Characteristics of the MiniDisc in the overview
System data of the MiniDisc
Buffer for branch bypass
Magnetic field modulation
Since 1970 Sony develops technologies for optical disk
memories with the target, preference/advantage of the mode of
operation without mechanical contact between carriers and recording
read head with high memory density, long life span, to begin optional
rapid access and low memory costs for recording and playback. The
application steps were video disk, CD and CD ROM. 1988 the
series by repeats recordable magnetooptic disk for the data storage
completed, the MiniDisc (MD). The possibilities, which the MiniDisc
within the area of the audiovisual media in the future opens, are
hardly to be over-looked today.
Base of the digital storage medium is a disk-shaped carrier
with substantially smaller dimensions than the CD, however with the
same advantage of the rapid, optional access to any place of the
recording, like that as it the users today from that CD is used.
During playback and recording the utilization of the modern mechano
optical or the magnetooptic technique and the effective audio data
reduction leads too uerst compact dimensions of disk and device. The
contactless, wear-free mode of operation ensures a high in connection
with the digital audio technique, of the number of the playing or
recordings independent..tone quality. During playback the effects of
Sten or vibrations on the device electronically balanced and so the
high quality of the digital technique with easier, comfortable and
unproblematic handling achieved, like that as it for a trend-setting,
portable system is indispensable.
Characteristics of the MiniDisc in the overview
System data of the MiniDisc
|Spieldauer in min||max. 74|
|Cartridge dimensions in mm||72 x 68 x 5|
|Diameter in mm||64|
|Thickness in mm||1,2|
|Diameter centering borehole in mm||11|
|Diameter intake area in mm||29|
|Diameter beginning of modulation in mm||32|
|Track upward gradient in m||1,6|
|Recording or scanning speed in m/s||1.2... 1,4|
|Frequency range in cycles per second||5...,20,000|
|Dynamics in dB||105|
|Sampling rate in kHz||44,1|
|Source quantization in bits||16|
|Error correction system||CIRC|
|Wavelength of the laser light in Nm||780|
|Diameter of the laser mark in m||0,9|
|Laser performance during recording in mW||5 (max.)|
|Recording system||magnetic field modulation|
|Buffer to the Sprungberbrkung
With the practical An.wendung of trans portable optical memory has itself branching the scanner than sequence of Erschtterungen or Sten, which affect from auen the playing Gert, when much strend herausge places. By Einfgen of a semiconductor memory into the signal path it succeeded to reduce the effects of such Strun towards crucial (Shock Resistant MEMORY /. Fr the function of this buffer is helpful the Datenkom pression on approximately 1/5 the origin data, available in the system, by the ATRAC module. The selection of the information - each sector trgt its own address -, cherten on the disk in sectors gebndelt gespei, takes place with a Signalflu from 1,4 Mbit/s. Due to the data reduction however only a continuous Signalflu of approximately 0.3 Mbit/s is necessary. This means, since the selection points more normally effected in Schben. These data flieen into a buffer, fig. 8. A semiconductor buffer with a Speicherkapazitt of 1 Mbit knows the signals f about 3 s spei chern, with 4 Mbit is 10 s berbrck bar. Fig. 9 clarifies in another representation and gives a comparison to CD, with which that betrgt signal flu constantly 1.4 Mbit/s. Lt the optical scanner as consequence of one of auen pltzlichen Erschtterung its correct position kenden on the Gert einwir, then remains without Einflu on the signal more cher given from the Spei, however the memory empties thereby. After Abklin towards the Strung looks up the scanner the last correct position well-known with the help of the address and fllt the memory with the max. signal flow of 1,4 Mbit/s without Unterbre chung, in most Fllen after 1 s of the buffers is again so there gefllt already and reading in in intervals is again taken up (fig. 10).
F the system MiniDisc is the optional access of special
importance, because he help not only to a high convenient operation,
but is functionally necessary also in connection with the beforehand
described Sprungberbrkung. Only playback MDs of industrieller
production carry a passing through time code and a table of contents
(TOC, Table OF Con tent) like the CD. That protects the rapid and
direct access. The f the recording " Leer" Discs intended (Recordable
MiniDisc) contain injected grooves f the Frung of the recording
playback laser (Pre Groove) with the production of the carrier. The
spiral formed by the groove are erlagert additional deflections (some
tenth micrometers) in seitenschrift, fig. 11. They form a time code
with a Auflung of 13,3 ms in coded form. Thus is the optional access
independently of the actual recording secured. The special system is
called ADIP (ADDRESS in Pre Groove). For comfortable handling area of
the track a field f contents a directory UTOC (user Table OF
CONTENTS), which can be arranged from the user, is contained of the
recorded disk in the initially, which structures on the time code. As
is shown in the fig. 12, modifications in the table of contents are
parallel realizable to modifications of the stored information in a
simple manner by editing.
The different type of the signal storage of both disk types of the MD requires a special, on the different request at fit optical scanner. A normal optical scanner f CD fails to a MO recording with the scanning. With the help of a polarizing beam splitter detecting the different polarization directions succeeds to a MO recording with the scanning. According to fig. 15 reflects on the surface (country) a CD focused laser beam (about 0.5 mW) with nonexistence of a pit such as light, while in the pit the quantity of light is clearly reduced (for instance on 25 %). this difference f the signal production used, whereby the actual information in the ergaengen between country and pit is stored (EFM modulation, transition = 1; no transition = 0 [ 1)1). The signals, which supply the two optical recipients with the scanning of a recording, are added, s. fig. 16. During the scanning of a magnetooptic disk, recordingable MD, is thus used appearance the called Kerr effect out, with which a polarized light beam under the influence of different directions of magnetization at point of focus is turned with reflection into its polarization direction (some degrees), s. fig. 17. As consequence the relation of the quantities of light as a function of the direction of rotation, which meet in both photo detectors, shifts.
Small Amplitudenauflung frt to Ouantisierungsrau. One
ensures ever nevertheless daf, since this Quantisierungsgerusch
remains unhbar, then the playback quality corresponds to that that CD
therefore is a primary task with ATRAC to minimize the Hbarkeit of
this noise by being hidden Ouantisierungsrau in frequency ranges, in
which high signal levels occurs. The maximum of ear sensitivity is
situated in the frequency area by 4 kHz, with other frequencies is
partly substantially more insensitive the ear. A..tone, which is
assumed with max. sensitivity even, is unhbar with same intensity, but
other frequency. Basically two width unit of same intensity, but
different frequency are unequally loud felt. A quiet sound can become
unhbar with presence of a loud. This effect is defined as covering
(masking) and is the more pronouncedly, the more near the width unit
in its frequency together to be situated and the more grer their
intensity difference is.
|The frequency and time partitioning applied by ATRAC show
fig. 19. The unequal width of the frequency bands is remarkable.
This allocation is based on a further psychoakustischen effect, the
groups of frequencies (Critical of tape), which with the human go were
determined. The width of these groups increases with rising
frequency, it amounts to e.g. with 100 cycles per second B=160Hz;
with 1000 cycles per second B=160 cycles per second and with 10,000
cycles per second B = 2,500 cycles per second. The groups of
frequencies are thus, like also fig. 18 show, in the lower frequency
range substantially more closely together than with heren frequencies.
The transfer of this allocation into the system ATRAC helps to achieve
a high accuracy also with small transfer capacity.
Music signals constantly change, and the ear adapts its sensitivity the rate of these modifications. In lively passages e.g. ear sensitivity changes rapidly, in carried sections against it slowly. Therefore analyzes. ATRAC constantly in short time periods the input signal and adapts signal processing to the ear behavior. In lively passages Zeitblke are formed by 1,45 or 2,9 ms, in carried as far as 11.6 ms. Longer Zeitblke the ermlichen application of narrow frequency bands and results in high Frequenzauflung with high reproducible..tone quality (fig. 20). This signal-dependent Flexibilitt is a Schlsel f high effectiveness of the since tenkompression with simultaneous minimization of quantization noise. This unequal time and allocation of frequency are implemented with ATRAC by the combination of filters and Transfarmationsprozessen, s. fig. 20. The input signal is divided in three Bnder: low 0... 5.5 kHz, means 5.5... 11 kHz, highly 11... 22 kHz and with a modified discrete cosine transformation (MDCT) continues to process. Before it determined, whether the signal modification takes place rapidly or slowly, and accordingly the Zeitblke will become selected.
If the signal is divided into spectral regions, the MDCT values are divided according to the 52 unequal groups of frequencies. In these groups the bit rate reduction takes place in agreement to the masking and sensitivity conditions of each group. A special algorithm serves the avoidance of unnig high bit values. Thus the data item length will become kept small, at the same time however hbare modifications of the music avoided.
With the Rkwandlung of the signals in the decoder first the MDCT Frequenzwerte is erfrt into current values by inverse MDCT function. Schlielich are combined the three section tapes, in order to receive a normal digital 16-bit-Audiosignal. The real data stream betrgt 256 Kbit/s f the channelchannel channel.
The Komplexitt and the high level of these technological Lung become clear in the fact, since entire ATRAC signal processing is implemented already with Systemeinfrung in only one LSI.
|Magnetic field modulation
The overwriting is a basic condition f the continuous
recording already beforehand a disk recorded by audio signals in real
time on. The application of the laser modulation, how it is used in
optical memory of the computers, is not f the MD suitably, because the
Lchen and recording take place here separately. Therefore f the
recordable MiniDisc the magnetic field modulation was developed.
With the magnetic field modulation the laser is constantly
switched on, and the magnetic field is modulated f the recording (fig.
22). The overwriting of available recordings through renouncement of
a separate Lchvorgang permits. This technique stzt itself with the MD
on a high-stable layer from terbium of ferrites cobalt, which a
magnetization modification permits with comparatively low field
strengths of 6,4 kA/m (80 Oe), so far was for instance the three-way
value necessary. The complex request of the recording carrier are
achieved among other things by the imbedding of the magnetic layer
into a multi-layer system (fig. 6). The low field strength ermlicht a
small solenoid actuator with small power requirement and practically
immediate Flurichtungswechsel (approximately in 100 lv) when
commutating the direction of magnetization.
|Signal pit of the herkmlichen CD produced by an ionized argon
laser with 460 Nm wavelength and focusing it a lens with WELL = 0,9.
That results in a diameter of the light spot of 0,4 GM. F the system
MD was only a diode laser with 780 Nm to the Verfung. Focuses it a
lens with WELL = 0.45 arises a light spot diameter of 0,9 m. thereby
appeared achieving memory density that CD first unmlich.
With experiments with D-RECORDING by means of conventional
laser modulation (rate of 1.2 m/s) up to 200 faulted Datenblke per
second were determined, a value the even still the request of the
D-STANDARD corresponds. With application the number went to the
magnetic field modulation on 20 per second zurk, fig. 23. Magnetic
field modulation is thus not only f the erschreiben suitably, it
results in also very error-poor recordings. The differences bezlich
the error behavior between the laser and the magnetic field modulation
find their assertion in the pit forms left really in the track. With
the magnetic field modulation the diode laser produces constantly a
performance of approximately 4.5 mW, and during focusing on the
magnetic layer this achieves the curie temperature (about 180 c1.
After leaving the light spot the temperature drops. When repeating
this process with presence of a magnetic field with two different
orientation directions dependent on orientation either 0 or 1 is
recorded. Thus develop in the fig. 24 of pit shown. If the magnetic
field can be switched sufficiently rapidly, then it is mlich, areas
with a length of 0,3 pm also with a laser with 780 Nm wavelength,
focuses it a lens with WELL = 0.45 to produce. Thus the demand is
after one with that CD ereinstimmenden memory density erflt!
Characteristically f the magnetic field modulation is in polarity
switching (of + after -) begrdete the high symmetry of the pit
structure. In contrast to it the laser modulation results in very
|The magnetic field can be oriented here only in a direction,
with the allocation a 1 = laser lighting and 0 with switched off laser
(unaufgezeichneter area), s. fig. 24. With laser modulation. e.g.
the second half of a pit is always thicker, because the temperature
rises. By controlling of the laser performance a balance would be
mlich. On the other hand a different laser performance influences the
at the beginning or terminator point of a pit. Thus distortions
develop and erhte asymmetry. Simultaneously time error (jitter)
occurs. That is particularly critical, because by the application of
the EFM the length forms a tracing the basis of the data bearing for
that of pit or that.
The magnetic field modulation does not permit fluctuations of the laser performance up to 20 %. the laser beam during the recording with magnetic field modulation only to f the heating up of the layer ensures there, is also a tilt between disk and scanning unit critical. Despite the described advantages the application in the computer engineering fails to the magnetic field modulation because of that lichen there high linear speed to more ber 10 m/s and the high frequencies (some 10 mc/s), with which the magnetic field are over polarized mte. In contrast to it with the MD necessary frequency is to be implemented heading constructions suitable by 720 kHz easily with:
Magnetic field modulation permitted only the one-sided data recording on the disk, f consumer's application is however no disadvantage, because a bilateral Ausfrung wde to more than the double cost.
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