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TDA7407D

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Figure 21. Internal Stereoblend Characteristics

If any other source than the stereodecoder is selected the decoder remains muted and the MPX pin is

connected to Vref to avoid any discharge of the coupling capacitor through leakage currents.

6.5.2 Ingain + Infilter

The Ingain stage allows to adjust the MPX signal to a magnitude of about 1Vrms internally which is the

recommended value. The 4th order input filter has a corner frequency of 80KHz and is used to attenuate

spikes and nose and acts as an anti allasing filter for the following switch capacitor filters.

6.5.3 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 TDA7407D offers an

section. If the tuner attenuation at 38kHz is in a range from 7.2% to 31.0% the TDA7407D needs no ex-

ternal network in front of the MPX pin. Within this range an adjustment to obtain at least 40dB 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 sep-

aration and the fieldstrength control are trimmed.

6.5.4 Deemphasis and Highcut.

The lowpass filter for the deemphasis allows to choose between a time constant of 50

µs and 75µs (bit D7,

Stereodecoder byte).

The highcut control range will be in both cases

VHCH and VHCL) the LEVEL signal is converted into a 5 bit word which controls the lowpass time con-

stant between

absolute voltage range between the VHCH and VHCL values.

6.5.5 PLL and Pilot Tone Detector

The PLL has the task to lock on the 19kHz pilotone during a stereo transmission to allow a correct demod-

ulation. The included detector enables the demodulation if the pilot tone reaches the selected pilot tone

Fieldstrength Control

The fieldstrength input is used to control the high cut and the stereoblend function. In addition the signal

can be also used to control the noiseblanker thresholds and as input for the multipath detector.

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