Granted Patent
Utility
US 5,138,569 · App. 07/452,247
Dual tone multi-frequency detector
Inventor:
Reinaldo A. Valenzuela
Patented Case
View Patent ↗
Granted: Aug 11, 1992
Filed: Dec 18, 1989
Inventor
Reinaldo A. Valenzuela
Assignee (at issue)
Codex Corporation
This patent predates the USPTO's electronic file wrapper — prosecution documents from this era exist only in the Patent Office's paper records. A certified copy of the file history can be ordered from the USPTO's paper archives.