Contents

1 CD (Compact Disk) [7]

• History: Created by Sony and Philips in 1980.
• Physical medium: A thin plastic disk some 12 cm in diameter. Information is encoded in a plastic-encased spiral track. The spiral track is read optically by a non-contact head which scans approximately radially as the disk spins. The spiral is scanned at a constant linear velocity thus assuring a constant data rate. This requires the disc to rotate at a decreasing rate as the spiral is scanned from its beginning near the center of the disc to its end near the disc circumference. The spiral track contains shallow depressions, called pits, in a reflective layer. Binary information is encoded by the lengths of these pits and the lengths of the areas between them, called land. During reading, a low power laser beam from the optical head is focused on the spiral layer and is reflected back into the head. Due to the optical characteristics of the plastic disc and the wavelength of light used, the quantity of reflected light varies depending on whether the beam is on land or on a pit. The current CD scheme encodes one bits as transitions from pit to land or land to pit and zero bits as constant pit or constant land, using NRZI (Non Return to Zero, Inverted).
• Bit-rate: (1x) 1,411,200 bps (2 PCM channels, 16 bits/sample, 22,050 Hz/channel), 80 minutes.
• Capacity: 1,411,200×80 minutes×60 ××× = 807.4951171875Mbytes
• Channel (disk) encoding: 2 levels of CIRC (Cross Interleaved Reed-Solomon Coding) for (forward) error correction.
  1. C2 level encoding: Apply a RS(28,24) (2 error correcting) code. Inputs 24 audio-bytes and outputs 28 code-bytes.
  2. Interleave the 28 bytes: This spread the erroneous bits produced by a burst error (produced by a scratch, for example) along 28 bytes. RS codes can correct most of short errors.
  3. C1 level encoding: Apply a RS(32,28) (2 error correcting) code. Inputs 28 bytes and outputs 32 bytes (total redundancy = 8 code-bytes/24 audio-bytes).

2 DVD (Digital Video Disk) [2]

• History: Invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995.
• Physical medium: Similar to CD, but with a higher pit/land density.
• Bit-rate: (1x) 11.08 Mbps.
• Capacity: (1 layer) 4.7 Gbytes.
• Channel (disk) encoding: Similar to CD.

3 Blu-ray [1]

• History: Developed by the Blu-ray Disc Association, a group representing makers of consumer electronics, computer hardware, and motion pictures. The first BR player was available in 2003.
• Physical medium: Similar to DVD, but with a higher pit/land density.
• Bit-rate: (1x) 36 Mbps.
• Capacity: (1 layer) 25 Gbytes.
• Channel (disk) encoding: Similar to DVD.

4 Hard Disk [3]

• History: Introduced by IBM in 1956 (3.75 MB). Now HD are measured in TB’s (1 TB = 1024² MB).
• Physical medium: An HD records data by magnetizing a thin film of ferromagnetic material on a disk (can be a stack of disks). Changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization.
• Bit-rate: (SATA) ≈ 300 Mbps.
• Capacity: Up to 5 Tbytes.
• Channel (disk) encoding: A non-standard number of RS redundant bits are used in order to correct non-burst errors.

5 SSD (Solid State Disk) [5]

• History: SSDs had origins in the 1950s with two similar technologies: magnetic core memory and card capacitor read-only store. Until the beginning of this century, only been used as high-performance disks in supercomputers. Now are al real option for desktop computers.
• Physical medium: A data storage device using integrated circuit assemblies as memory to store information persistently. SSD technology uses electronic interfaces compatible with traditional hard disk drives. Current core technologies are flash memories and DRAM (Dynamic Random Access Memory). Flash-based SSD are shipper but slower than DRAM-based. DRAM needs an external power supply in order to avoid data loss after turning off the system.
• Bit-rate: (SATA) Up to 600 Mbps.
• Capacity: Up to 2 Tbytes.
• Channel (disk) encoding: The same used in main RAM systems (in general, parity bits).

6 Flash memory via USB [6]

• History: Trek Technology and IBM began selling the first USB flash drives (8 MB) commercially in 2000.
• Physical medium: A data storage device that includes flash memory with an integrated USB interface.
• Bit-rate: 12 Mbps (USB 1.0), 480 Mbps (USB 2.0), 4 Gbps (USB 3.0), 10 Gbps (USB 3.1). However, USB flash drives does not read data higher than 400 Mbps and write data higher than 64 Mbps.
• Capacity: Up to 1 Tbytes.

7 Magnetic Tape [4]

• History: Magnetic tape was first used to record computer data in 1951 on the Eckert-Mauchly UNIVAC I (7.2 Kbps, 225 KB).
• Physical medium: A magnetic tape registers the bits.
• Bit-rate: Up to 240 Mbps.
• Capacity: Up to 5 Tbytes.
• Average access time: sequential access.

References