TDA7461N Datasheet, PDF (18/31 Pages) STMicroelectronics – CAR RADIO SIGNAL PROCESSOR

TDA7461N
by reading the status byte of the TDA7461N via
I2C bus.
Figure 19. Internal stereoblend characteristics
Fieldstrength Control
The fieldstrength input is used to control the high
cut and the stereoblend function. In addition the
signal can be also used to control the noise-
blanker thresholds.
LEVEL Input and Gain
To suppress undesired high frequency modula-
tion on the highcut and stereoblend function the
LEVEL signal is lowpass filtered firstly. The filter
is a combination of a 1st order RC lowpass at
53kHz (working as anti-aliasing filter) and a 1st-
order switched capacitor lowpass at 2.2kHz. The
second stage is a programmable gain stage to
adapt the LEVEL signal internally to different IF.
The gain is widely programmable in 16 steps
from 0dB to 10dB (step = 0.67dB). These 4 bits
are located together with the Roll-Off bits in the
”Stereodecoder Adjustment” byte to simplify a
possible adaptation during the production of the
carradio.
Stereoblend Control
The stereoblend control block converts the inter-
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 to be 33%, 42%, 50% or
58% of REF5V (see fig. 20).
To adjust the external LEVEL voltage to the inter-
nal range two values must be defined: the LEVEL
gain LG and VSBL. To adjust the voltage where
the full channel separation is reached (VST) the
LEVEL gain LG has to be defined. The following
equation can be used to estimate the gain:
L_G
=
Field
REF5V
strength voltage
[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 33, 42, 50 or 58%
of REF5V.
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 TDA7461N offers this TC
for the reference voltages, too. The TC is select-
able with bit D7 of the ”stereodecoder adjust-
ment” byte.
Figure 20. 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
VSBL
58%
50%
42%
33%
SETUP OF VMO
LEVEL INTERN
18/31
VMO
VST
t
FIELDSTRENGHT VOLTAGE
D97AU639
VMO
t
VST
FIELDSTRENGHT VOLTAGE