Yep ... it's the digital (meaning on-off) version of the slow moving line you see if you point your camera at a TV (or computer) screen - the speed that it happ

Yep ... it's the digital (meaning on-off) version of the slow moving line you see if you point your camera at a TV (or computer) screen - the speed that it happens at is the sum and the difference of the two frequencies (and perhaps at the sum/differences of their harmonics). In the CATV industry, we used to send 110 analog channels across roughly 800MHz of spectrum (54MHz to 860MHz). Other than background and ingress noise, a major source of noise (effectively) was distortion - non-linearities in the amplifiers that kept the signal at a reasonable level. Composite Second Order (CSO) and Composite Triple Beat (CTB) distortions were two values that equipment transporting these signals would generally call out - they are analogous to the summing and differencing of two signals and three signals (respectively) as described above for the lightbulb, but imagine doing that with 110 different frequencies simultaneously.