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TDA7460ND

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Stereo decoder part TDA7460ND
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5.5.2 Input stages
The In gain stage allows to adjust the MPX signal to a magnitude of about 1Vrms internally
which is the recommended value. The 4.th order input filter has a corner frequency of 80kHz
and is used to attenuate spikes and noise and acts as an anti-aliasing filter for the following
switch capacitor filters.
5.5.3 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 TDA7460ND offers an I
2
C bus programmable roll-off adjustment which is able to
compensate the lowpass behavior of the tuner section. If the tuner attenuation at 38kHz is in
a range from 20.2% to 31% the TDA7460ND needs no external network before the MPX pin.
Within this range an adjustment to obtain at least 40d B channel separation 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
car radio where the channel separation and the fieldstrength control are trimmed.
Figure 22. Signals during stereo decoder's soft mute
5.5.4 De-emphasis and high cut
The lowpass filter for the de-emphasis allows to choose between a time constant of 50ms
and 75ms (bit D7, Stereo decoder byte).
The high cut control range will be in both cases t
HC
= 2 · t
Deemp
. Inside the high cut control
range (between VHCH and VHCL) the LEVEL signal is converted into a 5 bit word which
controls the lowpass time constant between t
Deemp
...3 · t
Deemp
. There by the resolution will
remain always 5 bits independently of the absolute voltage range between the VHCH and
VHCL values.
The high cut function can be switched off by I
2
C bus (bit D7, Fieldstrength byte set to "0").
SOFTMUTE
COMMAND
STD MUTE
V
O
t
D97AU638
t
t
Obsolete Product(s) - Obsolete Product(s)
TDA7460ND Stereo decoder part
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5.5.5 PLL and pilot tone detector
The PLL has the task to lock on the 19kHz pilotone during a stereo transmission to allow a
correct demodulation. The included detector enables the demodulation if the pilot tone
reaches the selected pilottone threshold VPTHST. Two different thresholds are available.
The detector output (signal STEREO, see Figure 1: Block diagram) can be checked by
reading the status byte of the TDA7460ND via I
2
C bus.
5.5.6 Fieldstrength control
The fieldstrength input is used to control the high cut and the stereo blend function. In
addition the signal can be also used to control the noise blanker thresholds.
5.5.7 Level input and gain
To suppress undesired high frequency modulation on the high cut and stereo blend function
the LEVEL signal is lowpass filtered firstly. The filter is a combination of a 1
st
order RC
lowpass at 53 kHz (working as anti-aliasing filter) and a 1st-order switched capacitor
lowpass at 2.2 kHz. 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 0 dB to 10 dB (step = 0.67 dB). These 4
bits are located together with the roll-off bits in the "Stereo decoder Adjustment" byte to
simplify a possible adaptation during the production of the carradio.
Figure 23. Internal stereo blend characteristics
5.5.8 Stereo blend control
The stereo blend control block converts the internal LEVEL voltage (LEVEL INTERN) into
an demodulator compatible analog signal which is used to control the channel separation
between 0dB and the maximum separation. Internally this control range has a fixed upper
limit which is the internal reference voltage REF5V. The lower limit can be programmed to
be 33%, 42%, 50% or 58% of REF5V (see Figure 24.).
To adjust the external LEVEL voltage to the internal range two values must be defined: the
LEVEL gain L
G
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:
REF5V
Field stren
g
ht volta
g
e
[
STE
R
------------------------------------------------------------------------
-
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Stereo decoder part TDA7460ND
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The gain can be programmed through 4 bits in the "Stereo decoder-Adjustment" byte.
The mono voltage VMO (0 dB channel separation) can be chosen 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 3300 ppm. The TDA7460ND offers this
TC for the reference voltages, too. The TC is selectable with bit D7 of the "stereo decoder
adjustment" byte.
Figure 24. Relation between internal and external LEVEL voltage and setup of stereo
blend
5.5.9 High cut control
The high cut control setup is similar to the stereo blend control setup: the starting point
VHCH can be set with 2 bits to be 42, 50, 58 or 66% of REF5 V whereas the range can be
set to be 17 or 33% of VHCH (see Figure 25.).
Figure 25. High cut characteristics
5.6 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 noise blanker part is to cancel the
audible influence of the spikes. Therefore the output of the stereo decoder is held at the
actual voltage for 40 μs.
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 implemented. Behind the trigger stage a
pulse former generates the "blanking" pulse. To avoid any crosstalk to the signal path the
noise blanker is supplied by his own biasing circuit.
INTERNAL
VOLTAGES
t
D97AU639
VSBL
REF 5V
SETUP OF VST
INTERNAL
VOLTAGES
t
33%
REF 5V
SETUP OF VMO
LEVEL
LEVEL INTERN
FIELDSTRENGHT VOLTAGE
VSTVMO
LEVEL INTERN
42%
50%
58%
VSBL
VST
VMO
FIELDSTRENGHT VOLTAGE
LOWPASS
TIME CONSTANT
D97AU640
τ
Deemp
FIELDSTRENGHTVHCHVHCL
3•τ
Deemp
Obsolete Product(s) - Obsolete Product(s)
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TDA7460ND

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Buy TDA7460ND
Manufacturer:
STMicroelectronics
Description:
Audio DSPs Car Radio Signal
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