TDA7400 ST Microelectronics, TDA7400 Datasheet - Page 16

• Manufacturer Part Number: TDA7400
• Description: ADVANCED CAR SIGNAL PROCESSOR
• Manufacturer: ST Microelectronics

TDA7400
Figure 13. Signals During Stereodecoder’s
Deemphasis and Highcut.
The lowpass filter for the deemphasis allows to
choose between a time constant of 50 s and
75 s (bit D
The highcut control range will be in both cases
(between VHCH and VHCL) the LEVEL signal
is converted into a 5 bit word which controls the
lowpass
5 bits independently of the absolute voltage
range between the VHCH and VHCL values.
The highcut function can be switched off by I
bus (bit D
The setup of the highcut characteristics is de-
scribed in 2.9.
PLL and Pilot Tone Detector
The PLL has the task to lock on the 19kHz pilo-
tone during a stereo transmission to allow a cor-
rect demodulation. The included detector enables
the demodulation if the pilot tone reaches the se-
lected pilot tone threshold V
thresholds are available. The detector output (sig-
nal STEREO, see block diagram) can be checked
by reading the status byte of the TDA7400D via
I
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 and as input for the multipath
detector. These additional functions are de-
scribed in sections 3.3 and 4.
16/28
2
HC
Deemp
C bus.
SOFTMUTE
COMMAND
= 2
STD MUTE
. There by the resolution will remain always
7
V
time constant
, Fieldstrength byte set to ”0”).
O
Deemp
7
Softmute
, Stereodecoder byte).
. Inside the highcut control range
between
PTHST
. Two different
D97AU638
Deemp
t
t
t
...3
2
C
Figure 14. Internal Stereoblend Characteristics
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. 11, 12).
To adjust the external LEVEL voltage to the inter-
nal range two values must be defined: the LEVEL
gain L
voltage where the full channel separation is
reached (VST) the LEVEL gain L
fined. The following equation can be used to esti-
mate the gain:
G
and VSBL (see fig. 12). To adjust the
G
has to be de-