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Old 31st Jan 2017, 4:47 pm   #188
Station X
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Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
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Default Re: Technology related anachronisms on TV and in films etc.

That was a blip of 2280Hz from the AC9 signalling system which was only used on long distance circuits with no DC signalling path.

You still hear it today on some modern radio drama productions along with follow on dial tone at the called party when the calling party hangs up. That hasn't been used for years either. You still hear old fashioned 50Hz square wave dial tone too.
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