TDA7284 datasheet(9/14 Pages) STMICROELECTRONICS

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Usually such an increase drives the op-amp into

saturation and the time for which it remains in this

condition is called the limiting time(T1).

T1 depends on the relationship between the ex-

ternal capacitances, the time constant T=R1

• C1,

the supply voltage and the signal variation.

The criteria for choosing the length of T1 are the

result of several compromises. In particular if T1

is too long, there will be audible distortion during

playback (during T1 the output is a square

wave),and if it is too short, the sensation of in-

creased level will be lost while dynamic compres-

sion phenomena and instability may occur.

The time for return to nominal level is defined as

the total time between the instant To and the in-

stant in which the output reassumes the nominal

value. This time (Ts) is roughly equal to 5

• T1.

On the basis of tests carried out it has been found

that a musical signal with high dynamic range

(

∆VI =+40 dB) is to be recorded, the best value of

Ts is between 200 and 300ms.

Recovery time.

let us now suppose that at the instant To the input

signal decreases of

∆Vi (fig. 18).

The recovery time (Trec) is defined as the time

between the instant To and the instant in which

the output signal returns to the nominal level.

This time depends essentially on the discharge

time constant of R2

• C2 ( see fig. 16) and on the

size of the step -

∆Vi. In this case too, if this time

is too long the signal to noise ratio on the tape de-

teriorates.

If it is too short the sensation of the low signal

level is lost during playback.

The ALC system of the TDA7284

Fig. 16 becomes the following (fig. 19) where the

Figure 17: Limiting and Level Setting Time

Figure 18: Recovery Time

Figure 19

TDA7284

9/14

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