By P Jalvinger 970403
Last update 2000-01-11
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Before you start
Welcome to my webpage about the MiniDisc. Before you start I have to tell you how the links are working in the text. You find a word, let's say
Curie and wanna have more information, just click on it. When you are done, just klick back (the one that
belong to the word) and just like that, you are back at the same place where you came from, piece of cake don't you think! Enjoy...
If you wanna print this page
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ATRAC compressor
A single MD holds the same amount of music at the same sampling frequency as a CD (74 min and 44,1 kHz) but does so in 1/5 as many bits by using
a compressing method Sony calls
ATRAC (Adaptive TRansform Acoustic Coding). ATRAC is a form of data reduction technique that attempts to encode only the information audible
to the human perceptual system. The input signal is divided into three subbands
which are then transformed into the frequency domain using a variable block length. Transform coefficients are grouped into nonuniform bands to reflect the
human auditory system, and then quantized on the basis of dynamic sensitivity and masking characteristics.
ATRAC is also the basis of Sony's SDDS cinema digital audio system.
back
ATRAC history
| IC generation | IC Chip number |
MD Deck | Introduction date |
| ATRAC 1 | CXD-2527 | MDS-101 | 2/93 |
| ATRAC 2 | CXD-2531 | MDS-102
MDS-501 | 11/93
2/94 |
| ATRAC 3 | CXD-2536 | MDS-S30/S35/S1
MDS-302/303 | 11/94
12/94 |
| ATRAC 3.5 | CXD-2536A | MDS-JA3ES | 6/95 |
| ATRAC 3.5 | CXD-2536B | MDS-503 | 10/95 |
| ATRAC 4.0 | CXD-2650R | MDS-JE500/S37
MDS-JE700 | 8/96 |
| ATRAC 4.0 | CXD-2652AR | MZ-R50
MDS-JE510 | 8/96 |
| ATRAC 4.5 | CXD-2537R | MDS-JA50ES | 12/96 |
| ATRAC 4.5 | CXD-2654R | MDS-JE520 | 9/98 |
| ATRAC 5.0 | - | - | 99 |
How does it record it
Recordable MiniDiscs use a variation on conventional magneto- optical methods that Sony calls "Magnetic Field Modulation", in which
data is recorded using a semiconductor laser at high power together with a magnetic head. As the disc sweeps past the laser, a tiny area on the MD's
magnetic recording layer (Terbium, Iron and Cobolt) is heated to its
Curie temperature of 180°C while the
field of the magnetic head
is in contact with the other side of the disc switched back and forth to write a data pattern. When the area the head has magnetized moves away from the laserspot
, it cools below the Curie point to become "cast" in a string of N and S magnetized regions on the disc, spaced 30 millionths of a centimeter
apart, and corresponding to the stream of bits being recorded. Playback is accomplished using the same laser at a lower power, taking advantage of the
Kerr effect, in which the polarization angle of the reflected laser light is affected by whether it was reflected from an N or S magnetized region. The MD
optics detect these polarization differences to reconstruct the recorded bit stream.
Shock memory
The biggest problem in using optical discs in portable applications has been that of skipping due to shock or vibration. With MD, this is dramatically minimized
by using a semiconductor memory. The memory acts as a buffer, holding digital data equivalent to about three seconds of playing time before it's sent for
conversion into analog signals for playback. If the player is exposed to shock or sudden movement, jarring the pickup from its position on the disc, the
memory, which is unaffected by shock or movement, will continue to output digital data to maintain playback. Since the pickup's position is constantly monitored, it
can quickly continue tracking. Efficient operation of the shock-resistant memory is enabled using digital data compression. The pickup reads data from
the disc at the rate of 1.4 Mbit/second, but playback requires a rate of only 0.3 Mbit/second because the ATRAC system uses only about 1/5th of the
amount of data normally required. If the pickup loses its position, and the flow of data into memory is interrupted, data continues to flow from the
memory at a rate of 0.3 Mbit/second, enabling playback to continue from 3 to 10 seconds (vary from player to player, new players have up to 40 seconds of shockmemory). When the pickup resumes its position, it reads data from the
disc at 1.4 Mbit/second, replenishing the memory's data in less than a second.
back
SCMS
A system developed to limit digital dubbing to first generation copies only. Digital dubbing from a digital copy is not possible.
However it's possible to make an analogue dubbing of the digital MD, and with that analogue MD you can make a new digital dubbing and so on.
But how does the system knows what generation it is and if it's a digital recording, well here's how it works.
| Bits | Meaning |
Explanation |
| 00 | Permitted | No restrictions at all |
| 11 | Restricted | Allow 1 generation |
| 10 | Prohibited | Do not allow copies |
| Source | Recorded on copy |
| Analouge input | 11 |
| CD | 10 |
| Digital 00 | 11 or 00 depending on model |
| Digital 11 | 10 |
| Digital 10 | Will not record |
Specifications
Major specifications
- Recording/Playbacktime 74 min
- Cartridgesize 72*68*5 mm
- Shock memory 3, 6 or 10 sec. (Vary from player to player, new players have up to 40 seconds of shockmemory)
Disc specifications- Diameter 64 mm
- Thickness 1.2 mm
- Diameter centerhole 11 mm
- Diameter begining of program 32 mm
- Diameter beginning of lead-in area 29 mm
- Trackpitch 1.6 microns
- Linear velocity 1.2-1.4 meter/sec.
Audio characteristics
- Channels 2 (stereo/monaural)
- Frequency range 5-20 000 Hz
- Wow and flutter Unmeasurable
Signal format
- Sampling frequency 44.1 kHz
- Compression system ATRAC
- Modulation system EFM
- Error correction system CIRC
Optical parameters
- Laser wavelength 780 nm (standard)
- Laser diameter 0.45 (standard)
- Recording power 5 mW (max)
- Recording system Magnetic field modulation
Tip before recording
ANALOGUE
If you recording using analogue cables you should record around -4 dB (
this goes for any type of digital equipment (DAT, DCC) ). By using Level-Sync, the MD automatically divide two different tracks into two different songs. A warning
using this feature though - if there is a silent part in the song longer than 2 seconds, will cause that the song is divided into two different tracks. Aware of this problem -
always have the Level-Sync turnd on.
DIGITAL
When using an optical/digital cable (I use an optical cable between my Pioneer PD-S505 CD player and the MD recorder) you don't have
to worry about that a silent part in the song will divide it into two different tracks (Level-Sync problem). The MD recorder gets all the information it needs from the CD,
such as when the song start and end, what recordinglevel it should use and so on. When copying digital to digital the SCMS is coded into
the song and you can't make a digital copy of your recording.
Glossery
ADIP - ADdress In Pre-groove
Addresses are formatted at fixed time intervals on a recordable MD which has been stamped with grooves that have been formed in a wobble pattern
created by a sine wave signal under constant linear velocity (CLV) control.
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Lead-in Area
The area on a disc before the program area, starting at an inside diameter of 29 mm, in which table of contents
(TOC) data is recorded.
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Link Sector
Found only in recordable MiniDiscs, this sector links the beginning and end of a recording. There are three link sectors in one cluster.
Magnetic Field Modulation
The high-speed modulation of magnetic field in magneto-optical recording to present an input signal by magnetic orientation.
Masking Effect
A psychoacoustic phenomenon in which certain sounds are rendered inaudible by higher-level sounds on adjacent frequencies.
MDCT - Modified Discrete Cosine Transform
A technique to convert time-domain signals into spectral signals for each fixed time block. MDCT assures highly efficient coding with little connection
distortion between blocks because conversion is performed using a special window function for overlapped halves between adjacent blocks.
Pre-groove
A groove molded into recordable MiniDiscs which assists in tracking control. The pre-groove is meandered in a certain pattern in order to create
addresses throughout the entire disc.
Program Area
The area on the disc for the recorded audio.
Sampling Frequency
The number of divisions on an audio signal made in one second when digitizing an analog signal.
MD and CD use 44,1 kHz.
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Sampling Rate Converter
Convertes other frequencies when recording on a MD, like a DAT-player (48 kHz).
SCMS - Serial Copy Management System
A system developed to limit digital dubbing to first generation copies only. Digital dubbing from a digital copy is not possible. However it's possible to make
an analogue dubbing of the digital MD, and with that analogue MD you can make a new digital dubbing and so on. back
TOC - Table Of Contents
The generic term for all subdata (tracknumber, playingtime etc) apart from the audio data. Link sections of audio scattered about the disc into a
continuous stream, allowing tracks to be trivially segmented, combined. moved or deleted, all with a accuracy of 60 milliseconds. Space
freed by deleted material becomes available for further recording.
back
Wow and Flutter
Frequency fluctuation due to inconsistent speed in recording and playback. Slow fluctuation is called "wow", and rapid fluctuation is called
"flutter". back
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