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Old 19th May 2019, 10:33 am   #18
Join Date: Jul 2007
Location: Flintshire, UK.
Posts: 460
Default Re: GPO/BT Call Progress Tones?

Originally Posted by AC/HL View Post
As I remember, the tones were generated by the ringing generator. The cam operated spring sets for the cadence were fascinating to watch, in an anoraky sort of way. All electronic now, enabling voices as well as tones.
As late as the 1960/70's as the provision of STD was extended, some smaller exchanges had the tones produced by 'vibrating' relays but to have tones that would pass over amplified circuits, the very low frequency tones were increased and 'standardised' Small rural exchanges were changed over from the vibrating generation to small 'Ringer 2A' machines that produced the tones and ringing.
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