JVC XU-301 manual Information on MDs Minidiscs

English

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.

Cartridge

Disc

Shutter

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.”

SOUND

Play-only MD

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.

		(dB)
Sharp	+20
\	0
	-20
Sensitivity of
our ears	-40
\	-60
Dull
	20		50	100	500	1k	5k	10k	15k

(Hz)

Low Sound range \High

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.

Optical pickup

STOP

Sound jump guard memory

During normal play When vibration has occurred

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.

52

Recording MD

UTOC	Contents
Track details	1.	Track A: Page 9
	2.	Track B: Page 6
	3.	Track C: Page 10
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