TDA8740 Datasheet, PDF (8/24 Pages) NXP Semiconductors – Satellite sound circuit with noise reduction

Philips Semiconductors
Satellite sound circuit with noise reduction
Product specification
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.
ABBREVIATIONS
f_MOD = modulating frequency.
∆f_M = frequency deviation of the main Channel.
∆f_S1 = frequency deviation of secondary Channel 1 (left).
∆f_S2 = frequency deviation of secondary Channel 2 (right).
f_OM = carrier frequency of main Channel.
f_OS1 = carrier frequency of secondary Channel 1.
f_OS2 = carrier frequency of secondary Channel 2.
LPF = Low-Pass Filter.
NR = Noise Reduction.
PLL = Phase-Locked-Loop
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).
October 1994
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