Tascam MD-801RMKII owner manual Introduction, About the MiniDisc format, R3.12.2Laser pickup time

This manual covers the two models of the MD-801 Mk II product line: the MD-801R Mk II, which is capable of recording and playing back on MiniDisc media, and the MD-801P Mk II, which is for play- back only of such media. Apart from the recording and associated facilities provided on the MD-801R, the functionality and specifications of the two units are identical. For brevity, the unit will be referred to as the “MD-801” in instances where there is no dif- ference between the recording and playback models.

When a section of this manual describes a function which is included as part of the MD-801R only, the section will be marked as below:

R3.12.2Laser pickup time

This indicates how long…

There are no features which are unique to the MD- 801P model.

1.1 About the MiniDisc format

This section provides some basic information regard- ing the MiniDisc format. If more detailed informa- tion is required, the reader is referred to one of the many books on the subject. John Wilkinson, The Art of Digital Audio 2nd ed. Oxford: Focal Press, 1994 is an excellent reference source, particularly Chapter 12, which deals with the use of optical discs in audio applications.

The MiniDisc format (MD) is a convenient form of digital audio media, using optical playback, similar to CDs.

The format of the MD is substantially smaller than that of the CD, and the disc itself is housed in a car- tridge with a shutter, similar to that of a 3.5” floppy disc drive. The disc platter should never be removed from this cartridge.

Pre-recorded MDs, which are manufactured in the same way as commercial CDs, using a stamper pro- cess, are read using a laser beam to read the pits stamped onto the disc.

Recordable MDs are also available, and the technol- ogy used for recording on a recordable MD is magento-optical, allowing the disc to be re-used many times. Shutters on both sides of the disc allow access to the disc surface by the recorder’s laser and magnetic head.

There is a third kind of MD, the hybrid, which con- tains pre-recorded pits at the center of the MD, together with an area reserved for user recording.

1 - Introduction

Full explanations of the principles of magneto-opti- cal recording may be found in a number of excellent reference works, but briefly, during the recording process, the disc is exposed to a strong laser beam which heats the disc’s magnetic medium so that a magnetic field can be applied to change the orienta- tion of the magnetic particles. On cooling, this orien- tation is permanently fixed, unless the disc is reheated and re-recorded.

The MD format therefore provides the convenience and durability of an optical disc format with the reus- abilty characteristics of tape.

1.1.1 ATRAC compression

If a MiniDisc were to be recorded using exactly the same techniques as a CD, the playback time would be only 15 minutes of stereo playback.

However, using a technique known as ATRAC (Adaptive TRansform Acoustic Coding), 74 minutes of stereo audio at 44.1 kHz sampling frequency with a 16-bit word length can be recorded on an MD.

ATRAC uses two important pyscho-acoustic princi- ples to reduce the size of the recorded data: the greater sensitivity of the human ear to mid-range sig- nals, relative to bass and treble signals, and the “masking” effect, whereby quieter signals are hidden by louder ones.

The ATRAC used in the MD-801R/P Mk II is a very recent version of this encoding scheme (4.5), result- ing in an almost inaudible loss of signal quality.

1.1.2 Buffer memory

The ATRAC compression scheme has an additional advantage, as a RAM buffer must be provided for the compressed data, before it is decoded and sent to the D/A converters.

On the MD-801R/P Mk II, this buffer memory holds about 10 seconds’ worth of data. The advantage is that, unlike a CD player, the MD-801R/P Mk II is not vulnerable to mechanical shocks and vibration, etc., which can cause a CD player playback pickup to jump and cause audible skipping.

1.1.3 Random access and U-TOC

The format of a recordable MD is much closer to that of a computer disc than of conventional audio media. The random access nature of the disc media allows a logical “track” to be composed of sections of audio data recorded at different points on the disc.

In the same way that a computer’s operating system hides the details of the way that data is stored on the

TASCAM MD-801R/P Mk II 1–1