E-TDA7461ND View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'E-TDA7461ND' PDF : 48 Pages View PDF
TDA7461
Stereo decoder part
4.5.3
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 TDA7461 offers an I2C bus programmable roll off adjustment which is able to
compensate the lowpass behavior of the tuner section.
If the tuner attenuation at 38 kHz is in a range from 20.2 % to 31 % the TDA7461 needs no
external network before the MPX pin. Within this range an adjustment to obtain at least
40 dB 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 carradio where the channel separation and the
fieldstrength control are trimmed.
Figure 21. Signals during stereo decoder’s soft mute
SOFTMUTE
COMMAND
t
STD MUTE
t
VO
4.5.4
4.5.5
D97AU638
t
De-emphasis and high cut
The lowpass filter for the de-emphasis allows to choose between a time constant of 50 μs
and 75 μs (bit D7, Stereo decoder byte).
The high cut control range will be in both cases tHC = 2 * tDeemp. 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 tDeemp...3 × tDeemp.
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
I2C bus (bit D7, Fieldstrength byte set to "0").
PLL and pilot tone detector
The PLL has the task to lock on the 19 kHz 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 block diagram) can be checked by reading the status
byte of the TDA7461 via I2C bus.
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