What symptoms indicate hum on an upstream digital modulation channel?

The presence of bit errors that resemble impulse noise is a key symptom indicating hum on an upstream digital modulation channel. When hum is present, it introduces a disturbance in the signal that can lead to random bit errors, reflecting the way impulse noise affects data transmission. Impulse noise typically results in brief bursts of erroneous data due to its random nature, which is similar to the errors caused by hum interference.

The other symptoms listed do not accurately capture the specific impact of hum on digital modulation channels. For instance, interference patterns with low frequency may suggest other types of noise rather than the effects of hum. Lowered overall signal strength can result from various issues and doesn't specifically pinpoint the problem as being hum-related. Long periods of silence may indicate issues such as loss of signal or equipment malfunction, which are different from the distortion effects caused by hum. Therefore, recognizing bit errors akin to impulse noise effectively identifies the disruption caused by hum in the signal transmission.