Optical Storage

1. Optical Storage Amy Kate MahaneyShakar Zachary McNulty Graham Northup Jeremy Whinnery

2. History In the early 1980s, digital storage had transitioned from principally magnetic tapes to circular platters of magnetic storage, such as “hard” discs and “floppy” discs. • Magnetic tapes, however, were still in widespread use for non-professional audio recording and playback. They still suffered some issues, however, notably: • They could not easily be seeked by track (some multi-track designs permitted this, but were of limited practicality). • They had to be wound for a relatively long period of time to reach the beginning or end.

3. History Magnetic tapes were still preceded by another, circular spiral-track format—the gramophone record, which had none of these difficulties, though the equipment to play one proved to be too bulky and sensitive to motion to easily mobilize. Early attempts to use optical discs to encode videos go back to 1958, when a patent was placed for an analog optical disk. Similar attempts were later placed by other companies, such as Philips, who eventually created the LaserDisc (for video), a precursor to the Compact Disc.

4. History The Compact Disc was initially designed from LaserDisc technology by Philips and Sony independently (the name coming from Philips’ Compact Cassette, which had the same diagonal measure). In late 1979, the two companies collaborated to produce the “Red Book,” a digital audio encoding standard that would later be adopted by the IEC. In 1983, the Digital Audio CD (CD-DA) was launched in Europe and North America, and was well-received, with over 400 million being produced per year by the end of the decade.

5. History • Further attempts to exploit the versatility of the media resulted in a number of standards publications, each referred to by color: • The “Green Book” described the CD-interactive, a format that was supposed to operate on players more complex than an audio player but less complex than a computer. • The “Yellow Book” described the CD-ROM (Read Only Memory), intended to describe CDs that contain computer-readable data. • The “Orange Book” described CD-Recordable and CD-Rewritable, a combination of the CD-DA and CD-ROM. • The “White Book” described Video CDs. • The “Blue Book” described “Enhanced CDs” with data and audio tracks. • The “Beige Book” described the Photo CD. • The “Scarlet Book” described the Super Audio CD. • The “Purple Book” described the Double Density CD. • Only the Red, Yellow, and Orange Book standards attained significant market share of technology, though some SACDs still hold a small market niche. These works were collectively referred to as the “Rainbow Books,” and their colorful nature lent the idiom that some non-standard disc formats were referred to as “Black Book” formats.

6. History CDs are considered “First Generation” optical media. While they contain many of the features used by optical media today, numerous advances in various technologies have caused them to mostly become superseded by 2010. CD-DA, the original audio standard, has largely been supplanted by digital audio codecs that can fit on any digital storage device, including flash drives, SD cards, or network shares, and can be played by a variety of small computer-like devices (including smartphones). Improvements to the CD have so far resulted in the Digital Versatile Disc (DVD) and HD-DVD/BluRay standards, discussed later. (and numerous others…)

7. Operation All optical discs using present technology focus the energy from a light source onto a substrate that is either reflective or absorptive. The light must be highly collimated, in phase, pure in wavelength, and focused; thus, a laser is the preferred source. Different lasing media produce different wavelengths of light, which correspond to different colors (in the visible spectrum). While various techniques exist to further modify the properties of light generated by a laser, they are beyond the scope of this presentation and often impractical to fit in a small reader/writer.

8. Operation One of the earliest practical and widespread optical discs, the Compact Disc (CD), is read with an infrared laser, with a wavelength too large to be seen by the eye. The data is recorded onto the disk as a series of pits (indentations) and lands (flat areas) in the polycarbonate layer just above the reflective layer (of some metal, usually aluminium) in a spiral track starting at the inner rim. Unwound, the track is about 5.8km in length and is capable of storing slightly more than 700 MiB of (octet) data. 5.8km is about the distance from here to exit 7 on I-890. Courtesy Google Maps.

9. Operation The smaller pits entail a smaller pitch, which means more of the writable area of the disk is usable. Consequently, the spiral track is much longer—about 12.6km, storing about 4.9GiB of data. Increases in optical disc capacity are frequently correlated with the design of the laser and optical components. The Digital Versatile Disc, a “second-generation” disk, uses a visible red laser with a shorter wavelength—permitting smaller pits. However, the DVD specification also allows for recording and reading from a second layer under the first by changing the focus point of the laser; this doubles the writable area and nearly doubles the track length—about 25km, at about 8.9GiB. 12.6km is about the distance from here to the intersection of 155 and Central Avenue near Colonie. Courtesy Google Maps. 25km is about from here to Galway via NY-147. Courtesy Google Maps.

10. Operation Blue lasers—with even shorter wavelengths—permit even higher data densities. A single Blu-ray or HD-DVD disk has a single track length of about 28km (25GiB, with the smaller pits increasing per-revolution density); each added layer adds one further (complete) track for a total of 56km (50GiB) on a two-layer disc (the current industry HD standard), and over 100km (200GiB) on experimental four-layer discs. 28km is about the distance from here to the center of Albany. Courtesy Google Maps. 56km is about the distance from Ballston Spa to Albany via I-87. Courtesy Google Maps.

11. Red Laser CD, DVD Moving

12. “Groove”

15. On Off Do not detect light When half on and half off On

16. Encoding 0 1 1 0 1 0 1 1 1 0 0 1 1 1 0 1 0 Land Pit 0 1 1 0 1 0 1 1 1 0 0 1 1 1 0 1 0 Start Bit Thus, there are two permissible encodings of the same input bitstream depending on the start bit, and they are always complementary. The conversion from octets to data stored on the pits and lands of the disc is not direct; a number of encoding schemes are used to correct and reduce error (common in damaged discs) as well as aid decoding and recovery. To ensure a predictable pattern of transitions that assist the reader in tracking and recovering data, a scheme called EFM (for CDs) and EFMPlus (for most other discs) converts 8-bit octets into 14-bit streams (or 17-bit with padding, in the case of EFM) with the property that there are at least two zeroes (three pits/lands in a row) and no more than ten zeroes (11 pits/lands) in any given run. At the lowest level, the data on the disc represents a Non-Return-to-Zero Inverted encoding, which changes from pit to land or vice versa upon encountering a 1 bit in the input bitstream.

17. Encoding Error causes loss of most of the green byte. Error causes loss of one bit of the orange, yellow, and green bytes. Above the bitstream level, another encoding is used to interleave the input bytes, which ensures that a single, contiguous error (such as a scratch) will only affect, on average, a couple of bits per byte. In practice, these correction schemes work very well; the most frequent mode of failure for damaged optical disc is losing tracking (much like a skipping record). In addition, some error-correcting bits are added, which ensures that the couple of lost bits can be recovered or compensated for.

18. CD Digital Audio In a CD-DA (a music CD), the input bitstream (the data actually encoded), by the Red Book standard, represents linearly sampled interleaved-stereo PCM data at 44.1kHz (which has since been referred to as “CD rate”; it was inherited from the approximate sample rate possible on a NTSC/PAL video cassette, which happened to be the translation media for the first CDs).

19. CD Digital Audio • Each CD consists of the following organizations, in order from most to least specific: • Frame (6 two-byte stereo samples, eight error correction bytes above the on-disk error correction scheme, and a subcode byte) • Sector or timecode frame (98 frames, about 1.3 seconds of audio), corresponding to the smallest addressable unit with which one can define… • Indices (singular index), a logical marker in the table of contents, up to 100 of which can be encoded into each… • Track, the largest entity, of which 99 can be defined. x96 Index Index Track Index Stereo LPCM Samples x6 Index Error Correction Index Though indices were defined, most non-professional CD players don’t support seeking by index. Typically, Index 0 (the first) is the pre-gap (silence) and Index 1 is music data, although there were obscure uses for other indices. The track/index scheme was also inherited by the DVD, with the names title and chapter instead; unlike index seeking, chapter seeking is not uncommon in DVD players.

20. CD Digital Audio The logical structure of the CD-DA comprises three parts, in order: the lead-in (audio silence usually with subcode data, such as the Table of Contents), the program area (all of the audio data on the disc), and the lead-out (audio silence for the rest of the unused disc). Though all frames contain subcode bytes, they are used sparingly except in the lead-in for the Table of Contents. Each subcode byte is divided into eight bits as “channels”, with each successive frame containing one further bit in that channel. Only P and Q are standardized to contain ToC and timing data; other channels were used for non-standard purposes, like karaoke text.

21. CD Read Only Memory The CD-ROM contains many specifications in parallel to the Red Book CD-DA—in particular, it borrows the concept of a sector from the timecode frame, which contains 2352 bytes across 98 frames. However, unlike CD-DA, these frames contain digital data and some header information. This header information specifies the layout of the data: In Mode 1, a third layer of error correction is added, which is appropriate for most applications and filesystems, where having even a couple of bits wrong can result in a total malfunction. In Mode 2, no extra layer of error correction is added, slightly increasing the storage capacity. This is appropriate for data which has its own error-correction built in, or for non-critical data whose integrity can instead rely on the frame- and disc-layer codes. These modes must be homogenous within a track, but may otherwise be mixed, even with CD-DA data, as is the case with mixed-mode CDs.

22. Writable CDs • All of the logical details of a CD-R and CD-RW are the same as for CD-ROMs and CD-DAs; however, the spiral track of writable discs contain a fixed variation in the spiral track called a “wobble” that also encodes some additional data outside of the track, such as: • The fact that this is a writable disc, and if it is rewritable. • Location information, to permit accurate landing of recorded data on the track. • Some metadata, including the size of this disc, the manufacturer, the dye type, and the maximum permissible recording speed. • Because these are outside of the reading track, they are ignored by compliant CD-DA players and CD-ROM readers, safely permitting backward compatibility.

23. DVD The next step in the evolution of the optical disc came with the popularity of the CD-ROM—a format that was entirely based on the usual storage idioms of computer systems instead of audio players. All (conforming) DVDs contain a filesystem, usually the Universal Disc Format or some subset thereof, as well as an ISO-9660 bridge to permit some backward compatibility with CD-ROM software (which frequently used this format). In general, this means that conforming DVDs can be written by computers with little required software for conversion, and reliably played back on any other DVD-conforming device.

24. DVD DVD-Video is the most common format for mass-produced DVDs, but DVD-Audio is also supported. Both, encoding digital files in a digital filesystem, contain formats as diverse as the multimedia formats on a modern computer—far too many to list completely here, but most generally conform to MPEG standards (including MPEG Part 3 “MP3” audio and MPEG Part 2 video) and are combined into a single MPEG program stream. Similarly, other files with other formats are used to store metadata, such as “IFO” and “BUP” files (which contain title/chapter information).

25. HD-DVD/Blu-ray • In the move to High Definition content, very little has changed in the overall structure of the Blu-ray disc: • A MPEG Transport Stream is used instead of a Program Stream (since a transport stream may contain many programs). • These discs also use a (more robust) version of the Universal Disc Format, permitting usage as general-purpose data discs as well as multimedia discs. • The codecs used permit much higher bandwidths and resolutions of information, as is required in HD content.

26. Further Developments The capacity of “usual” multi-layered discs are rapidly reaching size limits which would require modification of the technology to surpass. Currently, the trend of using narrower-wavelength lasers has essentially stalled; higher frequencies entail higher energies, and at some point these lasers will become both unsafe for consumer use and too powerful to read any small media without risk of damaging it. One of the alternatively proposed technologies is the Holographic Versatile Disc, which would store data as holographic interference patterns in three dimensions, permitting much higher data density (and, in effect, an arbitrary amount of layers). Another technology involves using flourescent materials instead of reflective ones, preventing the interference that normally plagues multilayer discs with reflective coatings. A newer proposal involves using bacterially-produced proteins as luminescent areas, but it is essentially hindered by the same laser issue.