TDA7407 Datasheet, PDF (17/30 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

TDA7407
Figure 18. 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 19. Highcut Characteristics
LOWPASS
TIME CONSTANT
3•τ_Deemp
τ_Deemp
VHCL
VHCH FIELDSTRENGHT
D97AU640
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 low-
pass 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
device (see Testmode section 5 LEVELINTERN).
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 between 29.2% and 58%, of
REF5V in 4.167% steps (see figs. 14, 15).
To adjust the external LEVEL voltage to the inter-
nal range two values must be defined: the LEVEL
gain LG and VSBL (see fig. 15). 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:
LG
=
REF5V
Fiel d strength v ol t age[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 TDA7407 offers this TC for
the reference voltages, too. The TC is selectable
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. 19).
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