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TDA8741 Datasheet(PDF) 7 Page - NXP Semiconductors |
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TDA8741 Datasheet(HTML) 7 Page - NXP Semiconductors |
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7 / 24 page  October 1994 7 Philips Semiconductors Product specification Satellite sound circuit with noise reduction 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. |
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