TDA7400 datasheet(15/28 Pages) STMICROELECTRONICS

15 / 28 page

will be set automatically to the high-ohmic mute

condition after the audio signal has been soft-

muted.

Hence a checking of alternate frequencies could

be performed. To release the system from the

mute condition simply the unmute command must

be sent: the stereodecoder is unmuted immedi-

ately and the audioprocessor is softly unmuted.

Fig. 13 shows the output signal VO as well as the

internal stereodecoder mute signal. This influ-

ence of Softmute on the stereodecoder mute can

be switched off by setting bit 3 of the Softmute

byte to ”0”. A stereodecoder mute command (bit

0, stereodecoder byte set to ”1”) will set the

stereodecoder in any case independently to the

high-ohmic mute state.

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

attenuate spikes and nose and acts as an anti al-

lasing filter for the following switch capacitor fil-

ters.

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

TDA7400D offers an I2C bus programmable roll-

off adjustment which is able to compensate the

lowpass behaviour of the tuner section. If the

tuner attenuation at 38kHz is in a range from

13.8% to 24.6% the TDA7400D needs no exter-

nal 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 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.

INGAIN

3.5 ... 11dB

STEP 2.5dB

INFILTER

LP 80KHz

4.th ORDER

DEMODULATOR

- PLOT CANC

- ROLL-OFF COMP.

- LP 25KHz

PLL +

PILOT-DET.

F19

NOISE BLANKER

F38

STEREO

HOLDN

SB CONTROL

DEEMPHASIS

+ HIGHCUT

t=50 or 75

µs

REF 5V

VSBL

MPINFL

LEVEL INTERN

CONTROL

VHCCH

VHCCL

MULTIPATH

DETECTOR

MP_OUT

LEVEL INPUT

LP 2.2KHZ

1.th ORDER

MP_IN

LEVEL

FM_L

FM_R

MPX

100K

D98AU855

GAIN 0..10dB

MPLEVELOUT

QUALITY DETECTOR

QUAL

NOISE

Figure 12. Block Diagram of the Stereodecoder

TDA7400

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