Philips EM3E, Service Manual, Page: 55 / 120

Circuit Descriptions and Abbreviation List GB 107 EM3E 9.
Both channels contain an 'automatic carrier mute' function,
which automatically mutes the output of the analogue section
when no carrier is detected.
After demodulation, the FM-signals are subjected to a de-
emphasis operation. After that the matrix of the stereo
system is applied.
9.12.5 Audio Processing
The sound processing in EM3E is completely done by the
MSP's:
•
Volume control is done by the user via the SOUND
menu.
• Tone control in 'Stereo' sets is done via the BASS/
TREBLE control, in 'Virtual Dolby' sets via the 5-band
equaliser.
• Headphone control, the MSP has a separate Headphone
output so separate sound control is possible.
Automatic Volume Levelling (AVL)
One of the features of the MSP-family is Automatic Volume
Limiting (AVL). If used, it limits the big volume differences in
the 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 WRMS to 2
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 cutoff 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.
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.
Figure 9-15
9.12.7 Headphone Amplifier (diagram A7)
The headphone amplifier is a straight forward OpAmp
amplifier (7770, NJM4556). It is supplied with +/- 16 V.
9.13 Teletext and NexTView
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.
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
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