QS7785CF datasheet(11/15 Pages) NPC

11 / 15 page

QS7785PF/CF

NIPPON PRECISION CIRCUITS—11

Center Control

The center signal output on COUT is enabled when

AC is HIGH.

This section describes the output function whereby

ﬁxed frequency sound sources, such as vocal and

speech signals, can be output on the center signal. As

mentioned for “Type C” and “Type D” speaker

arrangements, the center signal can be output by

direct connection to a speaker or by adding the signal

to the front-channel output using a resistor connec-

tion between CIN and COUT.

Using a center speaker

When using a speaker, a capacitor Ca can be con-

nected between LPC and COUT, forming a lowpass

ﬁlter. The

−3dB cutoff frequency of the ﬁlter is given

by the following equation.

Frequencies higher than voice tones are cut from the

signal sent to the center speaker for clear reproduc-

tion of voice-band signals.

Adding to the front-channel output

With this connection, the bass frequency components

can be removed from a signal. Capacitors Ca and Cb

and resistor Ra can be connected between CIN and

COUT to form a highpass ﬁlter. The

−3dB cutoff fre-

quency of the ﬁlter is given by the following equa-

tion.

The input signal on CIN is added equally to both

front-channel outputs FLOUT and FROUT. Frequen-

cies lower than voice tones are cut from the signal

added to the front-channel outputs for clear repro-

duction of voice-band signals.

Note: Internal resistance value, R1 and R2, may vary ± 15%.

πCa R1

---------------------------

πCb Ra R2

()

----------------------------------------

Center output connection

FROUT

CIN

COUT

FLOUT

LPC

10k

R3:10k

R2:10k

Ra:10k

R1:20k

Front Right

Speaker

Cnter

Speaker

Front Left

Speaker

Close when MUTE=H

Open when MUTE=L

Close when AC=H

Open when AC=L

VREFOUT

Center output frequency response

−6dB

3dB

Gain

Freq.

V(COUT)/V(RIN)@V(LIN)=0

V(FROUT)

or V(FLOUT)

fL=

πCb(Ra+R2)

fH=

πCaR1

GC=20 log

Ra+R2

()−6dB

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