Digital Audio Media - Optical Media

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This kind of support makes use of the principle of light-reflection to access stored data. At a later stage in this section we'll be taking a look at the most commonly known optical support, namely, the Compact Disc. Before we move on let's briefly give a definition of the rotation speeds we'll be encountering in the next paragraphs.

  • CAV (Constant Angular Velocity): in this case the disc containing the information rotates at a constant speed. This allows quick access to data but doesn't make efficient use of the space available for memorization. Indeed the data is disposed along various concentric circumferences inside the disc; to always have the same amount of data read by the head in the time-unit, we will need to put less data in the outermost circumferences and more in the innermost ones (computer Hard Discs work this way).

    Digital Audio Media  - Data format in CAV mode

    Data format in CAV mode

  • CLV (Constant Linear Velocity: in this case the distribution of data need not be constant because the rotation speed varies according to the position of the head (data-reading in Compact Discs takes place this way). The data shall thus be evenly distributed over the support's entire surface:

    Digital Audio Media  - Data format in CLV mode

    Data format in CLV mode

    The support is engraved by a laser beam which creates what are called pits on its surface; the areas that don't get engraved are named lands. The pattern of the engravings corresponds to that of the digital signal to be stored, more specifically a transition from pit to land (or vice versa) corresponds to the state of 1 whereas the absence of transitions corresponds to the state of 0. The following diagram illustrates this:

    Digital Audio Media  - Recordings on an optical support

    Recordings on an optical support

    The pits' depth is λ/4 where λ is the incident waveform's wave-length. This way the wave that penetrates the inside of a pit and is then reflected, covers a λ/2 (2*λ/4) path, which implies phase-cancellation. Therefore, the reflected wave cancels the incident one. When, on the other hand, the recording wave encounters a land, it simply gets reflected. This way the two states 1 and 0 are reproduced through the presence, or lack thereof, of a reflected wave. The reading of optical support thus occurs by means of a laser beam which gets beamed onto the support's surface and whose reflected wave is measured with a detector called photo-diode[41 ]:

    Digital Audio Media  - Data-reading on an optical support

    Data-reading on an optical support

    The data on the CD is spread out on a single spiral-track which unfolds from the centre of the CD outwards. As already said, the type of rotation speed is CLV. The following are the minimum and maximum speeds:

    Internal: 500 rpm

    External: 200 rpm

    The spiral-track's width is 0.6 μm, whilst the recorded pits' width is 1.6 μm.

19.3.1. Tracking

The optical head that reads/writes the/on-the CD must point directly towards the vertical line the data is disposed along. It may be, considering the tiny dimensions we're dealing with, that this alignment isn't perfect, which might lead to incorrect reading of the data. This may happen either because the data-track shifts slightly in relation to the vertical line, or because the whole disc may lean out of position for some reason, altering the correct alignment. That's why two correction systems are used that constantly realign the laser's target with the data-track.

  • Horizontal tracking: this operates when the data-track shifts out of line with the laser's vertical beam. It is brought about by adding two lateral laser beams to the central one. The intensity of the lateral lasers' reflection is constantly monitored and must always remain the same. As soon as the intensity varies, the head is readjusted until correct alignment is restored.

    Digital Audio Media  - Horizontal tracking example

    Horizontal tracking example

  • Vertical tracking: this operates when the whole disc moves out of position. This kind of situation is monitored by adding two cross-firing lasers:

    Digital Audio Media  - Vertical tracking example

    Vertical tracking example

In this case too, if correct alignment is lost, the head moves until the correct position is reestablished.



19.3.2. CD printing

Printing a CD is a very expensive process which entails many manipulation phases, which we will illustrate shortly. The CD burning process that is carried out using a computer equipped with a burner, is totally different and incomparable in terms of quality. There's no doubt: burnt CDs that are copies of original CDs are far inferior in quality. The main reason is that the precision of the recording resulting from the printing process is far greater compared to when you burn a copy from an original CD, where a laser is used to create the recording. Having set the record straight (excuse the pun), let's take a detailed look at the different phases composing the printing process of a CD with reference to the following diagram:

Digital Audio Media  - CD printing process

CD printing process

  • A plastic disc is smoothed.

  • The disc is covered with photo-resistant material which allows laser beam recording.

  • The external layer of the disc is recorded with a laser beam. This translates the data pertaining to the audio signal into optical signals.

  • The engraved layer is cleaned.

  • The resulting master is covered with a layer of silver.

  • A layer of nichel is added.

  • The master that has now been obtained is called father and it is a negative copy. This copy is used to create a positive master (called mother) made of nichel and silver.

  • Casts are made from the mother.

  • Every cast is used as a master for the serial printing of the normal CDs. These then get pressure-engraved and subsequently covered in a layer of aluminum ensuring good reflection. The mother's "new born" casts are then delivered all over the world (by a white stalk) to the various distribution-chain branches who in turn will be able to print off their own CDs out of this matrix.

  • A final layer of polycarbonate is added to the disc with the double scope of protecting the disc's surface from scratches and oxidation as well as to act as a magnifying glass for the laser that reads the engravings on the disc. This phase is carried out locally at each branch, and effectively 'finishes the job off' producing the CDs that are then sold in shops.



19.3.3. Data format of a CD

Data is stored onto CD in a format that subdivides it into three sections:

  • Lead In: it is located in the inner part of the disc and contains a description of the tracks: their number, length and overall length of the CD.

  • Data Block: essentially samples of the digital signal transporting the actual sound information.

  • Lead Out: consists in a series of bits indicating the end of the CD.

As we have said, the Data Block section contains the data pertaining to the stored audio signal. The organization of the data is quite elaborate in order to obtain different results. Let's take a closer look. First of all the bit-flow is lengthened, seeing that laser systems do not allow excessively close transitions between two states (0 and 1). So, each 8 bit word is converted into a 14 bit word by way of an algorithm defined as 8-14 modulation applied both in the writing (encoding) phase and in the reading (decoding) phase, in order to reduce the frequency of transitions. The data to be stored (before the above-mentioned algorithm is applied) is divided into frames (sections). The first 8 bit of each frame are available for the constructor to insert data regarding the track (its number and length). Thereafter, 6 audio samples are inserted into the frame, 3 for the left channel and 3 for the right (namely, 6 x 16 bit = 96 bit). Finally the parity bits are added [42 ].

A standard frame looks like this:

Digital Audio Media  - Organization of the data within a frame

Organization of the data within a frame

The data is distributed down a spiral, not sequentially but scattered over various areas of the disc. The data pertaining to a single music track is thus spread all over the disc in different areas. This way if a speck of dust or anything else prevents the reading of the data in a certain area of the disc, the damage done is brought to a minimum. This data-distribution system is called CIRC (Cross Interleaving Reed-Solomon Code).

When we listen to a dusty or ruined CD we sometimes don't hear any sound degradation whatsoever (in actual fact there is, but in order to pick it up you need appropriate equipment and a very careful ear). This is possible thanks to an error-correction system that is present in CD players which recalculates the missing samples of a sequence (due to the player not being able to read them because of dust, scratches or disc skipping), by inserting new samples which likely resemble the original ones. If for example, a sample is missing in a sequence, its amplitude can be extrapolated as the average of the previous and the following samples. It is clear that the more samples are missing, the more their reconstruction will be inexact. When the calculation cannot be made, because of too many missing samples, silence is produced until a correct data reading is once again available.



19.3.4. Definitions of CD formats: the Grovening Books

The same CD Media is used to store different types of data, including audio data. The specifics pertaining to each format are contained in a set of official reference documents called the Grovening Books. Each book has its own colour and defines the specifics of a particular format. Let's get to know them:

  • Red Book

    Audio CD: these specifics include the encoding of the 16 PCM-type quantization-bits and the sampling frequency, which is set at 44.1 KHz.

    CD+G: used for Karaokes. They allow track-lyrics to be incorporated in the audio data.

  • Yellow Book

    CD-ROM: CDs used for storing data in different formats (audio, video, texts, images). One of its standard parameters is its capacity, fixed at 650 Mb.

    CD-ROM XA (eXtended Architecture): the data is distributed throughout the disc in a way that is similar to CIRC with audio-CDs.

  • Green Book

    CD-I: interactive CDs which contain information in different formats (audio, video, images).

    Sony PlayStation: for games played on the famous games-console.

  • Orange Book

    Photo CD: a format that was created by Kodak for the storage of photo images.

    CD-R: Compact Disc Recordable, CDs on which data can be written only once.

  • White Book

    Video CD: storage supports for MPEG format-compressed films.

    LaserDisc: 12" film-storage media. Allows the use of Dolby Prologic [Dolby Pro Logic and Dolby Digital ] .

  • Blue Book

    Enhanced Music CD: Often only called CD-Enhanced, CD-Extra or simply CD-Plus or CD+. CD enhanced Music is a CD containing two sessions. The first session includes the audio data as it is defined by standard Audio CDs (Red Book), the second session contains data (Yellow Book). So, on the same CD we can store both audio data and raw data.



19.3.5. DVD

DVDs are optical discs that are similar to normal CDs but have a far greater data-storage capacity (8.5 GB, which is the equivalent of 13 CDs). A DVD disc is made up of 4 main layers: one thick layer of polycarbonate on which the rest of the layers lie. Then we have a finer opaque layer made of reflecting material. On top of this we have a thin transparent layer, and finally a protective layer made of plastic. Pits and lands are located in the two intermediate layers, but compared to CDs the pits in DVDs are far smaller, which allows more data to be stored. This is why the laser beams used for writing and reading data have an inferior wave-length than CDs.

The DVD is effectively employed for the audio-video reproduction of live events. The adopted audio encoding is the Dolby Digital format [Dolby Digital 5.1 ] which can be configured in many ways, such as the 5.1 surround or the simple stereo 2.0. This audio format is based on the AC-3 compression algorithm and then the quality of the audio tracks is not outstanding, even if this can be perceived only using top quality equipment and with well-trained ears.

Sometimes, the audio is encoded in the DTS format [Digital Theatre System (DTS) ] , which allows a better channel separation and a better quality on the compressed data.

It's worthwhile pointing out that DVD if effectively employed for the reproduction of lyric operas, where subtitled act as the libretto allowing to fully enjoy of the beauty they hold.



19.3.6. Blu-ray Disc

This kind of optical support have been firstly released by Sony in 2002 and is employed for the storing and reproduction of high-definition television formats. As for the other optical supports, it is read by a blue-violet laser (hence the name) with a wavelenght (405 nm) inferior to the DVD readers (which employ a red-colored laser with a wavelenght of 650 nm) and then is able to decode much smaller pits and lands [Optical Media ] . This allows for more information to be stored onto this optical support, which has the same physical dimensions of CD and DVD.

There are many kind of blu-ray discs with different data capacity. The available space is around 50GB even if technology is evolving fast, allowing for lasers with even smaller wavelenght and for multiple layers within the support. There are already some prototypes allowing for the storage of a bigger amount of data.

From the audio point of view, the interesting thing is that with this support the audio tracks are available in a non-compressend format. The entire 5.1 matrix is available in PCM, 48K / 24 bit format for every single channel, which means a far better quality compared to audio CDs.

The Playstation 3 was the first commercial device featured this technology.





[41 ] A photo-diode is an electronic component capable of generating a current when it is hit by a beam of light (photons).

[42 ] Parity check is used to verify the integrity of the byte sequences. It is brought about by adding a set of extra controlling bit to the tail-end of a bit-sequence. For example, say we send the following 3 bytes:

00100101

11100100

01001010

If we carry out a binary addition on the columns we can calculate whether the result is odd or even and indicate it with an additional bit. For example the first column starting from the left is 010, therefore the addition gives us an odd number which we will indicate with the number 1. If we follow the same procedure with each column, we'll have the following parity bits:

10001011

The moment they are received, the parity bits are compared to the received bytes and if there is no correspondence it means that an error has occurred and therefore the sending of the byte sequence is re-requested. This kind of control is very quick and easy to put into practice, although it doesn't guarantee the detection of all errors. By adding other control bits we can carry out more sophisticated algorithms for the correction and controlling of errors.








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