A MiniDisc (MD) is
a magneto-optical disc-based data storage device initially intended for
storage of up to 80 minutes of digitised audio. The technology was
announced by Sony in 1991 and introduced January 12, 1992, and is capable
of storing any kind of binary data. The music format is based on
ATRAC/ATRAC3, using digital rights management, various bitrates, and
sampling directly from the digital or analogue input. Minidiscs are
popular in Japan as a digital upgrade to cassette tapes, but have not been
as popular in the United States despite multiple marketing efforts by
Sony. Minidiscs were also marginally popular for a time in the United
Kingdom between 1998 and 2001, when a limited selection of Minidisc albums
were available alongside CD and Cassette albums. Minidiscs are now
primarily used for recording.
Market
history
Along with Philips and Matsushita' Digital
Compact Cassette (DCC) system, the MiniDisc was targeted as a replacement
for analogue cassette tapes as the recording system for Hi-Fi equipment
but, as a consumer format, MiniDisc has met with only limited success,
though it has enjoyed a loyal niche following in some circles. It did not
catch on as well U.S. and Europe as Sony had hoped; in Japan, it is still
relatively popular, though quickly being replaced by flash and HDD-based
audio players like Apple's iPod. The low initial uptake of the format was
attributed to the small number of pre-recorded albums available on MD as a
relatively small number of publishers embraced the format. The initial
high cost of equipment was also a factor. The pre-recorded disks
disappeared from the market rather suddenly in the late
90s.
The company avoided the mistake
that it had made in the 1970s with the Betamax video recording system, and
this time licensed the MD technology to other manufacturers, with JVC,
Sharp, Pioneer, Panasonic and others all producing their own MD systems.
In recent years MiniDisc has faced new competition from CD-Recordable,
solid-state memory recording (flash memory), and hard disk recording,
while the popularity of traditional cassette tape refuses to wane in
certain quarters. MiniDisc is widely respected as being a very reliable
format when it comes to portable audio storage, such as field
recording.
MD
Data
MD Data, a version for storing
computer data was announced by Sony in 1993, but it never gained
significant ground, so today MDs are used primarily for audio storage. The
format was able to store 140 MB on a special disc, but was plagued by low
write speeds and slow seek times. MD Data drives also could not write to
audio-MD's, only the considerably more expensive data blanks. MD-Data2
blanks, which held 650 MB of data, were introduced around 1997, but were
only used in Sony's short-lived MD-based camcorder.
The Hi-MD format, introduced in
2004, marked a return to the data storage arena with its ability to act as
a USB hard drive.
Design
The disc is permanently housed
in a cartridge (68 × 72 × 5 mm) with a sliding door, similar to the casing
of 90 mm floppy diskettes. This shutter is opened automatically by a
mechanism upon insertion. The audio discs can either be recordable (blank)
or premastered. Recordable MiniDiscs use a magneto-optical system to
record data. A laser heats one side of the disc to its Curie point, making
the material in the disc susceptible to a magnetic field. A magnetic head
on the other side of the disc alters the polarity of the heated area,
recording the digital data onto the disk. Playback is accomplished with
the laser alone: taking advantage of the Faraday effect, the player senses
the polarisation of the reflected light and thus interprets a 1 or a 0.
Recordable MDs can be recorded on repeatedly; Sony claims up to one
million times. As of May 2005, there are 74-minute and 80-minute discs
available. 60-minute blanks, which were popular in the early years of the
format, were phased-out long ago and are rarely seen. Premastered
MiniDiscs use a mastering process and optical playback system that is very
similar to CDs, making them physically dissimilar to recordable discs. The
recorded signal of the premastered pits and of the recordable MD are very
similar to that of the CD. Eight-to-Fourteen Modulation (EFM) and a
modification of CD's CIRC code, called Advanced Cross Interleaved
Reed-Solomon Code (ACIRC) are employed.
Differences
from cassette and CDs
MiniDiscs use rewritable
magneto-optical storage to store the data. Unlike the Digital Compact
Cassette, or the (analogue) compact audio cassette, the disc is a
random-access medium, making seek time very fast. Minidiscs can be edited
very quickly even on portable machines. Tracks can be split, combined,
moved or deleted with ease. At the beginning of the disc there is a table
of contents (TOC), which stores the start positions of the various tracks,
as well as meta information (Title, Artist) about them and free blocks.
Unlike with the conventional cassette, a recorded song does not need to be
stored as one piece on the disk, it can be stored in several fragments.
Early MiniDisc equipment had a fragment granularity of 4 seconds audio.
Fragments smaller than the granularity are not kept track of, which may
lead to the usable capacity of a disc actually shrinking. Also, no means
of defragmenting the disc are provided in consumer grade equipment.
Defragmentation would require either two discs, or enough RAM to store the
full contents of a MiniDisc, and computing power to rearrange the
fragments so that each song is stored on the disc in one fragment
only.
All consumer-grade MiniDisc
devices feature a copy-protection scheme known as Serial Copy Management
System. An unprotected disc or song can be copied without limit, but the
copies can no longer be digitally copied.
Compression
The audio on a MiniDisc is
compressed using the ATRAC format (Adaptive TRansform Acoustic Coding),
whereas a CD contains uncompressed 16-bit stereo linear PCM audio. ATRAC
is a psychoacoustic lossy audio compression scheme, so decompression of
the compressed signal will not yield the original signal, although the
compressed signal may sound identical to the original to the listener. The
latest version of Sony's ATRAC is 'ATRAC3plus' (Sharp, Panasonic, Sanyo
and Pioneer have their own (but fully interoperable) ATRAC codecs).
Original ATRAC3 at 132 kbit/s (also known as ATRAC-LP mode) is the format
used by Sony's Connect audio download store. ATRAC3plus is not used in
order to retain backwards compatibility with earlier NetMD
players.
Anti-skip
MiniDisc has an advantageous
feature that prevents disc skipping. Older CD players had once been a
source of annoyance to users as they were prone to mistracking due to
vibration and shock. MiniDisc solved this problem by reading the data into
a memory buffer at a higher speed than was required (the size of the
buffer varies from model to model) before being read out to the
digital-to-analogue converter at the standard rate required by the format.
If the MiniDisc player was bumped, playback could continue unimpeded while
the laser repositioned itself to continue reading data from the disc. If
the memory buffer is sufficiently full, this feature allows the player to
stop the spindle motor for long periods, increasing battery life. The
memory buffer concept was incorporated shortly afterwards into portable CD
players as well.
Note that a buffer of at least
ten seconds is required on all MiniDisc players, be they portable or not.
This is needed to ensure uninterrupted playback in the presence of
fragmentation.
Operation
The data structure and operation
of a MiniDisc is similar to that of a computer's hard disk drive. The bulk
of the disc contains data pertaining to the music itself, and a small
section contains the Table of Contents (TOC), providing the playback
device with vital information about the number and location of tracks on
the disc. Tracks and discs can be named. Tracks may easily be added,
erased, combined and divided, and their preferred order of playback
modified. Erased tracks are not actually erased at the time, but are
marked so. When a disc becomes full, the recorder can simply slot track
data into sections where erased tracks reside. This can lead to some
fragmentation but unless many erasures and replacements are performed, the
only likely problem is excessive searching, reducing battery
life.
The data structure of the
MiniDisc, where music is recorded in a single stream of bytes while the
TOC contains pointers to track positions, allows for gapless playback of
music, something which the majority of competing portable players,
including most MP3 players, fail to implement properly. (notable
exceptions are CD players and ipods produced after sep
2006.)
At the end of recording, after
the 'Stop' button has been pressed, the MiniDisc may continue to write
music data for a few seconds from its memory buffers. During this time, it
may display a message ('Data Save', on at least some models) and the case
will not open. After the audio data is written out, the final step is to
write the TOC track denoting the start and endpoints of the recorded data.
Sony notes in the manual that one should not interrupt the power or expose
the unit to undue physical shock during this period. Sony actually advises
using the AC power supply when recording, if possible, as the added power
requirements during recording (noted above) drain batteries much quicker
than playback. (But one strength of the MiniDisc lies in its portability
for field recording, so sometimes batteries will be
used.)
Format
extensions
MDLP
What became a very brief format
war ended when DCC was phased out in 1996. In 2000, Sony announced MDLP
(MiniDisc Long Play), which added new recording modes based on a new codec
called ATRAC3. In addition to the standard, high-quality mode, now called
SP, MDLP adds LP2 mode, which allows twice as much recording time (160
minutes on an 80 minute disc) of good-quality stereo sound, and LP4, which
allows four times more recording time (320 minutes on an 80 minute disc)
of medium-quality stereo sound.
The bitrate of the standard SP
mode is 292 kbit/s, and it uses separate stereo coding with discrete left
and right channels. For most people the sound quality is indistinguishable
from a CD. LP2 mode uses a bitrate of 132 kbit/s and also uses separate
stereo coding. For most people the sound quality is almost as good as SP.
The last mode, LP4 has a bitrate of 66 kbit/s and uses joint stereo
coding. The sound quality is noticeably poorer than the first two modes,
but is sufficient for many uses.
Tracks recorded in LP2 or LP4
mode play back as silence on non-MDLP players.
NetMD
NetMD recorders allow music
files to be transferred from a computer to a recorder (but not in the
other direction) at high speed over a USB connection. In LP4 mode, speeds
of up to 32× real-time are possible and three Sony NetMD recorders:
MZ-N10, MZ-N910, and MZ-920 are capable of speeds up to 64× real-time.
NetMD recorders all support MDLP.
NetMD is a proprietary protocol,
and it is currently impossible to use it without proprietary software,
such as SonicStage. Thus, it cannot be used under Linux, *BSD, and so on.
A free implementation, libnetmd, is being developed, yet it cannot be used
to upload music (as of December 2005).
Hi-MD
The latest extension to the
MiniDisc format is Hi-MD. Hi-MD players and recorders feature a new
ATRAC3plus codec and have the ability to store files of any type on their
discs. Hi-MD players work with three different types of
discs:
Conventional
discs
- Hi-MD devices have the same capabilities of NetMD and
MDLP devices when working with these, but no more.
Conventional discs reformatted to
Hi-MD
- These have a raw data capacity
of 305 MB, compared to 160 MB (140 MB in data-mode) for standard
conventional discs.
Hi-MD discs - These have the
same form factor as conventional discs but a new recording medium gives a
raw data capacity of approximately 1 GB.
The latter two disc types have
the following new features and characteristics:
- A new
ATRAC3plus codec, available in bitrates of 256 kbit/s, 64 kbit/s and 48
kbit/s, plus a Linear PCM recording mode. A new, higher bitrate of 352
kbit/s was officially enabled for all HiMD hardware in latest version
SonicStage 3.4.
- When connected to a computer, a
Hi-MD formatted disc can be used to store computer files such as photos
and text files.
- A conventional MD disk can be read
and written to by a NetMD, in the original 160 MB format, but it becomes
unreadable when written in Hi-MD format (305 MB)
- A
1 GB Hi-MD is unreadable by a NetMD player regardless.
In 2005 Sony presented an update
to its Hi-MD devices: native support for the popular MP3 format.
Unfortunately, this did not include drag-and-drop capability, but rather
required the use of the Sony software to wrap the MP3 files in DRM. A
further drawback was that the MP3 playback could be best described as
'crippled' — Sony implemented a lowpass on MP3 playback, which could make
MP3 sound dull in comparison to ATRAC. Some users say that this was done
in hopes of fooling listeners that ATRAC is superior to MP3 to trap their
music in ATRAC format. 2005 also saw an update to the SonicStage music
transfer software, with version 3.2 allowing users to upload their
analogue recordings an unlimited number times.
More recently in 2005, Sony
announced the ability for their Hi-MD players to store and view photos;
hence the name 'Hi-MD Photo'. This was perhaps to rival the iPod Photo,
which was available just before its release.
A 1 GB Hi-MD disc can hold
between 94 minutes (PCM) and 45 hours (48 kbit/s ATRAC3plus) of
music, 48 kbit/s
ATRAC3plus is equal to FM quality.
Prior to the release of
Sonicstage 3.4 (Sony's music management program for the Minidisc format)
only Hi-MD recordings from analogue sources could be uploaded from the
Hi-MD recorder to a computer; as of Sonicstage 3.4 however,
digital-optical sourced recordings can be uploaded as
well.
With the latest introduction of
the Hi-MD MZ-RH1 in Japan at Mar 2006, Sony officially enabled full upload
functionality to computer for any audio recorded on Hi-MD or traditional
MD discs.
Recording
modes
Modes marked in green are available for recordings
made on the player, while those marked in red are only available
for music downloaded from a PC. Capacities are official Sony figures; real world figures are usually
slightly higher. Second generation Hi-MD players also support MP3 compression natively, in a
multitude of bitrates. Recently, 352 kbit/s and 192 kbit/s ATRAC3plus have also been made
available for 1st and 2nd generation H-MDs.
Wikipedia information about
Minidisc. This article is licensed under the GNU Free Documentation License . It uses material
from the Wikipedia article 'Minidisc'
