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Name: Chris
Status: Other
Grade: Other
Location: CA
Country: United States
Date: October 2007


Question:
Dear Scientist, Recently, I have come across a flickering phenomenon between an LCD display with a Florescent light lamp. This lamp actually utilizes 50Hz at 220V (from Europe). After investigation, I believe it has to do with the interference between the screen frequency and the lamp. If I were to adjust the default value from 77.5 Hz to approximately 99 Hz, then the phenomenon will therefore disappeared. I was wondering would you be able to provide some information in regards to the relationship between these type of frequencies?



Replies:
Chris,

It sounds to me like you are viewing a form of phase interference. Whenever there are two sources, (dual helicopter rotors, speakers, irplane engines, etc.) operating at relatively close frequencies, you get the interference. If I were to set up two speakers, one to play a 50 hz tone, and the other a 75 hz tone, then we could hear 'beats'. The rate of the beats is the difference between the two tones, so in this case, 25 hz.

That flickering effect is still there, and is quite unavoidable. while your monitor was set to 77.5 hz, events (where the lamp was not lit between power cycles, and the screen was refreshing) matched up about 27.5 times per second, and were noticeable with a human eye. When you changed to 99 hz, you increased the frequency of the flickering to almost 50 hz, and you can no longer notice them.

Ryan Belscamper



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