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Service Manual CP-785F
In case of NICAM - mode, the phase samples are decoded according the DQPSK - coding
scheme. The output of this block contains the original NICAM bitstream.
DSP section
All audio baseband functions are performed by digital signal processing (DSP). The DSP
section controls the source and output selection, and the signals processing.
Sound Mode switching
In case of NICAM transmission, the controlling software read the bit error rate and the operation
mode from the NICAM Decoder. When the set is in Auto detection mode ( default mode after
ATSS ) the controlling software set automatically the sound mode ( NICAM mono, NICAM Dual
1 or NICAM Dual 2 ) depending on the transmitted mode.
In case of 2 Carrier FM transmission, the controlling software read the transmission mode and
the signal quality level from the Stereo Detection Register. When the set is in Auto detection
mode the controlling software set automatically the sound mode ( mono, Stereo, Dual 1, Dual 2
) depending on the transmitted mode.
In Auto detection mode the controlling software evaluate the signal quality and automatically
switch to the analogy sound carrier 1, if the transmission quality is too poor. To avoid unwanted
automatic switching the threshold levels mono to stereo and stereo to mono is different. In
forced mono mode ( Red OSD in recall section ), the controlling software configure the
MSP3415D to demodulate only the analogue (FM or AM) sound carrier 1, no matter the signal
quality.
The sound mode forced or Autodetect is stored for each programme.
5-6 Sound amplification
The TDA8946J is a stereo BTL audio amplifier capable of delivering 2 x 7 W output power to an
8
load at THD = 10%, using a 12 V power supply and an external heatsink. The voltage gain
is fixed at 32dB.
With the three-level MODE input the device can be switched from standby to mute and to
operating mode.
The TDA 8946J outputs are protected by an internal thermal shutdown protection mechanism
and short-circuit protection.
Power amplifier
The power amplifier is a Bridge Tied Load (BTL) amplifier with an all-NPN output stage, capable
of delivering a peak output current of 1.5 A.
The BTL principle offers the following advantages :
- Lower peak value of the supply current.
- The ripple frequency on the supply voltage is twice the signal frequency.
- No DC-blocking capacitor
- Good low frequency performance
Mode selection
The TDA894xJ has several functional modes, which can be selected by applying the proper DC
voltage to pin MODE.
Summary of Contents for DTE-29U1TH
Page 67: ...Service Manual CP 785F 66 7 Exploded view ...
Page 68: ...Service Manual CP 785F 67 8 PCB Layout 8 1 Main PCB ...
Page 69: ...Service Manual CP 785F 68 8 2 AV PCB ...
Page 70: ... 69 Service Manual CP 785F 9 Circuit Diagram ...
Page 71: ...NEC Corporation 7 1 SHIBA 5 CHOME MINATO KU TOKYO 108 8001 JAPAN ...