A 1983 review of this remarkable system for the Institute of Electrical and Electronic Engineers (IEEE) attributes no fewer than eight "firsts" to SIGSALY. They are as follows:

1) The first realization of enciphered telephony

2) The first quantized speech transmission

3) The first transmission of speech by Pulse Code Modulation (PCM)

4) The first use of companded PCM

5) The first examples of multilevel Frequency Shift Keying (FSK)

6) The first useful realization of speech bandwidth compression

7) The first use of FSK - FDM (Frequency Shift Keying-Frequency Division Multiplex) as a viable transmission method over a fading medium

8) The first use of a multilevel "eye pattern" to adjust the sampling intervals (a new, and important, instrumentation technique)

From https://www.jproc.ca/crypto/sigsaly1.html

I think that the relevant patents remained classified until 1975.

https://en.wikipedia.org/wiki/SIGSALY#Operation if anyone just wants to get a feeling for how it worked. Pretty good writeup IMO.

> I think that the relevant patents remained classified until 1975.

If they were classified, they can’t have worked as patents. Patents, by their nature, must be open and available for others to read.