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Old 18th May 2019, 11:32 am   #8
Dave Moll
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Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,118
Default Re: GPO/BT Call Progress Tones?

Quote:
Originally Posted by Station X View Post
Dial tone was changed from 50Hz square wave to 400Hz sine wave because MF4 dialing didn't work well in the face of 50Hz harmonics which were present until the first keyed digit was accepted.
I hadn't realised that was the reason for the change. I had thought of it as simply a by-product of the move from mechanical to electronic exchanges.
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