical reasons.
A unit that is both a CD burner and a DVD reader and is imprinted with the label 52X32X52X16 means that it is a 52X32X52 CD burner and reader and a 16X DVD reader. The reading speed for DVD is about three times that of CD for this unit.
DVD Formats
There are two global organizations responsible for developing the DVD standard:
- The DVD Forum
- The DVD+RW Alliance
The DVD Forum is responsible for the specification of DVD-R and DVD-RW. Not all burners are compatible with the two formats. In 2004, the DVD+R DL appeared.
Flash Memory Types
Flash Memory and its Applications
Flash memory is a type of memory whose main characteristic is that it is non-volatile, i.e., it maintains its contents even when power is interrupted. The evolution of this type of memory is closely linked to the development of electronic devices like mobile phones, digital cameras, and PDAs. They are small in size and have low power consumption.
USB Pens: Not a memory card format but a flash memory that plugs directly into the computer via the USB port. They provide the convenience of not needing any reader device, as in the case of previous cards. They are also known as USB keys or USB pen drives, among other names.
Interfaces for Storage Units
- The IDE interface
- The internal Serial-ATA
- The SCSI interface
A peculiarity of this type of interface is that it is used interchangeably to connect both hard drives and CD and DVD drives. In the case of SCSI, it also applies to connecting other devices like scanners or printers.
The IDE Interface
What is the IDE interface? It is also known by the names ATA, ATAPI, EIDE, and Ultra-DMA. This interface provides very high performance at a relatively low cost. The current trend, however, is the generalization of the Serial ATA interface as a replacement.
Most motherboards feature two IDE interfaces that allow the connection of up to four different devices.
Connecting and Configuring the Interface with the Devices: Control of the IDE interface is one of the many features developed by the chipset. Each IDE interface connector is composed of 40 pins on the motherboard.
To connect the different devices, an interface cable is used with a strip of 3 connectors:
- One for the motherboard
- Two for storage devices
Like the cable for floppy drives, one of the drivers is marked in red ink across its length, corresponding to pin number 1, which should be placed on both the interface and the device attached to it.
The cable currently has 80 drivers, 40 of which transmit information from the interface, and 40 more that are connected to the ground and play a role in reducing interference between different signals, thus allowing higher transfer rates. Formerly, IDE cables only had the 40 drivers that transmitted the signal.
Of the two devices connected to the same IDE channel, one must be configured as master and the other as slave. If there is only one device on the channel, it will be configured as master. This is done with a jumper-type connector on the back of IDE devices, next to the IDE connector.
Evolution of the IDE Standard: Differences between versions of the IDE standard are based on how information is transferred from the storage unit to the system memory.
Initially, there was the PIO method, which required the intervention of the CPU. The next evolution in the method of transmission was the introduction of DMA, which consists of transmitting information between a device and memory without requiring CPU intervention.
As we can see, the DMA method did not increase the rate of information transfer but assumed a significant improvement because it freed the CPU from the burden of managing access to IDE devices. The Ultra-DMA mode of transmission was based on DMA but with some improvements that made it possible to increase transfer rates.
The Serial ATA Interface
Data transmission is carried out serially for Serial ATA, while for IDE, the transmission takes place in parallel. The data cable has 7 drivers. It had two enhancements: improved ventilation inside the computer case and access to the various components, such as memory or microprocessor. It allows data rates up to 150 MB/s. Serial ATA allows up to 300 MB/s. It eliminates the distinction between master and slave. With Serial ATA, a single device is connected.
The SCSI Interface
Allows connection of hard drives, CD/DVD drives, scanners, and printers. SCSI offers the possibility to connect more devices per interface, as well as working with higher transmission speeds. The IDE and Serial ATA interfaces are at a lower price. In the case of servers, the interconnection capacity of many devices provided by the SCSI interface is essential.
Installation of the Storage Units
On one side is the data connector, which is different depending on the type of interface used by the hard disk, and on the other is the power cord. SCSI devices have two data connectors.
In the case of the IDE interface, each storage device must be configured as master or slave. The case of the SCSI interface works differently; the devices are connected to form a chain. Each device, including the controller, is connected to a preceding and a following device. This gives a string in which we connect elements called terminators at the ends.
The case of the Serial ATA interface is simpler since each interface connects to one device.