Demodulation And De-Emphasis; Audio Baseband And Signal Routing - Pace MSS100 Series Service Manual

DEMODULATION AND DE-EMPHASIS
All of these functions are carried out by the audio and video processor
and switch matrix chip US00. The baseband input from the emitter of
Q101 is filtered by C556, C555, L503, R539 and R538 which form a
simple high pass filter with a chroma trap at 4.43 MHz. This strips most
of the video off the baseband and the filtered signal enters U500 at
pin 23. All functions are then carried out within the chip under control
of the microprocessor U700.
PERIPHERAL COMPONENTS AND CONTROL LINES
U500 is controlled via the ?C clock and data on pins 30 and 31 respec-
tively, It needs a 273 kQ. 1% resistor between pin 50 and ground to
produce an accurate current reference for biasing of amplifiers within
the chip and it provides accurate roll-off frequencies for the audio
filters. If this resistor is the wrong value or missing U500 will not
function. There is one general purpose Input/Output port on pin 29
which is used as a 22 kHz source for the tone switching option (see
page 19).
AUDIO BASEBAND PROCESSING AND SIGNAL ROUTING
The desired subcarrier frequency is loaded into a frequency synthesiser
which sets the frequency of an internal local oscillator. When the
frequency is reached the microprocessor (U700) switches in a PLL and
demodulation starts.
The AMPLK pins 41 and 46 are amplitude detectors which measure
approximately 3 V DC when locked to a subcarrier and measure 1.5 V
DC when no carrier is present.
The DET pins 47 and 39 are the output of the left and right FM phase
detectors and have audio superimposed on approximately 180 kHz.
The 180 kHz is a beat frequency which appears when two subcarriers
spaced by 180 kHz are input to the PLL.
The CPUMP pins 49 and 38 are the output of the frequency synthesiser
which drives the voltage to pull the VCO to the target frequency. As
the audio frequency is increased, the voltage on the CPUMP pins
increases.
The PKOUT pins 51 and 52, form the PANDA control loop peak detec-
tor. The DC level on these pins rises during loud passages of audio and
falls during quiet passages.
The FC pins 1 and 55, define the min/max roll off frequencies
repectively. Audio can be seen on these pins.
The J17 pins 32 and 33 set the de-emphasis for J17. Audio can be seen
on these pins.
Switch K3 selects between PANDA and non-PANDA de-emphasis.
Panda de-emphasis is the ANRS plus 75 ys. Switch K2 selects between
317,50 ps or 75 us. Switch K1 selects the audio source for the TV
SCART, internal satellite, external decoder or VCR. Switch K5 selects
the audio source for the VCR SCART, internal decoder or external
decoder. The audio source for the decoder SCART is fixed to internal.
The signal routing is shown in the diagram on page 22, so if an
external decoder is connected and pin 8 goes high then the decoder
audio will be switched by K5 out to the VCR SCART and by switch K1
out to the TV SCART. As can be seen, the audio output on the DE-
CODER and VCR SCARTs is of a fixed level whereas the audio output
on the TV SCART has volume control. Similarly if a VCR is connected
and pin 8 goes high then the VCR audio return will be switched by K1
outto the TV SCART.
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Circuit Description