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TEA6886HL データシート(PDF) 10 Page - NXP Semiconductors |
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TEA6886HL データシート(HTML) 10 Page - NXP Semiconductors |
10 / 92 page 2000 Nov 21 10 Philips Semiconductors Product specification Up-level Car radio Analog Signal Processor (CASP) TEA6886HL 7 FUNCTIONAL DESCRIPTION 7.1 Stereo decoder The MPX input is the null-node of an operational amplifier with internal feedback resistor. Adapting the stereo decoder input to the level of the MPX signal, coming from the FM demodulator output, is realized by the value of the input series resistor RIN. To this input a second source (AM detector output) can be fed by current addition. The input amplifier is followed by an integrated 4th-order Bessel low-pass filter with a cut-off frequency of 80 kHz. It provides the necessary signal delay for FM noise blanking and damping of high frequency interference at the stereo decoder input. The output signal of this filter is fed to the soft mute control circuitry, the output is voltage-to-current converted and then fed to the phase detector, pilot detector and pilot canceller circuits, contained in the stereo decoder PLL block. A PLL is used for regeneration of the 38 kHz subcarrier. The fully integrated oscillator is adjusted by means of a digital auxiliary PLL into the capture range of the main PLL. The auxiliary PLL needs an external reference frequency (75.4 kHz) which is provided by the TEA6840H. The required 19 and 38 kHz signals are generated by division of the oscillator output signal in a logic circuit. The 19 kHz quadrature phase signal is fed to the 19 kHz phase detector, where it is compared with the incoming pilot tone. The DC output signal of the phase detector controls the oscillator (PLL). The pilot present detector is driven by an internally generated in-phase 19 kHz signal. Its pilot dependent DC output voltage is fed to a threshold switch, which activates the pilot indicator bit and switches the stereo decoder to stereo operation. The same DC voltage is used to control the amplitude of an anti-phase internally generated 19 kHz signal. The pilot tone is compensated by this anti-phase 19 kHz signal in the pilot canceller. The pilot cancelled signal is fed to the matrix. There, the side signal is demodulated and combined with the main signal to the left and right audio channels. Compensation for roll-off in the incoming MPX signal caused by the IF filters and the FM demodulator is typically realized by an external compensation network at pin MPXIN, individual alignment is achieved by I2C-bus controlled amplification of the side signal (DAA). A smooth mono-to-stereo takeover is achieved by controlling the efficiency of the matrix with the help of the SNC peak detector. The matrix is followed by the FM noise suppression gates, which are combined with FM single poles and High Cut Control (HCC). The single pole is defined by internal resistors and external capacitors. Audio is fed from the gate circuits to the switchable de-emphasis, where the demodulated AM stereo signal can be fed in. After de-emphasis the signal passes to the output buffers and is fed to the radio input of the source selector. For HCC, the time constant of the single pole contained in the output buffer can be changed to higher values. This function is controlled by an average detector contained in the multipath and fading detector. 7.2 FM noise blanker The input of the ignition noise blanker is coupled to the MPXRDS input signal and to the LEVEL input. Both signals are fed via separate 120 kHz filters and rectifiers to an adder circuit. The output signal of the adder circuit is fed in parallel to the noise detector and the interference detector. The noise detector is a negative peak detector. Its output controls the trigger sensitivity (prevention of false triggering at noisy input signals) and the gain of the MPX high-pass filter. The output of the interference detector, when receiving a steep pulse, fires a single-shot trigger circuit, contained in the pulse former circuitry. The time constant of the single-shot trigger circuit is defined by an internal capacitor, and its output activates the blanking gates in the audio. 7.3 AM noise blanker The AM noise blanking pulse is derived from the AM audio signal which is fed into pin AMNBIN with the help of a peak-to-average comparator. The blanking time is set by a pulse former with external capacitor. The blanking pulse is fed to the gate in the AM audio path and out at pin AMHOLD to operate the gate built into the external AM stereo processor. 7.4 Multipath/fading detection and weak signal control For FM signal quality dependent controls there is a built-in combination of six detectors. These detectors are driven by the level information direct, by the AC components on the level via a 20 kHz band-pass filter (AM wideband) or by the high notes present at the FM demodulator output via a 60 kHz high-pass filter (ultrasonic noise). The relationship between the DC level and the AC components is programmable by the I2C-bus (2 bits each). The output of the level buffer, AM wideband detector and ultrasonic noise detector are analog-to-digital converted and readable by the I2C-bus. |
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同様の説明 - TEA6886HL |
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