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7-1.
Outline of Playback Circuit
Fig. 7-1. shows the structure of the playback circuit of the Mini Disc. The RF data read by the optical block is demodulated
by the EFM demodulator, and data errors are detected and corrected by the ACIRC decoder.
Since the Mini Disc data has the same structure as the CD-ROM, it is decoded in the CD-ROM decoder in the next step,
and the ATRAC data compressed to approximately 1/5 is stored in the shock proof memory (buffer RAM). Apart from the
ATRAC data, this memory also stores error information (C2PO) and TOC/UTOC information.
The ATRAC data is written in the memory intermittently, and read to the ATRAC decoder periodically. The compressed
data is expanded to 16-bit linear data by the ATRAC decoder, D/A converted by the D/A converter, and output as analog
signal.
MD
7-2.
Playback Operations
Fig. 7-2. shows the block diagram of the playback circuit and the waveform of the main parts.
The two RF signals (I and J signals) I-V converted inside the optical block are input respectively to Pins 6 (VI) and 7
(VJ) of the RF amplifier (IC501). The RF amplifier sets the gain and internal circuits according to the disc type (MO or CD)
using the serial data input form the system controller (IC801).
Disc discrimination and gain setting are performed together during focus search. (Details are provided in 8-3. Focus Search
and Disc Discrimination.)
Furthermore, the output of the RF amplifier (1) is switched according to the disc type written in the TOC. In the case of the
pit area of the playback only disc and MO disc, the additional signal of I and J is output, based on the Vref voltage (0.88V)
generated inside the RF amplifier, from Pin #¶ (RF-OUT) to the DSP/digital servo (IC601) via the RF amplifier (2) and EQ
(equalizer). In the case of the groove area (UTOC, program area) of the MO disc, the subtraction signal is output. (1)
The RF signal is converted to a 7-bit digital value at a sampling frequency of 4.23 MHz (96 fs:fs=44.1 kHz) by the
A/D converter inside the DSP/digital servo. In the EFM PLL circuit, clock extraction and generation are performed, and based
on the channel clock (4.23 MHz) obtained, EFM demodulation is performed in the next stage. In the ACIRC decoder, data
errors are detected and corrected, and the data is then written in the DRAM via the RAM controller. The accumulated data is
compressed to approximately 1/5 according to the transfer command from the system controller, and periodically sent to the
ATRAC decoder block. In the ATRAC decoder block, the compressed data is expanded, and the data is output continuously
to the D/A converter (IC301) from Rch and Lch alternately in synchronization with LRCK (L/R clock) and BCK (bit clock)
at the timing of the audio signal shown in the figure.(Refer to 2 to 4.)
7. PLAYBACK CIRCUIT
EFM
ACIRC
DEMODULATOR
DECODER
Fig. 7-1 Structure of Mini Disc (Playback Circuit)
CD-ROM
SHOCK PROOF
DECODER
MEMORY
— 32 —
ATRAC
D/A
DECODER
CONVERTER
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