Philips EM5E, Руководство по обслуживанию, Страница: 120 / 136

Circuit Descriptions and Abbreviation List EN 120 EM5E 9.
broadcast between e.g. news transmissions and commercials,
or within a movie.
To be able to get a Dolby approval (for the Virtual Dolby sets),
the AVL feature must be switchable. Therefore, the AVL
feature is customer switchable via the menu.
9.12.6 Audio Amplifier (Diagram A6)
The audio amplifier is an integrated 'class-D' power amplifier
IC, the TDA7490. It delivers an output of 2 x 15 W
RMS
to two full
range speakers. For the higher specified sets, a subwoofer and
centre speaker are easy to implement, by adding a second
amplifier.
Principle
Audio-power-amplifier systems have traditionally used linear
amplifiers, which are well known for being inefficient. In fact, a
linear Class AB amplifier is designed to act as a variable
resistor network between the power supply and the load. The
transistors operate in their linear region, and the voltage that is
dropped across the transistors (in their role as variable
resistors) is lost as heat, particularly in the output transistors.
Class D amplifiers were developed as a way to increase the
efficiency of audio-power-amplifier systems.
The Class D amplifier works by varying the duty cycle of a
Pulse Width Modulated (PWM) signal.
By comparing the input voltage to a triangle wave, the amplifier
increases duty cycle to increase output voltage, and decreases
duty cycle to decrease output voltage.
The output transistors of a Class D amplifier switch from full 'off'
to full 'on' (saturated) and then back again, spending very little
time in the linear region in between. Therefore, very little power
is lost to heat.
If the transistors have a low 'on' resistance R
DS(ON)
, little voltage
is dropped across them, further reducing losses.
The advantage of Class D is increased efficiency (= less heat
dissipation). Class D amplifiers can drive the same output
power as a Class AB amplifier using less supply current.
The disadvantage is the large output filter that drives up cost
and size. The main reason for this filter is that the switching
waveform results in maximum current flow. This causes more
loss in the load, which causes lower efficiency.
An LC filter with a cut-off frequency less than the Class D
switching frequency (200 kHz), allows the switching current to
flow through the filter instead of the load. The filter is less lossy
than the speaker, which causes less power dissipated at high
output power and increases efficiency in most cases.
The supply voltage is a symmetrical voltage of +/- 16 V (or +/-
19 V depending on the set execution), generated by the main
supply via L5512 (or L5506).
Mute
Sound-mute is done via the STBY-MUTE line connected to pin
6 (tri-state input) of the amplifier IC. This line is activated via the
SOUND-ENABLE line (software controlled) and/or POR line
(hardware controlled).
This signal is inverted by TS7701 and/or TS7703, as a result of
which at a high level of the SOUND-ENABLE signal, current is
sinked from pin 2 and the IC mutes.
Protection
A loudspeaker DC protection is implemented. The amplifier IC
TDA7490 has a 'standby-mute-play' feature via the tri-state
input pin 6. When the voltage on pin 6 is:
•
0 - 0.7 V, the IC is in 'standby' mode
• 1.7 - 2.5 V, the IC is in 'mute' mode
• 4 - 5 V, the IC is in 'play' mode (normal operation).
Note: this is a 'local' protection, not known to the
microprocessor.
Figure 9-22 Loudspeaker DC-protection
This works as follows:
•
During normal operation, V
X
is 'low' (0 V) and V
Y
is 'high'
(+5 V).
• When a positive DC-voltage is detected on a loudspeaker
output (= V
X
), TS7704, TS7706, and TS7707 will conduct,
making V
Y
'low'.
• When a negative DC-voltage is detected on a loudspeaker
output (= V
X
), TS7705, TS7706, and TS7707 will conduct,
making V
Y
also 'low'.
• When V
Y
is 'low' (read 0 V), the tri-state input (pin 6) puts
the amplifier IC in 'standby' mode.
9.12.7 Headphone Amplifier (Diagram B10)
The headphone amplifier is a straightforward OpAmp amplifier
(IC7681, NJM4556). It is supplied with +8 V.
9.12.8 Wireless Surround
A wireless surround-sound transmitter is obtainable as
accessory. For diagrams and circuit description see Service
Manual 'A10E-DPL' (order nr. 3122 785 10980).
9.13 Teletext and NexTView (Diagram B5)
9.13.1 Teletext
The TXT-decoder in the OTC gets its video signal directly on
pin 5 (from the HIP).
The RGB-outputs are available on pins 77/78/79. Fast blanking
is realised via pen 80.
The DRAM (IC7007) of the microprocessor is used for storage
of the TXT pages.
9.13.2 NexTView
NexTView allows the user to display a program guide on the TV
screen, which contains extensive information for each program
(only when supported by the programs provider).
This information can be displayed in a number of different
summaries:
•
DAY: The daily summary shows, from the current moment,
the program schedule for several stations for a short time
ahead.
• CHANNEL: The channel summary shows the program
schedule for one station.
• THEME: The theme summary shows, for each theme, the
program schedule of the various stations. These themes
CL 16532044_017.eps
140501
OUT
R
L
(RES)
C/SW
(PROT 1)
2778
7704
47K
47K
10K
+5V
47K
33K
7705
2705
7707
470N
VY
OUT
6 23
VX
7703
7706
7701
10K
10K
Sound Enable
POR
SOUND-IC's
7700/7702
STANDBY-MUTE
(TRISTATE)
100K
10K