TDA7407 Datasheet, PDF (16/30 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

TDA7407
Figure 16. Signals During Stereodecoder’s
Softmute
SOFTMUTE
COMMAND
t
STD MUTE
t
V_O
Figure 17. Internal Stereoblend Characteristics
D97AU638 t
If any other source than the stereodecoder is se-
lected the decoder remains muted and the MPX
pin is connected to Vref to avoid any discharge of
the coupling capacitor through leakage currents.
Ingain + Infilter
The Ingain stage allows to adjust the MPX signal to
a magnitude of about 1Vrms internally which is the
recommended value. The 4th order input filter has
a corner frequency of 80KHz and is used to attenu-
ate spikes and nose and acts as an anti allasing fil-
ter for the following switch capacitor filters.
Demodulator
In the demodulator block the left and the right
channel are separated from the MPX signal. In
this stage also the 19 kHz pilot tone is cancelled.
For reaching a high channel separation the
TDA7407 offers an I²C bus programmable roll-off
adjustment which is able to compensate the low-
pass behaviour of the tuner section. If the tuner
attenuation at 38kHz is in a range from 4.2% to
31.0% the TDA7407 needs no external network
in front of the MPX pin. Within this range an ad-
justment to obtain at least 40dB channel separa-
tion is possible.
The bits for this adjustment are located together
with the fieldstrength adjustment in one byte. This
gives the possibility to perform an optimization
step during the production of the carradio where
the channel separation and the fieldstrength con-
trol are trimmed.
The setup of the Stereoblend characteristics
which is programmable in a wide range is de-
scribed in 2.8.
Deemphasis and Highcut.
The lowpass filter for the deemphasis allows to
choose between a time constant of 50µs and
75µs (bit D7, Stereodecoder byte).
The highcut control range will be in both cases
τHC = 2 ⋅ τDeemp. Inside the highcut control range
(between VHCH and VHCL) the LEVEL signal
is converted into a 5 bit word which controls the
lowpass time constant between τDeemp...3⋅
τDeemp. There by the resolution will remain always
5 bits independently of the absolute voltage
range between the VHCH and VHCL values.
The highcut function can be switched off by I²C
bus (bit D7, Fieldstrength byte set to "0").
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 VPTHST. Two different
thresholds are available. The detector output (sig-
nal STEREO, see block diagram) can be checked
by reading the status byte of the TDA7407 via I²C
bus.
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.
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