CP-850FX Service Manual
5.7.4 NICAM DECODER
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.
5.7.5 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.
5.7.6 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 MSP firmware 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 MSP firmware read the transmission mode and the
signal quality level from the Stereo Detection Register. When the set is in ?Auto detection? mode
the firmware set automatically the sound mode ( mono, Stereo, Dual 1, Dual 2 ) depending on
the transmitted mode.
In ?Auto detection? mode the firmware 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.
When the sound mode change, the MSP firmware informs the microcontroller by rising pin 4.
This generates an interrupt to the controller, which then read MSP registers via I2C bus to know
the new sound status, and update OSD when needed.
In ?forced mono ? mode ( locker icon ), the controlling software configure the MSP341xG 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.
5.8 SOUND AMPLIFICATION
The TDA8946J is a stereo BTL audio amplifier capable of delivering 2 x 15 W output power to an
8 ? load at THD = 10%, using a 18 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.
5.8.1 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
Europe R&D 62 |
ACEC
