TDA8740 datasheet(8/24 Pages) PHILIPS

8 / 24 page

October 1994

Philips Semiconductors

Product speciﬁcation

Satellite sound circuit with noise reduction

TDA8740; TDA8740H

NOISE REDUCTION (NR)

The noise reduction can be regarded as an input

level-dependent low-pass filter (adaptive de-emphasis

system) followed by a fixed de-emphasis. With maximum

input level (0 dB) the frequency response of the first part

(i.e. without the fixed de-emphasis) is virtually flat. As the

input level is lowered by x-dB, the higher output

frequencies will be reduced an extra x-dB with respect to

the lower frequencies (1 : 2 expansion).

The NR output signal is fed to pin 36 (32) (left) and pin 32

(28) (right) via internal resistors.

Fixed de-emphasis is achieved by these resistors and

external capacitors connected to ground. The signals are

DC decoupled via pins 36/35 (32/31) and 32/33 (28/29)

and then routed to the output selectors.

OUTPUT SELECTION

With the output selector (see Table 2) the outputs at

pins 25 and 24 (21 and 20) can be switched to the different

channels. Both outputs can be switched to both secondary

channels, to the main channel and to the external inputs at

pin 28 and 27 (24 and 23) for IC chaining purposes.

Pin 23 (19) is a separate output which delivers the main

channel only, thereby creating the possibility of having

three different output channels simultaneously e.g. for use

in hi-fi VCRs.

The outputs at pins 25 and 24 (21 and 20) can be muted

by setting the MUTE signal at pin 13 (8) to logic 1 (switch

positions 6 and 7).

The output at pin 23 (19) can be muted by setting the

MUTE signal and the EXT/INT signal at pin 26 (22) both

logic 1 (switch position 7).

All outputs at pins 23, 24 and 25 (19, 20 and 21) are line

drivers with SCART level capability and are short-circuit

protected by 125

Ω output resistors.

Output level of all channels =

−6 dBV typical when

frequency deviation of FM signal is 54% of maximum

frequency deviation (i.e. 0.54

× 85 kHz = 46 kHz for the

main channel and 0.54

× 50 kHz = 27 kHz for the

secondary channels) at 1 kHz modulation frequency

(reference level).

ABBREVIATIONS

fMOD = modulating frequency.

∆fM = frequency deviation of the main Channel.

∆fS1 = frequency deviation of secondary Channel 1 (left).

∆fS2 = frequency deviation of secondary Channel 2 (right).

fOM = carrier frequency of main Channel.

fOS1 = carrier frequency of secondary Channel 1.

fOS2 = carrier frequency of secondary Channel 2.

LPF = Low-Pass Filter.

NR = Noise Reduction.

PLL = Phase-Locked-Loop

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