I have a basic understanding of how strong encryption works on digital data. Bits can be changed so that they appear completely random and map one-to-one back to the original set, given the correct decryption key (and possible other parameters).
But we always talk about digital encryption, never anything that would work on an analogue system as far as I know. Good encryption didn't exist until relatively recently (or cryptanalysis would not have been as useful as it was during World War II) while phones and televisions, both using mostly analog signals until the internet came along, have existed for much longer. To my knowledge, nobody seems to have ever come up with secure and practical ways of encrypting them without converting it to a digital signal first.
Upon searching for analog encryption systems I came across DigiCipher (the successor to Videocipher), but it seems to use binary data since Wikipedia mentions that it uses an mpeg-2 compressed stream. Other sites mention that scramblers were used to prevent television channels from being watched by non-paying consumers, but scramblers have little to do with encryption since they (as the name suggests) are not intended to make the message unintelligible. Wikipedia's secure voice article only briefly mentions analogue signals and then goes on to describe different digital systems.
The only real analogue encryption scheme I found was on that Wikipedia page about secure voice communications. It works by adding noise before transmission and subtracting it on the receiving end. This sounds similar to a one-time pad, but it probably wasn't used only once, which means that all encryption became useless if the phonographic record(s) with the noise were stolen.
Thinking this through, digital encryption algorithms may be used to generate new analogue records so that they're no longer a one time pad. A secret key and variable number (e.g. yymmdd) could be used as input to a digital encryption algorithm, which then produces the noise to be added to the analogue signal. Modern (secure) encryption algorithms are not doable by hand at a reasonable speed far as I know though, so I'm not sure how practical this idea is when all you have is mechanical or analogue technology. They would have needed a computer.
Have we not (yet) figured out how to securely encrypt signals in a truly analogue way (without the use of impractical one time pads), is it impossible (proven, somehow), or are existing algorithms just obscure because there are few applications for them (now that almost everything is digital)?