Due to a recently issued patent I now am the sole inventor on three different patents. For various personal reasons I am not very public about the details of two of them. However, the first one, 4,412,299, does not share those concerns.
The title is Phase Jitter Detector. This was an invention to help meet speech immunity requirements for a Touch-Tone receiver. What is “speech immunity?” It turns out that because the old analog phone lines used the same channel for voice and signaling (the Touch-Tone digits) background speech, music, and other sounds would sometimes simulate a Touch-Tone digit with sufficient accuracy to insert a false digit into your dialed number. This false digit would result in a wrong number.
The problem increased in severity as the dynamic range of the receiver increased. With some reasonable precautions the problem was kept under control with normal subscriber to central office communications. But in the late 1970s and 1980s companies were creating things which they wanted to be able to remotely control. Examples from that time are answering machines to private branch exchanges (remember “dial nine for an outside line?”) where you could call the “main number” and then dial “One for Parts, two for Service, three for Sales, etc.. The receivers used for subscriber-to-subscriber communications required nearly twice the dynamic range of subscriber to central office communication. With this requirement the speech immunity problems went way up.
You could design a receiver which would would avoid the speech hits but in doing so you reduced the noise immunity of the receiver. A small amount of noise would prevent the detection of valid Touch-Tone digits. In other words, there is a trade-off between speech immunity and noise immunity. In the subscriber-to-subscriber signaling case you needed far better than normal noise immunity while retaining whatever speech immunity you could.
This speech immunity versus noise problem inspired me to get my master’s degree in electrical engineering. The graph below is from my thesis:
IIRC, the minimum A-level for subscriber to C.O. signaling was about –28 dBm. For subscriber-to-subscriber signaling the minimum A-level was about –40 dBm. Using a standard test tape from AT&T and a receiver without any guarding against speech hits the number of speech hits nearly tripled for this lower signal strength acceptance level.
My invention enabled the accurate distinction between a true Touch-Tone signal and speech/music/etc. which imitated a Touch-Tone digit. The receiver utilizing this invention, TelTone’s M-937 Touch-Tone Receiver, was able to meet any published requirements for subscriber-to-subscriber receivers for both noise and speech immunity. To the best of my knowledge no other production receiver was ever able to make that claim.