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Modulated Light
Interference |
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I got a call from a
music listener. He was begging me to help him with a problem. A few years ago he had
purchased a very nice cordless audio headset. The system worked well and he used it
often. Then, he moved to a new house. Once he got settled, he tried using his
headset but it would not work. Or, more precisely, it would not work very well. It
made some terrible noises on top of the music. I asked him to describe the thing.
Based on his description it was clear that it was an infrared type and not an RF device.
It used an array of infrared LEDs to transmit the audio signals to the portable headset.
I asked him a few more questions about how he used it, what kind of room, the nature of
the walls and such. |
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I had my suspicions about the cause of his problems so I asked him what
kind of lamps were in use in the room where his audio player was located. He was puzzled
by the question but laid the phone down while he checked. About a minute later he
came back to the phone and said that all the lamps were compact fluorescent lamps (CFLs).
Bingo! I asked him if he ever used such lamps in his old home. No, he used
standard 60w incandescent lamps at his old house. I told him, I was fairly certain
that I knew the problem. As proof, I asked him to perform a simple experiment.
I told him to try his headset with all the lights turned off in his room. Since this
was in the middle of the day, he should have enough light in the room to see what he was
doing without the use of any of the lamps. Again, he laid the phone down and was
gone for a couple minutes. When he came back on line, he was excited. He said
with the lights off, the headset work perfectly. He then asked me what was causing
the problem. |
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Compact fluorescent lamps (CFLs) are terrible modulated light sources.
Some CFL devices are worse than others but nearly all splatter out all kinds of ugly light
pulses (see oscilloscope photos below). In some cases, their modulated light can
influence how infrared remotes work on TVs and DVD players. I have heard cases where
some nearby CFL lamps completely killed remote TV operation. What is the fix?
Don’t use CFLs in situations where their light can cause problems. |
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The infrared remotes on TVs and his cordless headset use light detectors
with visible light blocking filters but they can still be overwhelmed by some nearby CFLs.
As more and more of these lamps become standard hardware, the light spectrum will continue
to get noisier. Lights using LEDs should be much cleaner but their use has not yet
overshadowed CFL lamps. |
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Wireless IR Headset |
CFL
Lamp |
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Modulated Light from CFL |
50KHz
and 140KHz Optical Noise from CFL |
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