什么叫不连续发射?
参考答案:
GSM系统采用了DTX技术,基本原则是只在有话音时才打开发射机,这样可以减小干扰,提高系统容量;还可以节省无线发射机电源的耗电量。
要使笛子发出声音,必须用力来吹,使笛管中的______发生______.
— Did you find the missing couple in the mountain yesterday?
— No, but we ____ to get in touch with them ever since. [ ]
A. have tried
B. have been trying
C. had tried
D. had been trying