TDA7407 datasheet(21/46 Pages) STMICROELECTRONICS

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TDA7407

Multipath detector

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5.1

Description of the audioprocessor part

5.1.1

Input multiplexer

●

CD quasi differential

●

Cassette stereo

●

Phone differential

●

AM mono

●

Stereo decoder input.

5.1.2

Input stages

Most of the input stages have remained the same as in preceeding ST audioprocessors with

the exception of the CD inputs (see Figure 7). In the meantime there are some CD players

on the market having a significant high source impedance which strongly affects 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 available of the CD out-pins

5.1.3

AutoZero

In order to reduce the number of pins, there is no AC coupling between the In-Gain and the

following stage, so that any offset generated by or before 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 stereo

decoder, the input stage and the In-Gain (please notice that externally generated offsets,

e.g. generated through the leakage current of the coupling capacitors, are not cancelled).

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 audioprocessor is muted before the volume stage during

this time.

5.1.4

AutoZero remain

In some cases, for example if the

μP is executing a refresh cycle of the I2C 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 applications 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.

5.1.5

Multiplexer output

The output signal of the input multiplexer is available at separate pins (please see the block

diagram). This signal represents the input signal amplifier by the In-Gain stage and is also

going into the mixer stage.

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