In Canada and Europe it is standard for Approach Control and Enroute ARTCC frequencies to be linked via a frequency relay system, so that when a controller is working more than one frequency, the aircraft tuned to different frequencies will still be able to hear each other. The YUL ACC (Noranda sector) feed is a good example of this system, where you can hear the aircraft using many different frequencies while only monitoring 133.975.
I've never heard this system being used in the US before until today. Listening now to Cleveland Center on the ZOB Area 6 (Pittsburgh Area) feed it appears that frequency 135.175 and 134.475 are now linked via relay. The controller, which you can hear loud and clear on 134.475 is also working 135.175, when aircraft are transmitting on 135.175 but being recieved on 134.475 you can clearly hear the difference in transmission strength and clarity.
Anyone have any more information on this system being implemented in the US or heard it in use at any other US ARTCC facilities ?
DJ