The PCM (Pulse Code Modulation) sampling time is set at 125 microseconds to meet the Nyquist theorem's requirements. According to Nyquist, the minimum sampling rate should be at least twice the bandwidth of the signal to accurately represent the analog signal in digital form. In this case, 125 microseconds corresponds to 1/8000 seconds, which is equivalent to a sampling rate of 8,000 samples per second. This sampling rate is commonly used for telephone-quality audio, where the audible frequency range is approximately 300-3400 Hz