I have a small board based on 555 timer ic which can generate square wave signal with frequencies from ~30Hz up to ~64kHz. Here is the picture of this board hooked up to tv and oscilloscope (generating its maximum frequency), tv is shut off:

As one can see, the signal is almost clear square wave. When tv is turned on, some noise from pre-amplifier is getting mixed in, and main signal gets distorted. Also the pattern is visible on tv screen: