TDA7336 datasheet(8/20 Pages) STMICROELECTRONICS

部件名	TDA7336

功能描述	STEREO PREAMP AMS DOLBY B* NOISE REDUCTION PROCESSOR

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制造商	STMICROELECTRONICS [STMicroelectronics]

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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 unproper operations.

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

10

µA. 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 C9x 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::

Vin

=

Vright

+ Vleft

2

(4): the charging current at pin 8 is about 20

µA

(half input signal) so program detection time

constant is given by:

C7 x 2Vbe

20

µA

= 70 xC7 [µF] = [msec]

It is recommended to insert a resistor (R7) high

enough to prevent fast C7 capacitor discharging.

TDA7336

8/20

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