TDA7336 Datasheet, PDF (8/20 Pages) STMicroelectronics – STEREO PREAMP+ AMS+ DOLBY B* NOISE REDUCTION PROCESSOR

TDA7336
AUDIO MUSIC SENSOR (See Figure 2)
Aim of this section is to detect interprogram
spaces present on a recorded tape.
Both the blanks (interprogram spaces) and the
programs minimum detectable durations can be
easily set by means of 2 external and inde-
pendent time constants.
Also the minimum detectable input signal level
can be externally adjusted, by a dedicated gain
network.
Main blocks are:
Variable gain limiter amplifier
Signal detector circuitry
Logic blocks able to avoid unproperoperations.
Operations Description (refer to the simplified
schematic of Figure 2).
a)The two channels left and right mean signal
coming from the preamp chain (AC coupled)
are added (current) at the AMS input and pre-
sented to the limiter amplifier. The limiter am-
plifier gain is internally fixed to 40dB, and it
could be reduced by the R8 external resistor.
The AMSVth threshold is fixed at 1V. The fol-
lowing signal detector comparator informs of
the presence of music signal (High level out)
and avoids the erroneous detection of very
low signal (like noise) as real program.
b)The system moves in blank detection mode
everytime the supply is turned on or, with
power supplied, the AMS On/Off pin (to be
driven by an open collector output) is turned
off (T1 open). Once this condition has hap-
pened, the capacitor C15 charges, defining a
minimum reset time, long enough to reset the
Flip-Flops FF1 and FF2 and to descharge the
program detector external C7 capacitor. The
charging current at the AMS SW pin is about
10µA. It follows that the reset time (C15 x
Vpin7)/IC15 is given approximately by 0.122 x
C15 ns where C15 is in µF.
c)From now on where the sum input signal (left-
right) amplified by the limiter exceeds the sig-
nal detector threshold (3) the capacitor C₇ is
forced to charge. When the voltage across
this capacitor reaches the comparator
COMP1 threshold voltage Vth1 (approx. 2 Vbe)
FF1 is set and FF2 is reset. (4) It follows that
Q1 is turned on discharging the program de-
tection capacitor (C7).
d)Always when there is a recorded zone, Q1 is
on; it becomes off only if a blank section is
reached. When, with a time constant control-
led by the R9, C9 network the voltage at
space detection time pin reaches Vth2 thresh-
old (comparator COMP2) a true blank section
is detected and the FF2 is set. (The minimum
charging time is approximately given by:
0.69 x C9 x R9 [sec]
In this condition Q2 is on, forcing the open col-
lector AMS output to go low, informing the
user that a true blank section has been de-
tected. The state is then able to start from the
above point c). Figures 3-4 show the signal
behaviour.
The device is able to prevent false interspace de-
tection and the end of the tape. (see Fig. 4).
Note:
(3): minimum RMS input signal is given by:
Vin
=
AMSVth
⋅
1K + R8
101K + R₈
⋅
0.6
where Vin is the mean of left and right channels::
V_in =^Vright₂^{+ Vleft}
(4): the charging current at pin 8 is about 20µA
(half input signal) so program detection time
constant is given by:
C₇ x 2V_be
20µA
=
70
x
C₇
[µF]
=
[msec]
It is recommended to insert a resistor (R7) high
enough to prevent fast C7 capacitor discharging.
8/20