tda8740h NXP Semiconductors, tda8740h Datasheet - Page 7
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tda8740h
Manufacturer Part Number
tda8740h
Description
Satellite Sound Circuit With Noise Reduction
Manufacturer
NXP Semiconductors
Datasheet
1.TDA8740H.pdf
(24 pages)
Philips Semiconductors
FUNCTIONAL DESCRIPTION
Satellite sound
The baseband signal coming from a satellite tuner
contains the demodulated video signal plus a number of
sound carriers to facilitate reception of a
PAL/NTSC/SECAM satellite signal.
Nearest to the video signal is the main sound carrier which
carries the single channel sound related to the video. This
is an FM modulated carrier with a fixed pre-emphasis. The
carrier frequency can be in the range of 5.8 to 6.8 MHz.
Additionally, a number of optional secondary sound
carriers may be present which can be used for stereo or
multi-language sound related to the video, or for unrelated
radio sound. These carriers are also FM modulated, but for
better sound quality (improved signal-to-noise
performance) broadcast satellites (e.g. ‘ASTRA’) use a
noise reduction system (adaptive pre-emphasis circuit,
combined with a fixed pre-emphasis).
These secondary carrier frequencies can be in the range
of 6.30 to 8.28 MHz. The TDA8740; H contains all circuitry
for processing the main channel and for two secondary
channels, from baseband signal to line (SCART) output
drivers. The desired frequencies can be routed to the
TDA8740; H via bandpass filters.
Main channel (see Fig.1)
The lock-in range of the main channel PLL can be
switched between 5.5 to 7.5 MHz, PLL off and 10.0 to
11.5 MHz using the MCS signal at pin 7 (2) [when pin 7 (2)
is at logic 0, being a voltage from 0 to 1.2 V, the lock-in
range = 5.5 to 7.5 MHz; when pin 7 (2) is at logic 1, being
a voltage from 3.5 V until V
11.5 MHz; when pin 7 (2) is in the mid voltage position,
being a voltage from 1.8 to 2.8 V, the main channel PLL is
switched off]. The mid voltage position of the MCS pin can
also be obtained by a floating MCS pin if the circuit supply
voltage V
is then determined by the resistor divider at this pin
between V
If only one fixed carrier frequency for the main channel is
to be demodulated (e.g. 6.5 MHz), the lock-in range of the
PLL should be switched to 5.5 to 7.5 MHz. The baseband
signal is applied to the main channel input, pin 18 (14) via
a 6.5 MHz ceramic bandpass filter. Alternatively, if there is
a requirement to demodulate different main channel
frequencies, these frequencies can be transferred to a
fixed intermediate frequency (e.g. 10.7 MHz) using an
external mixer and oscillator-frequency synthesizer. In this
event the lock-in range of the PLL should be switched to
October 1994
Satellite sound circuit with noise reduction
p
P
is 10.8 to 13.2 V. The voltage on the MCS pin
and ground.
P
, the lock-in range = 10.0 to
7
10.0 to 11.5 MHz. The IF signal is applied to the main
channel input, pin 18 (14) via a 10.7 MHz ceramic
bandpass filter.
The filtered signal is AC-coupled to a limiter/amplifier and
then to a PLL demodulator. The PLL FM demodulator
ensures that the demodulator is alignment-free. High gain
and DC error signals from the PLL, which are
superimposed on the demodulator output, require DC
decoupling. A buffer amplifier is used to amplify the signal
to the same level as the secondary channels and
decouples DC using an electrolytic capacitor connected to
pin 41 (37). The demodulator output signal is fed to pin 20
(16) via an internal resistor. The output signal can be
de-emphasized by means of this resistor and an external
capacitor connected to ground.
Capacitor value = de-emphasis time constant per 1500
(for 50 s: 33 nF).
From here the signal is fed to the output selectors. The
signal is amplified to 500 mV (RMS) (i.e. 6 dBV) in the
output amplifiers.
Secondary channels
Up to eight secondary channel inputs are available at pins
2, 4, 6, 8, 10, 12, 14 and 16 (1, 3, 5, 7, 9, 11, 40 and 42).
External ceramic bandpass filters tuned to the required
secondary sound carrier frequencies route these signals to
the inputs.
For stereo applications the TDA8740; TDA8740H contains
two identical secondary sound processing channels. For
each channel it is possible to select from four inputs (IN-A,
IN-B, IN-C and IN-D) using the input selector (see Logic
Table 1). With the input switch several stereo signals or
languages can be selected for demodulation. It should be
noted that the inputs are identical and can be freely
interchanged. Secondary Channel 1 will also be referred
to as ‘LEFT’ or ‘LANGUAGE 1’ and secondary Channel 2
will also be referred to as ‘RIGHT’ or ‘LANGUAGE 2’.
From the input selector switch the signals are coupled to
limiter/amplifiers and then to the PLL demodulators.
Processing is similar to the main channel. The
demodulator output signal is amplified in a buffer amplifier
and DC decoupled using electrolytic capacitors connected
to pins 40 (36) (left) and 42 (38) (right). The output level is
set with a 220
capacitor.
High frequency components in the amplified PLL output
signal are filtered out in the audio LPF block (4th order
Butterworth low-pass filter) to prevent unwanted influence
on the noise reduction.
resistor connected in series with the
TDA8740; TDA8740H
Product specification