TDA7400D Datasheet, PDF (17/28 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

TDA7400D
Figure 15. Relation Between Internal and External LEVEL Voltage and Setup of Stereoblend
INTERNAL
VOLTAGES
REF 5V
VSBL
SETUP OF VST
LEVEL INTERN
INTERNAL
VOLTAGES
REF 5V
LEVEL
58%
50%
VSBL 42%
33%
SETUP OF VMO
LEVEL INTERN
VMO
VST
t
FIELDSTRENGHT VOLTAGE
D97AU639
VMO
t
VST
FIELDSTRENGHT VOLTAGE
Figure 16. Highcut Characteristics
LOWPASS
TIME CONSTANT
3•τ_Deemp
τ_Deemp
VHCL
VHCH FIELDSTRENGHT
D97AU640
nal LEVEL voltage (LEVEL INTERN) into an de-
modulator compatible analog signal which is used
to control the channel separation between 0dB
and the maximum separation. Internally this con-
trol range has a fixed upper limit which is the in-
ternal reference voltage REF5V. The lower limit
can be programmed between 29.2% and 58%, of
REF5V in 4.167% steps (see figs. 11, 12).
To adjust the external LEVEL voltage to the inter-
nal range two values must be defined: the LEVEL
gain LG and VSBL (see fig. 12). To adjust the
voltage where the full channel separation is
reached (VST) the LEVEL gain LG has to be de-
fined. The following equation can be used to esti-
mate the gain:
L_G
=
Field
REF5V
strengthvoltage[STEREO]
The gain can be programmed through 4 bits in
the "Stereodecoder-Adjustment" byte.
The MONO voltage VMO (0dB channel separa-
tion) can be choosen selecting VSBL
All necessary internal reference voltages like
REF5V are derived from a bandgap circuit.
Therefore they have a temperature coefficient
near zero. This is useful if the fieldstrength signal
is also temperature compensated.
But most IF devices apply a LEVEL voltage with a
TC of 3300ppm. The TDA7400D offers this TC
for the reference voltages, too. The TC is select-
able with bit D7 of the "stereodecoder adjustment"
byte.
Highcut Control
The highcut control setup is similar to the
stereoblend control setup : the starting point
VHCH can be set with 2 bits to be 42, 50, 58 or
66% of REF5V whereas the range can be set to
be 17, 22, 28 or 33% of VHCH (see fig. 21).
FUNCTIONAL DESCRIPTION OF THE NOISE-
BLANKER
In the automotive environment the MPX signal is
disturbed by spikes produced by the ignition and
for example the wiper motor. The aim of the
noiseblanker part is to cancel the audible influ-
ence of the spikes.
Therefore the output of the stereodecoder is held
at the actual voltage for a time between 22 and
38µs (programmable).
The block diagram of the noiseblanker is given in
fig.17.
In a first stage the spikes must be detected but to
avoid a wrong triggering on high frequency
(white) noise a complex trigger control is imple-
mented. Behind the triggerstage a pulse former
generates the "blanking" pulse. To avoid any
crosstalk to the signalpath the noiseblanker is
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