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Service manual WP 895/895F, CP885/885F
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 reads 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 sets
automatically the sound mode ( NICAM mono, NICAM Dual 1 or NICAM Dual 2 ) depending on the transmitted
mode.
In the 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 automatically sets
the sound mode ( mono, Stereo, Dual 1, Dual 2 ) depending on the transmitted mode.
In “Auto detection” mode the controlling software evaluates the signal quality and automatically switches to the analogue
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 are different.
In “forced mono “ mode ( Red OSD in status Display Window), the controlling software configures the MSP341xD to
demodulate onl y 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 W 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