TDA7407 datasheet(12/30 Pages) STMICROELECTRONICS

12 / 30 page

DESCRIPTION OF THE AUDIOPROCESSOR

PART

Input Multiplexer

CD quasi differential

Cassette stereo

Phone differential

AM mono

Stereodecoder input.

Input stages

Most of the input stages have remained the same

as in preceeding ST audioprocessors with excep-

tion of the CD inputs (see figure 4).

In the meantime there are some CD players in

the market having a significant high source im-

pedance which affects strongly the common-

mode rejection of the normal differential input

stage. The additional buffer of the CD input

avoids this drawback and offers the full common-

mode rejection even with those CD players.

The output of the Cd stage is permanently avail-

able of the Cd out-pins

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.

This stage is located before the volume-block to

eliminate all offsets generated by the Stereode-

coder, the Input Stage and the In-Gain (Please

notice that externally generated offsets, e.g. gen-

erated through the leakage current of the cou-

pling capacitors, are not cancelled).

The auto-zeroing is started every time the DATA-

BYTE 0 is selected and takes a time of max.

0.3ms. To avoid audible clicks the audioproces-

sor is muted before the volume stage during this

time.

AutoZero Remain

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 TDA7407 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.

Multiplexer Output

The output signal of the Input Multiplexer is avail-

able at separate pins (please see the Blockdia-

gram). This signal represents the input signal am-

plifier by the In Gain stage and is also going into

the Mixer stage.

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-

15K

100K

CD+

CD-

15K

PHONE+

PHONE-

15K

D98AU854A

100K

STEREODECODER

IN GAIN

CASSETTE

MPX

100K

CD OUT

Figure 4. Input stages

TDA7407

12/30

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