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Old 24th August 2005, 06:58 AM   #1
jenda is offline jenda  Switzerland
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Default Coupling Control signals into Audio Lines

Hi all,

I'am wondering how control signals are coupled into audio lines. For example like some walkman/MD player's remotes do.

Does someone have any idea?

Do some chips exist, that are able to do the modulation of control signals (eg. OOK or FSK modulators transmitting on audio lines)?

Kind Regards, j
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Old 3rd September 2005, 05:48 PM   #2
metaxus is offline metaxus  Australia
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The portable audio device remote controls don't inject a control signal, per se...

The remote's plug and unit's socket have an extra pair of contacts to carry the remote signals.

There is a different value resistor in series with each switch in the remote control, and the IC for the controls in the unit that measures the resistance across those two pins, compares them to an internal lookup table, and then acts accordingly.

See the attached image to clarify.
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File Type: gif mdctrl.gif (7.6 KB, 61 views)
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Old 6th September 2005, 10:14 AM   #3
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ie just done this as part of an electronics module in my design degree. I was able to superimpose 2 control frequencies on the top of a separate carrier wave, ie the sound, by using switched wein bridge oscilators at one end, then filtering them out from the music signal with a pair of opamp based band pass filters. the signals had to be above audio range so they didnt affect the music playing.
My attempt was pretty basic but it worked ok. Wasnt particularly compact either. Im sure theres a easy way to do it with a specialised IC tho!
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