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一、伴音制式转换的原理 A3机芯是以集成块LA7680为主体,能接收PALD/K、I制和NTSC—M制。为了能接收PAL制和NTSC制设置了可控带通滤波器,再由LA7680内部放大检波,并从其(44)脚输出AFT电压对高频头本振加以校正,使图像中心频率为38 MHz。以高路华2818为例,其电路图如图1所示,该机采用本振频率可转换的伴音二中频变频器IC171(LA7975)。通过IC703(LA7910)及外围电路组成电子制式开关电路。由CPU的(26)、(27)脚的电平组合,得出了A、B、C三组控制电平对伴音控制电路进行控制。其对应关系如表1所示。
First, the sound system conversion principle A3 movement is integrated block LA7680 as the main body, can receive PALD / K, I system and NTSC-M system. In order to receive PAL system and NTSC system set up a controllable band-pass filter, and then by the LA7680 internal amplification detection, and from its (44) pin output AFT voltage on the tuner to correct the tuner, the image center frequency of 38 MHz . To high Luhua 2818, for example, the circuit shown in Figure 1, the machine uses the local oscillator frequency conversion of two intermediate frequency inverter IC171 (LA7975). Through the IC703 (LA7910) and the external circuit composed of electronic switch circuit. By the CPU (26), (27) the level of the foot combination, drawn A, B, C three control levels of the sound control circuit. The corresponding relationship shown in Table 1.