TEA0678 データシートの表示（PDF） - Philips Electronics

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TEA0678

Dual Dolby* B-type noise reduction circuit, automatic music search, with differential outputs and mute

Philips Electronics 

Philips Semiconductors

Dual Dolby* B-type noise reduction circuit, automatic

music search, with differential outputs and mute

Preliminary specification

TEA0678

Short description ‘music search’

A system for ‘music search’ mainly consists of a level and

a time detection (see Fig.4). For adapting and decoupling

the input signal will be amplified (A), then rectified (B) and

smoothed with a time constant (C). So the voltage at (C)

corresponds to the signal level and will be compared to the

predefined pause level at the first comparator (D), the level

detector. If the signal level becomes smaller than the

pause level, the level detector(D) changes its output

signal. Due to the output level of the level detector the

capacitor of the second time constant (E) will be charged,

respectively discharged. If the pause level of the input

signal remains for a certain time, the voltage at the

capacitor reaches a certain value, which corresponds to

an equivalent time value due to the charging. The voltage

at the capacitor will be compared to a predefined

time-equivalent voltage by the second comparator (F), the

time detector. If the pause level of the input signal remains

for this predefined time, the time detector (F) changes its

output level for ‘pause found’ status.

handbook, full pagewidth

(A)

(B)

INA

AMPLIFIER RECTIFIER

INB

(C)

(D)

COMPARATOR 1

t1

VI

(E)

(F)

COMPARATOR 2

t2

Vt

OUT

LEVEL DETECTOR

TIME DETECTOR

MED772

Fig.4 Integrated ‘music search’ function.

In this IC the signals of both channels are first rectified and then added. The signal behind the adder is described by

Vadd = VchanA + VchanB, where:

VchanA: absolute value channel A

VchanB: absolute value channel B

This means, even if one channel signal appears phase shifted to the other channel (at worst cases inverted), the

TEA0678 will ensure the normal AMS function.

1996 Jun 06

11

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