TDA8741 Просмотр технического описания (PDF) - Philips Electronics

Номер в каталоге

Компоненты Описание

Список матч

TDA8741

Satellite sound circuit with noise reduction

Philips Electronics 

Philips Semiconductors

Satellite sound circuit with noise reduction

Product specification

TDA8741; TDA8741H

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. For accurate tuning to the many

sound carriers an external frequency synthesizer and

mixer is used to transpose the sound carriers to

intermediate frequencies of 10.7 and 10.52 MHz.

The TDA8741; H contains all circuitry for processing the

main channel and for two secondary channels, from IF

signal to line (SCART) output drivers. The desired

frequencies can be routed to the TDA8741; 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 VP, the lock-in range = 10.0 to

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 voltage is then determined by the

resistor divider at this pin between VP and ground.

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

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 µs: 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

An external mixer and oscillator frequency synthesizer

transfers the secondary sound carriers to fixed

intermediate frequencies e.g. 10.7 and 10.52 MHz. Two

secondary channel inputs are available at pins 8 and 16 (3

and 11). The signals are routed to the inputs via external

ceramic bandpass filters which are tuned to the required

intermediate frequencies.

For stereo applications the TDA8741; TDA8741H contains

two identical secondary sound processing channels.

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 inputs 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 resistor connected in series with the 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.

October 1994

7

Share Link: