TDA7402_07 Datasheet, PDF (42/71 Pages) STMicroelectronics – Car radio signal processor

Functional description of stereo decoder
TDA7402
channel separation and the fieldstrength control are trimmed. The setup of the stereo blend
characteristics which is programmable in a wide range is described in Chapter 6.8.
6.4
De-emphasis and highcut
The de-emphasis lowpass allows to choose a time constant between 37.5 and 100µs. The
highcut control range will be 2 x τ_Deemp or 2.7 x τ_Deemp dependent on the selected time
constant (see programming section). The bit D7 of the hightcut-byte will shift timeconstant
and range.
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 τ_Deemp...3 (3.7) x τ_Deemp.
Thereby the resolution will remain always 5 bits independently of the absolute voltage
range between the VHCH and VHCL values. In addition the maximum attenuation can be
fixed between 2 and 10dB.
The highcut function can be switched off by I²C bus (bit D₇, Highcut byte set to "0").
The setup of the highcut characteristics is described in Chapter 6.9.
6.5
PLL and pilot tone detector
The PLL has the task to lock on the 19kHz pilot tone during a stereo transmission to allow a
correct demodulation. The included pilot tone-detector enables the demodulation if the pilot
tone reaches the selected pilot tone threshold V_PTHST. Two different thresholds are
available. The detector output (signal STEREO, see the Block diagram) can be checked by
reading the status byte of the A619 via I²C-bus. During a Softmute the PLL can be set into
"Hold"-mode which freezes the PLL's state (bit D₄, Softmute byte). After releasing the
Softmute the PLL will again follow the input signal only by correcting the phase error.
6.6
Field strength control
The fieldstrength input is used to control the highcut and the stereo blend function. In
addition the signal can be also used to control the noiseblanker thresholds and as input for
the multipath detector. These additional functions are described in sections 6.3 and 7.
6.7
EVEL input and gain
To suppress undesired high frequency modulation on the highcut- and stereo blend-control
signal the LEVEL signal is lowpass filtered firstly. The filter is a combination of a 1st order
RC lowpass 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 devices (see Testmode section 5: LEVELHCC). The gain is
widely programmable in 16 steps from 0dB to 6dB (step=0.4dB). These 4 bits are located
together with the Roll-Off bits in the "Stereo decoder adjustment" byte to simplify a possible
adjustment during the production of the car radio. This signal controls directly the Highcut
stage whereas the signal is filtered again (fc=100Hz) before the stereo blend stage
(see Figure 35).
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