IIT JEE , AIEEE Forum - Physics , Electricity - when you want to take a film from tv screen by mobile phone u see noise bands on film why?

saeed

when you want to take a film from tv screen by mobile phone you see noise bands on film why?

sid.shah.90

I believe its mainly due to difference in refresh rates of the screen and the camera.

eg. if ur video device has refresh rate of 60Hz and the camera some higher refresh rate than u'll see bars going up or down.....

But I don't think this problem exists in LCD or TFT screens

srujana

First thing you need to know is, the TV screen or the video recorder doesn't display or record the entire picture at a time

i.e, it works like a printer ,scanning and displaying 1 line at a time.

But the time taken for the whole process is too small., lesser than 1/10 th of the second and hence our brain is not able to process it.

The video recorder also works on the same principle, if the time at which the first line is displayed in the TV coincides with that of the recorder, then you wont see any bands, else dark bands appear.

suresan

because of the difference in their frame rates

eistien

both the equipments we are speaking of consists of circuits inside withe the cell working on wireless communication which is based on tracking of resonant frequency as we pass by the tv with our cell phone the cell phone which detects waves matching the resonant frequency show high electric and magnetic field amplitude which affects the transmission of signals accross the cables nearby which causes distortion !!! suresan sir am i rite!!! comment on this sir!!! rate me if useful!!!!

eistien

you dont get this problem on LCD screens as they are not susceptible to field changes in surroundings!!!!

suresan

In fact you are right. What i thought was that when we take the video of a tv show with mobile we see the frames rolling. But the question was on the distortoin( due to interference) caused by mobile . You are absolutely right. But there is only a minor variation that i will add in your second answer. Even with LCD , interference occurs because the perpheral circuits are affected by stray signals. Ofcourse to a far lesser degree.

edison

We see 'flickering' because the refresh rates of the display device and the recording instrument (a camera) are closely matched. However, the cameras used by movie makers are not the same as the commercially available handycams etc. They are expensive and extremely high resolution devices. This would obviously imply that their refresh rates are very high as compared to the simple tvs and monitors that they capture while filming a movie.

edison

This is due to the slow refresh rate (25 Hz) of our TV screens, as u know for illusion of motion to our eyes it needs a refresh rate of atleast 25 Hz.

Now when u take a video of the TV screen the camcoder also has a similarly low refresh rate. Since while taking the picture the refreshing of the Tv and the camcoder are occuring differently at different times, we observe the flicker.

Ask & Discuss Questions with Community & Experts