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AM signals are derived from the amplitude information, whereas the phase information serves for FM and NICAM
demodulation.
5.6.4 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.
5.6.5 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.
5.6.6 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.
5.7 SOUND AMPLIFICATION
The TDA8944J (TDA8946J) is a stereo BTL audio amplifier capable of delivering 2 x 7 W (2 x 15 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 8944J outputs are protected by an internal thermal shutdown protection mechanism and short-circuit
protection.
5.7.1 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
5.7.2 MODE SELECTION
The TDA894xJ has several functional modes, which can be selected by applying the proper DC
voltage to pin MODE.
CP-850/F Service Manual
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