TDA7403 datasheet(6/25 Pages) STMICROELECTRONICS

6 / 25 page

Input stages

The input circuits are the same as in preceeding

ST audioprocessors with exception of the CD in-

puts (see figure 1).

The typical input impedance is 100k

Ω.

AutoZero

In order to reduce the number of pins there is no

AC coupling between the In-Gain and the follow-

ing stage, so that any offset generated by or be-

fore the In-Gain stage would be transferred or

even amplified to the output.

To avoid that effect a special offset cancellation

stage called AutoZero is implemented.

To avoid audible clicks the audioprocessor is

muted before the loudness stage during this time.

In some cases, for example if the

µP is executing

a refresh cycle of the I

2C bus programming, it is

not useful to start a new AutoZero action because

no new source is selected and an undesired mute

would appear at the outputs. For such applica-

tions the TDA7403 could be switched in the "Auto

Zero Remain" mode (Bit 6 of the subaddress

byte). If this bit is set to high, the DATABYTE 0

could be loaded without invoking the AutoZero

and the old adjustment value remains.

Softmute

The digitally controlled softmute stage allows

muting/demuting the signal with a I

2C bus pro-

grammable slope. The mute process can either

be activated by the softmute pin or by the I

bus. The slope is realized in a special S shaped

curve to mute slow in the critical regions (see fig-

ure 2).

For timing purposes the Bit 3 of the I

2C bus out-

put register is set to 1 from the start of muting un-

til the end of demuting.

D98AU951

100K

STEREODECODER

IN GAIN

CASSETTE

MPX

Figure 1. Input stages

DESCRIPTION OF THE AUDIOPROCESSOR

EXT.

MUTE

+SIGNAL

REF

-SIGNAL

2C BUS

OUT

Time

D97AU634

Figure 2. Softmute Timing

2dB

1dB

-1dB

-2dB

Time

D97AU635

VOUT

10ms

Figure 3. Soft Step Timing

Note: Please notice that a started Mute action is always terminated

and could not be interrupted by a change of the mute signal.

Note: For steps more than 1dB the softstep mode should be

deactivated because it could generate a 1dB error during the

blend-time

TDA7403

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