Circuit Descriptions, Abbreviation List, and IC Data Sheets
9.
AP China can have a Multi-System and I/DK version. For
India, it might only be BG standard.
9.8.2
Functionality
The features available in the Hercules are as follows:
•
Treble and Bass Control.
•
Surround Sound Effect that includes:
–
Incredible Stereo.
–
Incredible Mono.
–
3D Sound (not for AV Stereo).
–
TruSurround (not for AV Stereo).
–
Virtual Dolby Surround, VDS422 (not for AV Stereo).
–
Virtual Dolby Surround, VDS423 (not for AV Stereo).
–
Dolby Pro-Logic (not for AV Stereo).
•
Bass Feature that includes:
–
Dynamic Ultra-Bass.
–
Dynamic Bass Enhancement.
–
BBE (not for AV Stereo).
•
Auto-Volume Leveller.
•
5 Band Equalizer.
•
Loudness Control.
All the features stated are available for the Full Stereo versions
and limited features for the AV Stereo.
9.8.3
Audio Amplifier Panel (diagram SA3)
Introduction
This panel contains the audio filters and amplifiers necessary
for driving the speakers.
The audio inputs come from the SSB (via connector 1739).
The PSU delivers the positive and negative supply voltage of
16 V
DC
.
After being filtered and amplified, the signals go to the speaker
section, where the full range speakers are driven (load
impedance is 8 ohm).
Amplifier
The amplifier is an integrated class-D amplifier (TDA7490). It
combines a good performance with a high efficiency, resulting
in a big reduction in heat generation.
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
(RDS(ON)), little voltage is dropped across them, further
reducing losses.
A Low Pass Filter at the output passes only the average of the
output wave, which is an amplified version of the input signal.
In order to keep the distortion low, negative feedback is applied
(via R3723/3708).
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
disadvantag
e 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,
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.
Mute
A mute switch (item 7701) is provided at pin 6. This switch is
controlled by the SOUND_ENABLE line from the Hercules
(mute during operation).
Protections
Because of the symmetrical supply, a DC-blocking capacitor,
between the amplifier and the speaker, is not necessary.
However, it is still necessary to protect the speaker for DC
voltages. Therefore, the following protections are therefore
implemented:
•
Via R3765 and R3767, each stabilised supply voltage line
is checked on deviations.
•
Via R3718 and 3717, each amplifier output is checked for
DC-voltage.
9.8.4
Audio: Lip Sync (Optional)
A “lip sync” circuit with an audio delay can be added (not for all
models/regions), in order to synchronise with video delay due
to the complexity of the display processing. This video delay is
significant, due to memory based processing. For instance, the
“frame rate conversion” cause a delay of two frames, while the
LCD panel response also cause a delay.
The circuit is a (16 bit) FIFO based digital delay. E.g.: the
memory size required for a 80 ms delay (with a data clock of
1.024 MHz) can be calculated with: Memory size = delay time
* f_clk. This gives: 80 ms * 1.024 MHz = 81920 bits.
To calculate the memory size for a 16 bits mode I2S digital
audio stream we must use the following data:
•
f_s = 32 kHz, 16 bits, stereo
•
Data clock = 32 kHz * 16 * 2 = 1.024 MHz
•
Memory size for 1 ms delay = 1 ms * 1.024 MHz = 1024 bits
= 1 kbit
So, the delay time of 80 ms can be built with five steps of 16
ms, which is close to the frame rate. Therefore, a 128 kbit
SRAM (16 x 8) is chosen.
Note that above described calculation is just an example,
values in the set can deviate.
9.9
Control
9.9.1
Hercules
The System Board has two main micro-controllers on board.
These are:
•
On-chip x86 micro-controller (OCM) from Genesis LCD TV/
Monitor Controller.
•
On-chip 80C51 micro-controller from Philips
Semiconductors UOCIII (Hercules) series.
Each micro-controller has it own I
2
C bus which host its own
internal devices.
The Hercules is integrated with the Video and Audio Processor.
For dynamic data storage, such as SMART PICTURE and
SMART SOUND settings, an external NVM IC is being used.
Another feature includes an optional Teletext/Closed Caption
decoder with the possibility of different page storage depending
on the Hercules type number.
