TDA7406 Datasheet, PDF (37/64 Pages) STMicroelectronics – Car radio signal processor

TDA7406
Functional Description of Stereodecoder
8.4
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 time constant
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 Section 8.9.
8.5
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 Block diagram) can be checked by
reading the status byte of the TDA7406 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.
8.6
Fieldstrength Control
The fieldstrength input is used to control the highcut- and the stereoblend-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 5.3 and 6.
8.7
LEVEL-Input and -Gain
To suppress undesired high frequency modulation on the highcut- and stereoblend-control
signal the LEVEL signal is lowpass filtered firstly. The filter is a combination of a 1.st-order
RC-lowpass at 53kHz (working as anti-aliasing filter) and a 1.st-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 “Stereodecoder-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 stereoblend stage (see
Figure 26).
8.8
Stereoblend Control
The stereoblend control block converts the internal LEVEL-voltage (LEVELSB) into an
demodulator compatible analog signal which is used to control the channel separation
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