How To View Surveillance Dvr On A Specific Channel On My Tv?

hey, i need help on finding a device where i can view my surveillance DVR on a specific channel on my TV via coax cable. i don't want to run two coax cables just one so i can watch my basic cable channels and surveillance camera on a specific channel. this device is used in most apartment buildings where they can see the front entrance from there TV on a specific channel so that they can unlock the door.i need to know the name of this device...please

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