What Is Compression? What Is Atrac; What Does The Frame Indication Mean; What Does "Resolution Is 11.6 Ms" Mean; While Playing Back Four Tracks Simultaneously - Yamaha MD4 Frequently Asked Questions Manual

6.

What is compression? What is ATRAC?

Even though Mini Discs are only 64 mm in diameter, they can store up to 74 minutes of
stereo audio, which is the same as the standard 120 mm audio compact disc. To achieve this
data capacity, audio data is compressed using Sony's ATRAC (Adaptive TRansform
Acoustic Coding) system. Using the ATRAC system, the Yamaha MD4 provides a
four-track recording time of 37 minutes.
ATRAC uses established psychoacoustic principles to compress audio data to about one-fifth
of its original size, with virtually no loss in sound quality. For example, the sensitivity of the
human ear is frequency dependent. Two tones of the same level but at different frequencies
will not be heard at the same loudness. Another principle used is the "masking effect". That
is, softer sounds become inaudible when louder sounds at similar frequencies are present.
ATRAC uses both of these principles to compress the audio data.
ATRAC also employs a variable bit length technique, whereby bits are allocated to data
depending on signal amplitude. For example, a low level signal does not require as many bits
as a high level signal. So rather than waste 8 bits on a 5-bit value, ATRAC removes the 3
unused bits, thereby reducing the amount of data.
7.

What does the frame indication mean?

One second consists of 86 frames. Since the data is compressed by units of 11.6ms (512
sample @44.1 kHz) based on the ATRAC system, a frame is obtained by dividing one
second by 11.6 ms.
8.

What does "resolution is 11.6 ms" mean?

This is the unit of one frame (11.6 msec) of resolution. Since each compression block is
11.6ms (0.0116 second), this is the minimum unit for recording and editing.
9. Why is it possible to perform ping-pong recording while playing back
four tracks simultaneously?
The MD4 handles data compressed to about one-fifth of its size and length of time. In
addition, random access capability will locate the playback point immediately for recording.
Therefore, the Yamaha MD4 has enough room to record and play back data at the same point
in the same channel.

10. Doesn't data slip when overdubbing due to data compression?

The compression and expansion process takes some amount of time. The Yamaha MD4 has
a special dedicated circuit that compensates with a time offset for overdubbing and
ping-pong recording.
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