By L. SCHENKER. "A customer voice-frequency pushbutton signaling scheme involving a new two-group signal code, termed "four-by-four", is described. It is shown that the use of this code, with judiciously chosen frequencies, permits the detection of bona fide signals after transmission over any ordinary voice connection and facilitates discrimination against false signals resulting from speech. Apparatus for generating the tones is described, and the principles of reception are discussed." Contributed by Paul Fassbender.

Human Factors Engineering Studies of the Design and Use of Pushbutton Telephone Sets by R. L. DEININGER. July, 1960. Describes 18 pushbutton layouts tested. "Technological progress in recent years has brought pushbutton signaling from the telephone set within sight of economic feasibility. What, from the user's viewpoint, are the desirable operating characteristics of the key set which should guide development and manufacture? And how do people process information when they key a telephone number?" Referenced in Singing Wires, April 2020. Contributed by Paul Fassbender.

Bell System Technical Journal. 3A Speakerphone.  Missing pages 64 and 65.  Contributed by Gary Goff. 

Training simulator with multiple TSPS stations. Bell system Technical Journal, Page 1347. Contributed by Steve Cichorsky.  Ocr by Clint Gilliland.