TDA7402 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'TDA7402' PDF : 69 Pages View PDF
Functional description of stereo decoder
TDA7402
the 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 5.8.
5.4
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 τDeemp or 2.7 x τDeemp 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 τDeemp...3 (3.7) x τDeemp.
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
t(s between 2 and 10dB.
c The highcut function can be switched off by I2C bus (bit D7, Highcut byte set to "0").
du The setup of the highcut characteristics is described in Chapter 5.9.
te Pro 5.5
PLL and pilot tone detector
le The PLL has the task to lock on the 19kHz pilot tone during a stereo transmission to allow a
o correct demodulation. The included pilot tone-detector enables the demodulation if the pilot
bs tone reaches the selected pilot tone threshold VPTHST. Two different thresholds are
available. The detector output (signal STEREO, see Figure 2: Block diagram) can be
O checked by reading the status byte of the TDA7402 via I2C-bus. During a Soft Mute the PLL
) - can be set into "Hold"-mode which freezes the PLL's state (bit D4, Soft Mute byte). After
t(s releasing the Soft Mute the PLL will again follow the input signal only by correcting the
phase error.
roduc 5.6
Fieldstrength control
P The fieldstrength input is used to control the highcut and the stereo blend function. In
teaddition the signal can be also used to control the noiseblanker thresholds and as input for
lethe multipath detector. These additional functions are described in sections 5.3 and 6.
Obso 5.7
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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