Audio Processing - Philips LC7.1E Service Manual

Circuit Descriptions, Abbreviation List, and IC Data Sheets
9.7

Audio Processing

The audio decoding is done entirely via the Multistandard
Sound Processor (MSP) 4450P (item 7411).
This processor covers the processing of both analogue and
(NICAM) digital input signals by processing the (analogue) IF
signal-in to processed (analogue) AF-out (baseband audio). An
internal 40 ms (stereo) audio delay line (LIP SYNC) is foreseen
and therefore no external delay line is necessary.
All internal clock signals are derived from an external
18.432 MHz oscillator, which, in NICAM or I
turn is locked to the corresponding source.
The following functionality is included:
• Automatic Standard Detection (ASD) automatically detects
the actual broadcasted TV standard
• Automatic Sound Select (ASS) automatically switches
2
(without any I C-bus action) between mono/stereo/
bilingual mode when the broadcast mode changes.
9.7.1 Audio Application
ANALOGUE
2 nd SIF
FRONT END
ANA_IN1+
I 2 S1
I 2 S_DA_IN1
DVB / MOJO I2 S_WS
(if present)
I 2 S_CL
SCART 1 IN
SC1-IN
SCART 2 IN
SC2-IN
COMP IN
SC3-IN
SIDE IN
SC4-IN
HDMI IN
HDMI AUDIO
SC5-IN
IC DAC
Figure 9-8 Block diagram audio processing - EU application
In EU applications, the MSP features:
• Sound IF input for signals coming from the analogue front-
end
2
• Three I S-inputs for signals ("DATA", "CLK" and "WS")
coming from the MOJO in case of digital reception
• Five analogue inputs: for EXT1 to EXT4 and HDMI
• Loudspeaker output path
• Headphone output path
• SCART-1 output path (RF)
• SCART-2 output path (WYSIWYG = monitor).
Digital audio signals coming from HDMI sources are fed to a
digital-to-analogue converter and then fed to the MSP.
In case of reception of digital TV signals, digital audio signals
coming from the MOJO are directly fed to the MSP via the
I2S_DA_IN1, I2S_WS1 and I2S_CL1 lines. This ensures a
"true digital path".
The microprocessor (item 7311) controls the audio part with the
following control lines:
• MUTEn: used to mute the Class D amplifiers
• ANTI_PLOP: used to detect any DC failure in the Class D
amplifiers
• DC_PROT: used to detect any DC failure in the Class D
amplifiers.
9.7.2 Audio Amplifier
The audio amplifier is an integrated class-D amplifier
(TDA8932T, item 7A01). It combines a good performance with
a high efficiency, resulting in a big reduction in heat generation.
2
S-mode, on its
MSP 4450P
DACM
LOUDSPEAKER
CLASS D
AMPLIFIER
DACA
HP AMPLIFIER
SC1-OUT
SCART 1 OUT
SCART 2 OUT
SC2-OUT
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Principle
Figure 9-9 Principle Class-D Amplifier
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.
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. The main reason
for this filter is that the switching waveform results in maximum
090307
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, thus reducing the
overall loss and increasing the efficiency.
DC-protection
A DC-detection circuit is foreseen to protect the speakers. It is
built around three transistors (items 7A05 to 7A07) and
generates a protection signal (DC_PROT) to the
microprocessor in case of a DC failure in the Class D
amplifiers.
LC7.1E PA 9.
+V
-V
EN 73
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