TDA7400 datasheet(16/28 Pages) STMICROELECTRONICS

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Deemphasis and Highcut.

The lowpass filter for the deemphasis allows to

choose between a time constant of 50

µs and

µs (bit D7, Stereodecoderbyte).

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 TDA7400D via

2C 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.

LEVEL Input and Gain

To suppress undesired high frequency modula-

tion on the highcut and stereoblend function the

LEVEL signal is lowpass filtered firstly.

The filter is a combination of a 1st order RC low-

pass 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

device (see Testmode section 5 LEVELINTERN).

The gain is widely programmable in 16 steps

from 0dB to 10dB (step = 0.67dB). These 4 bits

are located together with the Roll-Off bits in the

”Stereodecoder Adjustment” byte to simplify a

possible adaptation during the production of the

carradio.

Stereoblend Control

The stereoblend control block converts the inter-

nal LEVEL voltage (LEVEL INTERN) into an de-

modulator compatible analog signal which is used

to control the channel separation between 0dB

and the maximum separation. Internally this con-

trol range has a fixed upper limit which is the in-

ternal reference voltage REF5V. The lower limit

can be programmed between 29.2% and 58%, of

REF5V in 4.167% steps (see figs. 11, 12).

To adjust the external LEVEL voltage to the inter-

nal range two values must be defined: the LEVEL

gain LG and VSBL (see fig. 12). To adjust the

voltage where the full channel separation is

reached (VST) the LEVEL gain LG has to be de-

fined. The following equation can be used to esti-

mate the gain:

SOFTMUTE

COMMAND

STD MUTE

D97AU638

Figure 13. Signals During Stereodecoder’s

Softmute

Figure 14. Internal Stereoblend Characteristics

TDA7400

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