Chapter 10: Magneto-Optical Recording Systems - MiniDisc
10.3 MiniDisc
The MiniDisc (MD) system, developed by Sony, offers both digital sound and random access features. In addition to these features, three types of MiniDiscs have been developed for various applications:
- Playback-only MiniDisc for prerecorded music;
- Recordable MiniDisc allowing up to 74 minutes of recording time; and
- Hybrid MiniDisc, a combination with premastered and recordable areas.
The intrinsic recording technology supporting the recordable MiniDisc is the magnetic field direct overwrite method, applied to a consumer product for the first time in the world. The distinctive features of the MiniDisc are:
- Overwrite function;
- Maximum 74 min. recording time on a disk only 64mm in diameter, achieved using data compression and high-density recording;
- Quick random access supported by address information in the wobbled groove; and
- Disk protection with the cartridge and shutter.
10.3.1 Introduction and Features of the MiniDisc System
The MiniDisc (MD) system, developed by Sony, offers both digital sound and random access features. In addition to these features, the following three types of MiniDiscs have been developed for various applications:
- Playback-only MiniDisc for prerecorded music;
- Recordable MiniDisc allowing up to 74 minutes of recording time; and
- Hybrid MiniDisc, a combination with premastered and recordable areas.
The intrinsic recording technology supporting the recordable MiniDisc is the magnetic field direct overwrite method, applied to a consumer product for the first time in the world. The distinctive features of the MiniDisc are:
- Overwrite function:
- Maximum 74 min. recording time on a disk only 64mm in diameter, achieved using data compression and high-density recording;
- Quick random access supported by address information in the wobbled groove; and
- Disk protection with the cartridge and shutter.
Figure 10.19 Probability of each error length: A graph showing the probability (frequency) on the y-axis (logarithmic scale from 10⁻⁶ to 10³) versus error length in bytes on the x-axis (0 to 60). Two curves are shown: 'Random error' and 'Burst error'. The random error probability decreases sharply with increasing error length, while the burst error probability decreases more gradually.
Figure 10.20 Defect management strategies: A flowchart illustrating defect management. It shows 'Read process' and 'Verify process' leading to 'Servo Error Check' for '2 ID error' (8 bytes error in a file) and '2~3 ID error' (4 bytes error in a file). 'Write process' also leads to 'Servo Error Check' for '2~3 ID error' (4 bytes error in a file). A 'Secondary Defect List' and 'Primary Defect List' are mentioned, along with a 'Spare Sector'. 'Media production' is also indicated.
10.3.2 System Concept and Specifications
Durability and reliability for the recordable MiniDisc have been proven with data storage media for computer peripherals, such as the magneto-optical disk. Figure 10.21 shows various MD systems.
The specifications of the compact disc (CD) were first proposed in 1982, described in the "Red Book." Technological developments for data and recording applications are specified in the "Yellow Book" and "Orange Book." The MiniDisc specifications, an extension of these, are given in the "Rainbow Book," as shown schematically in Figure 10.22. A block diagram and main specifications are shown in Figure 10.23 and Table 10.5.
Figure 10.21 Various MD systems: An image depicting different MiniDisc hardware units, likely players and recorders.
Figure 10.22 Relationships between industrial standards for optical disk products: A diagram showing how CD standards (Red Book, Yellow Book, ISO-10149) relate to MiniDisc standards (Rainbow Book). It illustrates the evolution from CD-ROM to MiniDisc, highlighting shared parameters and modifications like ACIRC data structure and addressing.
Figure 10.23 Block diagram of MiniDisc system: A flowchart showing the components of a MiniDisc system. Input audio goes through a 16-bit A/D converter, ATRAC encoder, shock-resistant memory controller, EFM CRC encoder/decoder, RF amplifier, address decoder, and magnetic recording head. The disk is read by an optical pickup and magnetic recording head. Output audio comes from a 16-bit D/A converter. A system control unit manages various operations, including display and key input.
| Major Specifications | |
|---|---|
| Recording/playback time | 74 min (max) |
| Cartridge size (WHD) | 72x68x5mm |
| Disk specifications | |
| Diameter | 64 mm |
| Thickness | 1.2 mm |
| Diameter (center hole) | 11 mm |
| Diameter (beginning of program) | 32 mm |
| Diameter (beginning of lead-in) | 29 mm |
| Track pitch | 1.6 microns |
| Linear velocity | 1.2–14 m/s (CLV) |
| Signal Format | |
| Sampling frequency | 44.1 kHz |
| Compression system | ATRAC *1 |
| Modulation system | EFM *2 |
| Error correction system | CIRC *3 |
| Optical Parameters | |
| Laser wavelength | 780 nm |
| NA | 0.45 |
| Recording power | 5 mW (max.) |
| Recording system | Magnetic field modulation |
*1 Adaptive transform acoustic coding
*2 Eight to fourteen modulation
*3 Cross interleave Reed-Solomon code
10.3.3 Random Access Functions
Figure 10.24 shows the cross section of a playback-only MiniDisc. Both the lead-in area and lead-out area are on the inner and outer circumferences, respectively.
Recordable MiniDiscs are formed with special pre-grooves that cover the entire disc recording area. The pre-grooves enable tracking and spindle servo control operations during both recording and playback, as illustrated in Figure 10.25. These pre-grooves meander slightly at 13.3ms intervals to maintain a specified linear velocity and to create addresses which allow very stable high-speed random access.
In addition to the meandering pre-grooves, a UTOC (User Table of Contents) also contributes to user-friendly quick random access. As shown in Figure 10.25, the lead-in area on the inner circumference of the disk is followed by the UTOC area, the program area, and finally the lead-out area, similar to the playback-only MiniDisc.
Each sector in the TOC (Table of Contents) in the lead-out and the UTOC is specified as in Table 10.6.
Figure 10.24 Cross section of a playback only MiniDisc: A diagram showing the layers of a MiniDisc, including the lead-in area, program area, and lead-out area.
Figure 10.25 Cross section of a recordable MiniDisc: A diagram illustrating the structure of a recordable MiniDisc, showing the lead-in area, UTOC area, program area, and lead-out area, with music data indicated.
| TOC | |
|---|---|
| Sector 0 | Generic terms for all subdata |
| Sector 1 | Track number, playing time, etc. |
| Sector 2 | Recording power and recordable time for recordable MD |
| Sector 3 | Generic terms, start and finish address |
| Sector 4 | Disk title, music title and artist Recorded date and time for disk and music Barcord for disk and ISRC for music Disk title, music title, and artist (ISO-8895-1) |
10.3.4 Signal Recording Format
The MiniDisc system uses the popular eight-to-fourteen modulation system (EFM) in writing data on a disk and the Cross Interleave Reed-Solomon Code (CIRC) for error correction. Audio data reduced by ATRAC is grouped into blocks for recording in a format very similar to the CD-ROM mode 2 standard, as in Figure 10.26.
The first three sectors of one 36-sector cluster are used as link sectors during recording, with the fourth sector reserved for subdata. In the remaining 32 sectors, the compressed digital data are recorded. When the last sector has been written, error correction data must be written in the first link sector and half of the second sector of the following 36-sector cluster.
Figure 10.26 MiniDisc data format configuration: A diagram showing a 36-sector cluster, composed of link sectors (L) and data sectors. It illustrates how sound groups (424 bytes each, comprising left and right channels) are distributed across sectors and how they relate to samples and time (512 samples = 11.6ms).
Figure 10.27 ATRAC operation layout: A flowchart depicting the ATRAC process. It shows 'Music signal input' going through 'Non-uniform frequency-time splitting', 'Encoder' (with 'Bit allocation'), then to the 'MD' (MiniDisc), followed by a 'Decoder' (with 'Non-uniform frequency-time synthesis'), resulting in 'Music signal output'. The descriptions highlight how music signal components are divided and allocated bits, and how they are reconstructed.
10.3.5 ATRAC Data Compression
MiniDiscs are recorded using Sony's Adaptive Transform Acoustic Coding (ATRAC) system, designed specifically for high fidelity audio using digital data compression technology. For each block of time, ATRAC analyzes the music signal and determines the sensitivity of each frequency region. Sensitive regions are recorded accurately with very little quantization noise. Less sensitive regions are recorded less accurately, but the quantization error is hardly noticeable. The result is high-fidelity audio at only one-fifth the bit rate, enabling MiniDiscs to offer up to 74 min of recording and playback time on a 64mm diameter disk.
During ATRAC encoding, audio data is compressed to one-fifth its original volume and handled in 424-byte units called “sound groups” with left and right channels allocated 212 bytes each. Eleven sound groups are distributed into two sectors. Recorded sound groups in the first sector comprise the left and right channels of five sound groups, plus the left channel of a sixth group. The right channel of the sixth group and the left and right channels of another five groups are recorded in the second sector. Each of the two sectors can be expressed as 425 x 5 + 212 x 1 = 2332 bytes. In this manner, 11 sound groups are written per every two sectors in each 32-sector cluster. ATRAC decoding restores the data block to its original volume and time axis, with one sound group becoming equivalent to 512 samples (512 x 16 x 2/8 = 2048 bytes) for both channels, with a playing time of 11.6ms.
10.3.6 Construction of Recordable MiniDisc
Magneto-optical technology is central to the functioning of recordable MiniDiscs. A magnetic recording head and a laser are used on opposite sides of the disk, with the shutter opening on both sides, as shown in Figure 10.28.
The construction of the recordable MiniDisc is illustrated in Figure 10.29. The wobbled groove is 1.2 µm wide and deep. A 1.6 µm track pitch is specified for the recording track to coexist with address information formed by meandering at intervals of every 13.3ms. These dimensions are adopted to satisfy servo characteristics, consider thermal crosstalk from adjacent tracks, and gain maximum C/N.
The design concept behind the MO layers has been reported by Y. Tamada et al. [4]. The current 5.25" and 3.5" MO disks employ a four-layer structure, and the MiniDisc has also adopted this structure.
Figure 10.28 Cartridge assemble: An exploded view diagram showing the components of a MiniDisc cartridge: upper shell, write protector, clamping plate, disk, shutter lock, shutter, and lower shell.
Figure 10.29 Structure of recordable MiniDisc: A cross-sectional diagram of a recordable MiniDisc showing its layered structure: Protective layer (~10 µm), Reflective layer [Al], Dielectric layer [SiN], MO layer [TbFeCo], Dielectric layer [SiN], and PC substrate (1.2 mm). It also indicates the wobbled groove and dimensions like 1.2 µm width and 1.6 µm track pitch.
10.3.7 Magnetic Field Modulation Overwrite
The recordable MiniDisc requires the same storage density as the compact disc, which is 0.85 µm/pit for the 74-min model. The magnetic field modulation method offers advantages in system stability and margin between media and drive compared to the laser modulation recording method. The edge of the marked pattern is determined by the flux reversal, intrinsically independent of laser power fluctuation. This method also inherently supports overwrite recording. For MiniDisc, the maximum required field modulation frequency is 720 kHz at a linear velocity of 1.2 to 1.4m/s.
10.3.8 Magnetic Head for Overwrite
The recording magnetic field for the recordable MiniDisc must be over 8 kA/m (100 Oe), and the magnetic head is allowed to contact the disk. For portability, a contact type head has been designed with the following conditions:
- Spacing is 150µm;
- The magnetic field is over 8 kA/m (100 Oe) within ±0.5mm from the center of the core.
Figure 10.30 shows the magnetic field distribution of a recording head at a spacing distance of 150 µm.
Figure 10.30 Magnetic field profile of a magnetic head: A diagram showing a magnetic head assembly with a laser and magnetic field. A graph displays the magnetic field profile (kA/m) against position (mm), indicating the field strength and distribution relative to the head's core. It shows a peak field strength of 15 kA/m at the center, decreasing to 0 kA/m at ±0.5 mm, with a current of DC 0.4 A and spacing of 150 µm.
10.3.9 Recording Characteristics and Film Properties
10.3.9.1 Magnetic Characteristics and C/N
Figure 10.31 compares Kerr loops near the Curie temperature for a recordable MiniDisc (a) and conventional MO (b) on a glass substrate. Remarkable differences in squareness and the required minimum magnetic field saturation (Hs) are noted.
The temperature dependences of residual θk, Hc, and Hs (saturation field) are shown in Figures 10.32 and 10.33. The difference in Hc between MD and conventional MO is clearly demonstrated. The magnetization of the MiniDisc is easily reversed by a field of less than 8kA/m (100 Oe) at temperatures near the Curie temperature.
Figure 10.34 shows the magnetic field dependence of the C/N for conventional MO and MiniDisc. The C/N of conventional disks at 8kA/m (100 Oe) is insufficient for MiniDisc specifications, gradually increasing with Hext up to 16kA/m, reaching about 49 dB. In contrast, the C/N of the MiniDisc is much higher even at 8kA/m (100 Oe), exceeding the minimum requirement of 46dB.
Figure 10.35 shows trends for carrier and noise levels. The MiniDisc carrier level saturates even at low fields, with noise being the dominant factor for C/N in low fields.
Figure 10.31 Kerr hysteresis loops at 170 and 180°C: Two sets of graphs (a and b) showing Kerr hysteresis loops (θk vs. H) at different temperatures (170°C, 180°C) for (a) Recordable MiniDisc and (b) Conventional MO disk. These illustrate magnetic properties under varying conditions.
Figure 10.32 Temperature dependence of residual θk and Hc: A graph plotting Hc (kA/m) and θk (A.U.) on the y-axis against Temperature (°C) on the x-axis. It compares MD (circles) and conventional MO (triangles) for both Hc and θk, showing how these magnetic properties change with temperature.
Figure 10.33 Temperature dependence of saturation fields (Hs): A graph plotting Hs (kA/m) on the y-axis against Temperature (°C) on the x-axis. It compares MD (circles) and conventional MO (triangles), illustrating how the saturation field changes with temperature.
Figure 10.34 Bias field dependence of C/N: A graph showing Carrier-to-Noise ratio (C/N in dB) on the y-axis against external magnetic field (Hext in kA/m) on the x-axis. It compares MD (solid line, circles) and conventional MO (dashed line, triangles) under specific conditions (v=1.22m/s, laser power=4.55mW, mark length=0.85µm).
Figure 10.35 Bias field dependence of carrier and noise: A graph showing C & N (dBm) on the y-axis against external magnetic field (Hext in kA/m) on the x-axis. It compares MD and conventional MO carrier levels and noise levels, illustrating their behavior under varying magnetic fields.
10.3.9.2 Power Margin
During application, laser power may fluctuate. This fluctuation must be accounted for in recording. The BLER (block error rate) must be less than 3 x 10⁻² even with ±20% laser power fluctuation.
Figure 10.36 (a) and (b) show the laser power dependence of C/N, jitter, and BLER. As the C/N remains above 46dB and BLER is below 3 x 10⁻² from 3.6mW to 6.5mW, this range is sufficient for practical applications. High BLER on the low power side is due to low C/N, while on the high power side, it's mainly from jitter caused by heat from adjacent tracks.
Figure 10.36 Laser power dependence: Two graphs. (a) shows C/N (dB) and Jitter (ns) on the y-axis versus Laser Power (Pw in mW) on the x-axis. (b) shows Block error rate on the y-axis versus Laser Power (Pw in mW) on the x-axis. Both graphs compare MD and conventional MO under specific conditions.
10.3.10 Reliability and Durability
Sufficient reliability is designed into the MiniDisc, similar to conventional MO disks. Figure 10.37 shows no change in BLER when the same area is repeatedly recorded a million times. The MiniDisc system allows the magnetic head to contact the disk. For durability, the recordable MiniDisc has a lubricating feature on the overcoat layer covering sensitive layers and facing the magnetic head. The magnetic head is specially designed to reduce friction. Figure 10.38 shows changes in friction when the head continuously contacts the same track under various circumstances, indicating sufficient durability for practical applications.
Figure 10.37 Durability in repeated recordings: A graph showing Block error rate on the y-axis (logarithmic scale) versus the number of Recordings (from 10⁰ to 10⁶) on the x-axis. It indicates that the block error rate remains constant over many recordings.
Figure 10.38 Friction force of a magnetic head: A graph showing Friction force (mN) on the y-axis against Running distance (km) on the x-axis. It displays curves for different environmental conditions: -15°C, 70°C, RH95°C, and an office environment, illustrating how friction changes with distance and temperature/humidity.
10.3.11 Future Applications
The recordable MiniDisc and MiniDisc system represent the world's first implementation of magnetic field modulation overwrite in a consumer product. This technology will find further use in a data storage model, the MD DATA, which has a storage capacity of 140MB with the same 64mm diameter disk. Specifications are shown in Table 10.7.
Moreover, this technology has been introduced into the Digital Audio Master Disc for application in professional recording studios. Recording densities for the Master Disc are presented in Table 10.8, comparing them with the MiniDisc. By setting the optical head to NA 0.5 and wavelength to 780nm, the resolution potential is raised. As a result, the recording density of the Master Disc is 20% greater than the 74 min version of the MiniDisc, in addition to using 2–7 modulation.
Figure 10.39 offers a comparison of recording densities for different magneto-optical disks.
In summary, the technology for magnetic field modulation has been realized with overwrite features for both consumer and professional applications, for audio entertainment, and for data storage.
| Format | |||
|---|---|---|---|
| User area (radius) | mm | 16-30.5 | |
| Sector size (mode 4) | bytes/sector | 2048 | |
| Configuration of track | Spiral | ||
| Track pitch | µm | 1.6 | |
| Direction of rotation | CCW (seen from optics side) | ||
| Mechanical characteristics | Outer diameter of disk | mm | 64 |
| Cartridge dimension (W/R/H) | mm | 72 x 68 x 5 | |
| Substrate thickness | mm | 1.2 | |
| Read/write conditions | Nominal write magnetic field strength | kA/m | 8-24 |
| Carrier-to-noise ratio | dB | >46 | |
| Block error rate | < 3 x 10⁻³ | ||
| Recording capacity | MB | 140 | |
| Reliability | Read cycle | > 10⁶ | |
| Write/read cycle (rewritable) | > 10⁶ |
| MASTER DISC | MD (74 min.) | |
|---|---|---|
| Disk format | ||
| Modulation system | 2-7 RLL | EFM |
| Track pitch | 1.55 µm | 1.6 µm |
| Smallest pit length | 0.78 µm | 0.847 µm |
| Optical pickup | ||
| Numerical aperture of lens (NA) | 0.50 | 0.45 |
| Laser wavelength | 7.80 nm | 780 nm |
| Comparison of recording densities | MASTER DISC / MD = 1/2 x 1/1.55 x 1/0.785 = 1.18 8/17 x 1/1.60 x 1/0.847 |
10.4 FUTURE PROSPECTS
10.4.1 Overview of Magneto-Optical Disks
As stated in Section 10.3, magneto-optical disks have the added feature of direct overwriting by the magnetic field modulation technique.
The characterization of magneto-optical disks in comparison with other rewritable media is discussed in Chapter 1, but a brief review is given here.
Figure 10.40 shows how the areal recording density among the major recording media has been changing over time. Although the magneto-optical disk is still superior in areal recording density, its performance in a complete system is generally inferior to other media.
Figure 10.39 Comparison of MO disk recording densities: A bar chart showing recording surface density (Mbit/in²) on the y-axis for various MO disk formats (5.25", 3.5", MDW-60, MDW-74, Master disc) on the x-axis.
Figure 10.40 Progress in areal recording density: A line graph showing Areal density (Mbits/in²) on the y-axis (logarithmic scale) versus Year (1975-2000) on the x-axis. It compares the progress of Magnetic tape, MO disk, and Hard disk technologies, noting the rapid improvement of hard disks due to their 'fixed' nature.
