Using electronic devices on board

[tec]

Hi,

It is forbidden to use a video cam on board a plane. At the start of every flight, it is asked to turn off all electronic devices. I understand the case of transmitters etc. but video cams? Can anybody explain what harm can a video cam cause - what is the real reason (technical) behind this requirement, if any?

Another thing: curious about short *bling* kind of signals when the plane takes off and decends. What do those signals indicate, do they mean reaching a certain altitude?

Thanks.