Digital Storage Media
DSM may be divided into 3 categories:
Magnetic
Optical
Magneto – Optical
and also 3 sub categories
Read and write ( CDRW, MO-Disk)
Read Only (DVD, CD)
Limited Write (CDR, DVDR)
1: Magnetic Digital Storage Media
Drawbacks of analogue magnetic recording does not apply, no erase heads and bias are needed
The storage medium does not dictate the quality of the audio, as it would with analogue media.
- Construction: A disk coated with iron particles
- Read / Write heads acts as an electro-magnet, inducing a field as it passes over the disk
- Reading: The polarity of particles on the disk creates a magnetic field that induces a current in the head.
- Formatting: Disk must be formatted by the operating system. I.e. Disk are divided in sections consisting of sectors, track and blocks.
Disadvantages:
Magnetic storage media are prone to interference (speakers, TV, Power Transformers.) Mechanical disturbances and Temperature causes problems.
Advantages:
Generally very fast at locating and transforming data. Constant angular velocity (speed does not change), the velocity of the disk may be removed from the equation in terms of access time.
2: Optical Digital Storage Media
a) Compact Disk (CD)
- Plastic disks 1.2 mm thick and 12 cm diameter
- A laser beam is used to read the data
- Access times and transfer rates are slower than hard disk’s
- Follows a Cd standard of Governing books
How CD’s work
Table of Content
Cd’s must contain a T.O.C
The T.O.C
- It is found in the Q-channel (in the lead-in area of the CD)
- Defines the type of tracks.
There are 8 sub code channels included with data stored in CD’s. The audio (REDBOOK) CD needs only the P or Q code.
CD Manufacture:
- a glass plate is polished until it is optically flat ( Totally flat )
- the plate is washed and coated with a chemical called “Photo resist“
- the coated glass plate is backed to harden the “photo resist”
- a laser beam recorder (LBR) etches (burn) the “photo resist” layer
- “photo resist” surface is developed to remove the areas etched by the laser.
- “Pits” in the surface are created
- “Pits” extend through the “photo resist” to the glass plate
- This is known as the Glass Master
- The Glass Master surface is then “metalised” either with nickel or silver
- The nickel alloy or silver layer is separated from the Glass Master.
- The negative copy of the Glass Master is known as the Metal Master or Father.
- From the Metal Master a positive copy is made (of the Glass Master)
- This copy is known as the Metal Mother or Mother.
- Stampers or Sons are made from the Mother.
- Pressing is done by injection molding.
- Resulting plastic disk is covered with a reflective aluminium layer
- Lacquer is applied to protect the date surface from wear and tear.
Cd Layout
Cd’s are split in 3 areas
Lead-in area: holds the T.O.C (in the first 4mm of disk)
Program area: Holds the audio or data
Lead-out area : Marks the end of the CD
b) CD-R
- Cd-R’s are based on the same basic principles as Cd’s
- CD-R’s contain a dye (colouring) that blisters when burned by a laser
- During reading the laser reflects back normally until it hits a bump.
c) CD-RW
- CD-RW use a process called “Phase Change”
- The medium has two states that are stable but can be altered.
- The recording layer is polycrystalline (reflectivity) when a laser is applied to it.
- During recording a high-powered laser is used (creating dark areas / less reflective)
- When the disk is over written a lower intensity laser restores reflectivity
d) MD (mini-disk)
- Share the basic properties of CD’s
- Laser of same wavelength, pits of same size.
- The audio is subjected to data compression 5:1
- ATRAC (compression) – Adaptive Transform Acoustic Coding works on the principle that sound may not be excluded from representation (masking)
e) DVD
DVD vs CD’s
Same dimensions
Same principles (smaller pit sizes)
Cd’s = single layer / single sided ............................
DVD’s = Single or dual layered / Single or double sided
f) DVD-5
- single layered / sided DVD
The layers are:
1: label
2: Polycarbonate
3: reflective layer
4: more Polycarbonate
g) DVD-9
- single sided / dual layered
- capacity 8.54 GB
- semi reflective opaque layer and a reflective layer
The layers are:
1: label
2: polycarbonate
3: reflective layer
4: semi-reflective layer
5: polycarbonate
h) DVD-10
- Double sided / single layered
- Capacity 9.46 GB
i) DVD-18
- Double sided / Double layered
- Capacity 17.08 GB
j) DVD-Audio
- audio data on either linear PCM or MLP format
MLP = Meridian loss less parking
- It supports sampling rates of 44.1 kHz, 48kHz, 88.2kHz, 96kHz and 192kHz with bit depths of 16, 20, 24 bits.
- It also supports 2 to 8 track
- Support for lossy compression, AC3, etc.
- DVD-Audio allows 2 channel groups using different sampling rates / bit depths. E.g., left, right and centre at 24bit / 96 kHz and surround at 16bit / 48kHz
- Other information can be stored onto DVD-A, pictures, text, etc.
k) DVD-Video
- Video is stored using MPEG 2 video data compression. The audio data can be in one of several formats.
- Linear PCM, 2 channels 24 bit / 96 kHz up to 8 channels 16 bit / 48 kHz.
- MPEG 1 or 2, 2 channels 24 bit / 96 kHz up to 8 channels 16 bit / 48 kHz.
- DTS , 8 channels 7.1
- Using MPEG 2 a 4.7 GB disc can hold 2 Hours and 13 minutes of audio.
l) Super Audio CD
- a Sony / Phillips standard
- contains a Redbook layer of standard cd audio
- contains a second semi-transposed layer with DVD data.
- Super audio CD’s offer no video, no images or interactivity.
3) Magneto – Optical
Principle: MO-Disks combine magnetic and optical properties.
Recording: The recorded layer is covered with a material that is magnetic pliable when heated to a high temp. ( the curie point ). The heating is achieved using a high powered laser.
Playback: Light polarisation is used (magnetic fields can change). A laser is passed through the polymer layer. As it does that, it’s angle of polarisation change with the magnetic field. A suitable photo-detector that is only sensitive to light polarized in a single direction interrupts the signal ( light – dark / on – off )
|
