TDA7407 datasheet(16/30 Pages) STMICROELECTRONICS

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If any other source than the stereodecoder is se-

lected the decoder remains muted and the MPX

pin is connected to Vref to avoid any discharge of

the coupling capacitor through leakage currents.

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 attenu-

ate spikes and nose and acts as an anti allasing fil-

ter for the following switch capacitor filters.

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

TDA7407 offers an I

2C bus programmable roll-off

adjustment which is able to compensate the low-

pass behaviour of the tuner section. If the tuner

attenuation at 38kHz is in a range from 4.2% to

31.0% the TDA7407 needs no external network

in front of the MPX pin. Within this range an ad-

justment to obtain at least 40dB channel separa-

tion 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 separation and the fieldstrength con-

trol are trimmed.

The setup of the Stereoblend characteristics

which is programmable in a wide range is de-

scribed in 2.8.

Deemphasis and Highcut.

The lowpass filter for the deemphasis allows to

choose between a time constant of 50

µs and

µs (bit D7, Stereodecoder byte).

The highcut control range will be in both cases

τHC = 2 ⋅ τDeemp. 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⋅

τDeemp. There by the resolution will remain always

5 bits independently of the absolute voltage

range between the VHCH and VHCL values.

The highcut function can be switched off by I

bus (bit D7, Fieldstrength byte set to "0").

The setup of the highcut characteristics is de-

scribed in 2.9.

PLL and Pilot Tone Detector

The PLL has the task to lock on the 19kHz pilo-

tone during a stereo transmission to allow a cor-

rect demodulation. The included detector enables

the demodulation if the pilot tone reaches the se-

lected pilot tone threshold VPTHST. Two different

thresholds are available. The detector output (sig-

nal STEREO, see block diagram) can be checked

by reading the status byte of the TDA7407 via I

bus.

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 noise-

blanker thresholds and as input for the multipath

detector. These additional functions are de-

scribed in sections 3.3 and 4.

SOFTMUTE

COMMAND

STD MUTE

D97AU638

Figure 16. Signals During Stereodecoder’s

Softmute

Figure 17. Internal Stereoblend Characteristics

TDA7407

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