signal信号是可数名词吗？

不是

an area with a poor [an excellent] TV signal

影象不清[极佳]的地区

A red light is usually a signal of danger.

红灯通常是危险的信号。

They went into action at a given signal.

在约定的信号发出时, 他们投入了战斗。

What he said was the signal for the argument.

他的那番话引起了这场争论。