SAA7806H NXP [NXP Semiconductors], SAA7806H Datasheet - Page 33

• Manufacturer Part Number: SAA7806H
• Description: One chip automotive CD audio device
• Manufacturer: NXP [NXP Semiconductors]

Philips Semiconductors
9397 750 13697
Objective data sheet
6.6.6.1 General
6.6.6.2 Q-channel subcode interface
6.6.6 CD decoding
furthermore a new sync is detected in the data shortly after a previous sync signal
(interpolated or real) no new sync signal will be generated, because this means the frame
has ‘slipped’. After enough data byte periods, the sync signals are allowed to pass again.
There is a small chance it is possible to detect ‘false’ syncs, causing corrupted EFM bits to
form by accident in the combination RL11-RL11. If 2 (or 3) of such false syncs are
detected at the correct distance from each other, this would cause a false resync of the
demodulator. Such resync could lead to a large number of samples being corrupted at the
output of the CIRC decoder. Chance of false sync detection is highest during defects
(black and white dots).
To prevent such false demodulator resyncs, two features have been built in, which are
both programmable via register DemodControl:
The decoder block performs all processing related to error correction and CIRC
de-interleaving and uses an internal SRAM FIFO which provides the necessary data
capacity for doing this. It also extracts the Q-channel subcode and the CD-TEXT
information from the data stream and delivers it to the application via a register interface.
The channel decoder contains an internal buffer which stores the Q-channel bytes of a
CD-subcode-frame. This subcode can be retrieved by the microcontroller by accessing
the registers SubcodeQStatus, SubcodeQData and SubcodeQReadend.
To start retrieving the subcode, the microcontroller must read the register
SubcodeQStatus first. This register contains various status bits that indicate the status of
the Q-subcode that may be read. When, after reading the register SubcodeQStatus, the
QREADY bit is found ‘1’, the Q-subcode interface will be blocked (indicated by QBUSY
going to ‘1’) to prevent a new subcode overwriting the current one. Bit QCRCOK indicates
if the current subcode frame had correct data content by a hardware CRC check.
After reading SubcodeQStatus with QREADY = '1', the microcontroller may retrieve as
many subcode bytes as required (max. 10) by issuing subsequent reads to register
SubcodeQData.
The content of the Q-channel subcode in the main data area is described in
description of the content during the lead-in area, see CD red book.
•
•
ROBUSTCNTRESYNC; this feature should always be turned on; when it is on, the
demodulator will look for 3 (instead of 2) consecutive syncs with correct in-between
distance before resyncing; this will improve robustness to false syncs substantially
SYNCGATING; when ‘1’, the sync-detection is turned off during a defect, to avoid the
detection of false syncs; when ‘0’, sync detection is left on all the time; it should be
noted that the defect detector needs to be setup properly before this feature can be
used; therefore this feature is turned off by default after reset.
Rev. 01 — 20 June 2005
One chip automotive CD audio device
© Koninklijke Philips Electronics N.V. 2005. All rights reserved.
SAA7806
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