Philips Semiconductors
Preliminary specification
I
2
C-bus controlled BTSC stereo/SAP decoder
FUNCTIONAL DESCRIPTION
Input level adjustment
The composite input signal is fed to the input level
adjustment stage. The control range is from
−3.5
to +4.0 dB in steps of 0.5 dB. The subaddress
control 4 of Tables 5 and 6 and the level adjust setting of
Table 10 allows an optimum signal adjustment during the
set alignment. The maximum input signal voltage is
2 V (RMS).
Stereo decoder
The output signal of the level adjustment stage is coupled
to a low-pass filter which suppresses the baseband noise
above 125 kHz. The composite signal is then fed into a
pilot detector/pilot cancellation circuit and into the MPX
demodulator. The main L + R signal passes a 75
µs
fixed
de-emphasis filter and is fed into the dematrix circuit. The
decoded sub-signal L
−
R is sent to the stereo/SAP switch.
To generate the pilot signal the stereo demodulator uses a
PLL circuit including a ceramic resonator. The stereo
channel separation is adjusted by an automatic procedure
to be performed during set production. For a detailed
description see Section “Adjustment procedure”. The
stereo identification can be read by the I
2
C-bus
(see Table 2). Two different pilot thresholds (data
STS = 1; STS = 0) can be selected via the I
2
C-bus
(see Table 14).
SAP demodulator
The composite signal is fed from the output of the input
level adjustment stage to the SAP demodulator circuit
through a 5f
H
band-pass filter. The demodulator level is
automatically controlled. The SAP demodulator includes
an internal field strength detector that mutes the SAP
output in the event of insufficient signal conditions. The
SAP identification signal can be read by the I
2
C-bus
(see Table 2).
Noise detector
TDA9850
The composite input noise increases with decreasing
antenna signal. This makes it necessary to switch stereo
or SAP off at certain thresholds. These thresholds can be
set via the I
2
C-bus. With ST0 to ST3 (see Table 6) the
stereo threshold can be selected and with SP0 to SP3 the
SAP threshold. A hysteresis can be achieved via software
by making the threshold dependent of the identification
bits STP and SAPP (see Table 2).
Mode selection
The stereo/SAP switch feeds either the L
−
R signal or the
SAP demodulator output signal via the internal dbx noise
reduction circuit to the dematrix/switching circuit. Table 8
shows the different switch modes provided at the output
pins OUTR and OUTL.
dbx decoder
The dbx circuit includes all blocks required for the noise
reduction system in accordance with the BTSC system
specification. The output signal is fed through a 73
µs
fixed
de-emphasis circuit to the dematrix block.
SAP output
Independent of the stereo/SAP switch, the SAP signal is
also available at pin SAP. At SAP, the SAP signal is not
dbx decoded. The capacitor at SDE provides a
recommended de-emphasis (150
µs)
at SAP.
Integrated filters
The filter functions necessary for stereo and SAP
demodulation and part of the dbx filter circuits are provided
on-chip using transconductor circuits. The required filter
accuracy is attained by an automatic filter alignment
circuit.
1995 Jun 19
7
PHILIPS [ NXP SEMICONDUCTORS ]
