这些研究者通过改变光脉冲的强度来重编电码，比如，用高强度来表示1低强度来表示0。

These researchers were encoding digital data by varying the intensity of a pulse of light. For example, high intensity would represent a 1 and low intensity would represent a 0.

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4D指四个数据符（2比特），PAM5表示分5个层级的脉冲幅度调制。

The 4D means four data symbols (two bits), the PAM5 is Pulse Amplitude Modulation with five signal levels.

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数字式，指一种信号。该信号在一定的时间内，振幅保持不变。通常由一系列的高低脉冲表示。

Digital, Digital refers to a signal whose amplitude remains constant for a finite period of time. Digital signals are commonly represented as a series of high and low pulses.

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