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Old 6th December 2007, 12:22 PM   #1
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Default Cable Issue

days ago i was producer of a small pa set up ....in this set up also a device used for translation by philips was involved

so the technician just wanted one connection from my yamaha mixer to connect this and then repeat and broadcast via infared the second language

i presume that some of us have the idea how confernce translation systems work

so he give a pice of cable that was actually fitted with rca plug but the cable it shelf was a rg 59 video cable 3 meters long

i say to him that this cable is not right for this use ....he said its a belden high quality video cable so it should work .....

in the set up involved ecept this infrared system small pa 4 speakers and amp wirlesss microphones normal microphones cd player video mixer with sound and dvd player dimmer lights, frameless plasma wall and so on ///

so while we test everything in the translation system was radio inderfirience you could here something like from a radio station in the headphones of the translator .....

then the first thing i said remove the video cable and use any normal audio cable

OOOOOOOPS problem solved

since i am extremelly very familliar with pa setups and how critical is earthing and earthloops .....eventhough very complicated installation all the rulls about power where perfectly taken care of by me ....so there was not a chance that something like that was involved

so what was it ???? the cable used was 3 meters long belden double sealed video cable costs i thing more than 2 euro per meter but designed for 75 R signals

any clu ???????
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Old 7th December 2007, 06:42 AM   #2
gootee is offline gootee  United States
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Maybe one end of the shield in the video cable was not connected, and the shield was acting as an antenna. After that, almost any semiconductor PN junction and a little low-pass filtering might give you the audio from AM radio.
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Old 7th December 2007, 07:25 AM   #3
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Default thought

about it and checked
it wasnt plugs in both sides looked very ok

simular thing happend to me in the past while checking one quitar preamp i didnt had any audio cable on my benc and picked up a piece of RG 58 CB antenna cable and constructed a jack to jack cable to connect the quitar ....

working but the cable it self worked as a microphone !!!!! if touch or vibrate the cable was actually audible in the speaker !!!!
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Old 7th December 2007, 07:33 AM   #4
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Maybe this cable had the exact amount of capacitance to bring a poorly designed output circuit into oscillation...
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Old 7th December 2007, 07:37 AM   #5
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Default truth is

that this is what i had in mind my shelf


for the quitar preamplifier figers 100% cause it was totally crapy made ....

but from professional philips equipment sounds really scary
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Old 7th December 2007, 04:32 PM   #6
gootee is offline gootee  United States
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Speaking of cable capacitance...

For your first example, if you could connect a low-frequency spectrum analyzer (i.e. able to cover AM radio band) to the output of the device that the cable's input was connected to (i.e. instead of the cable), when the radio station interference is present, maybe you could see if any "AM Radio" RF was present.

If so, maybe the capacitance of the cable, and the output resistance of the device, was enough lowpass filtering to remove the RF (AM carrier), to give you the AM station's audio. (Alternatively, maybe you could "simulate" it by using the good cable, but also inserting a custom-made adapter, or something, that has an equivalent capacitance soldered between the signal and ground conductors.)

There are other possibilities. But without knowing the details of all of the circuits and the construction of the equipment involved, it would be difficult to discuss. For example, maybe, with bad luck, the cable even helped to form a tuned resonant circuit, somehow.
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