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Information on MDs (Minidiscs)
This is a new digital audio disc format: it has a diameter of 64 mm and enables up to 74 minutes of playback and recording.
Function of the cartridge
The diameter of the actual disc is 64 mm, making it slightly smaller
than CD singles. Each disc is housed in a cartridge. The pocket-
size cartridge measures 68 x 72 mm and is 5 mm thick. It is easy to
carry about and store. Since the disc is protected by the cartridge, it
is kept free from dust and dirt, and there is no danger of the disc
being scratched or exposed to fingerprints due to the shutter which
is closed at all times except while the disc is being used. All in all,
these features make the minidisc extremely easy to use.
Two types of discs available
MDs come in two types: the recording MD which can be used for
recording, and the play-only MD which can be used for playback
only. Both types use the same system for playback: a laser beam is
directed onto the disc and the signals are read out by its reflection.
However, the system for recording is different.
Play-only MDs
This type of MD is used for pre-recorded MDs available on the
market. It cannot be used for recording. As with CDs, data is
recorded by the presence or absence of small indentations called
pits. A disc with this kind of recording system is called an “optical
disc.”
Recording MDs
This is the “blank” type of MD which can be used for recording. Its
data is recorded using magnetism which is easily processed so that
recordings can be made again and again. By using a laser beam to
heat spots on its surface to cancel out the magnetism, the data is
recorded using a magnetic head. A disc with this kind of recording
system is called a “magneto-optical disc.”
ATRAC (Adaptive Transform Acoustic
Coding)
Sound contains some elements which are not actually very audible.
For instance, when the sound is soft, it is hard to hear the bass and
treble. Also, during or immediately after a passage of loud sound,
soft sounds cannot be heard even though they may be there. A
technique called ATRAC (Adaptive Transform Acoustic Coding) is
used with MDs to select or dispense with sounds on the basis of our
acoustic characteristics so that the data is compressed into a small
quantity. It compresses the data to be recorded into about one-fifth
of the original data so that it can be stored even on a small-size MD.
Sound range and sensitivity of our ears
Sound jump guard memory
When an MD is played, a function (sound jump guard memory)
operates to store the data of the track being played instantly in a
memory to prevent jumps in the sound due to vibration. When the
disc signals cannot be read by the optical laser because of vibration,
the function uses the data in the “sound jump guard memory” to
ensure that there are no breaks in the sound actually heard.
UTOC (User Table of Contents)
In addition to the data of its tracks, a recording MD comes with the
UTOC contents facility. This records the position of each track, the
breaks between the tracks, the track sequence, etc., and tracks can be
searched quickly by viewing these contents. During editing, all that
changes is the UTOC contents: the tracks themselves need not be re-
recorded.
Shutter
Disc
Cartridge
SOUND
Play-only MD
Recording MD
Sharp
Sensitivity of
our ears
Dull
Low | Sound range \ High
0
+20
-20
-40
-60
(dB)
20 50 100 500 1k
5k 15k10k
(Hz)
Optical pickup
Sound jump
guard memory
During normal play When vibration has occurred
STOP
Track details
Contents
1. Track A: Page 9
2. Track B: Page 6
3. Track C: Page 10
•
•
•
UTOC
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10/p50-56 (E) 4/1/98, 5:11 PM52