Sound Amplification - NEC DTE-25G5THS Service Manual

Service Manual CP-785A
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.
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