TDA7541 datasheet(34/76 Pages) STMICROELECTRONICS

34 / 76 page

Functional description

TDA7541

34/76

compensation bits in byte14 to simplify a possible adaptation during the production of the

car radio.

Stereo blend control

The stereo blend control block converts the internal LEVEL-voltage into a 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 VREF1. The lower limit can be programmed between 29 and 58%

of VREF1 in 4% steps. In order to adjust the external voltage VFSTC to the internal control

range two values must be defined: the Level gain LG and VSBL. Full channel separation is

reached when the internal level voltage (VST) becomes bigger than VREF1. Therefore the

following equation can be used to estimate the gain:

The MONO-voltage VMO (0dB channel separation) can be chosen selecting SBC.

Figure 4.

Programming of stereo blend

Highcut control

The high cut control set-up is similar to the stereo blend control set-up: the starting point

VHCH can be set with 2 bits to be 42, 50, 58 or 66% of VREF1 whereas the range can be

set to be 11, 18.3, 25.7 or 33% of VHCH.

4.3.2

Noise blanker

In the automotive environment spikes produced for example by the ignition or the wiper-

motor disturb the MPX-signal. 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 a time between 22ms and 38ms (programmable).

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. An own biasing circuit supplies the noise

blanker in order to avoid any cross talk to the signal path.

Trigger path

The incoming FM demodulator output signal is taken in front of the weak signal mute, high

pass filtered, amplified and rectified. This second order high pass filter has a corner-

REF1

FSTC @ fullstereo

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