Uli, I disagree with part of your explanation: “For very high frequencies a simple cable is a very complex arrangement of capacitors and coils.” What you say about the charateristics of a cable is right on. The second part I have trouble with. “This can cause reflections and subsequent enhancement or weakening of some parts of the signal. Think of the ferrite core as something like a sponge that cleans up undesirable ‘dirt’ signals.” That is not the purpose of the ferrite core. They stop Radio Frequency (RF) noise and Electromagnetic Interference (EMI) from degrading the signal in cables. Long cables can act as an antenna (in effect). By changing the impedence (with a ferrite core) of the cable you suppress this interference. RF and EMI interference is not limited to tv/radio stations, but also household appliances, computers, cars, etc.
Respectfully, Kerry