TDA7284 Datasheet, PDF (9/14 Pages) STMicroelectronics – RECORD/PLAYBACK CIRCUIT WITH ALC

Figure 17: Limiting and Level Setting Time
TDA7284
(∆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).
Figure 18: Recovery Time
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
Figure 19
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
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