The story sorts this out. Absent an interpreter, you necessarily need to be able to talk to someone who knows the other language so that you can query them, etc. Realistically, there's no way to intercept or interpret communications as the protocols are almost impossible to figure out (that is another layer of 'language'). Let say an alien intercepts a TV signal, can they convert it back to video? It is almost impossible that they will be able to infer the entire set of protocols: signal -> video, aligning video, etc. Especially given that aliens may be so different from us that their technologies may have developed in an orthogonal direction to our own.
If I recall, this was a core idea in Godel Escher Bach. Is meaning inherent to information, or do you always need a compatible interpreter (eg. a human mind)?
Messages don't exist by themselves, but are layered. Eg. To understand that a radio signal is a video I need to also understand video protocols, RF communication, etc.
In GEB, Hofstadter talks about the Three Layers of Any Message:
1 - The Frame Message
2 - The Outer Message
3 - The Inner Message
He uses the example of a "message in a bottle" washed up on the beach. The "Frame" is the recognition that the bottle is sealed and contains a dry piece of paper. The "Outer Message" would be the fact that the writing on the paper is in French, and the "Inner Message" would be the actual contents of the French writing.
At each stage, the reader needs to have some set of concepts or language with which to grapple with the nested messages, if they have any hope of finally decoding the Inner Message.
For example, a wild boar wouldn't recognise the Frame Message in the first place, while a German person might recognise the Frame Message, and recognise the Outer Message ("This text is definitely French"), but be stymied by an in-ability to actually read French.
I think you forget that you have to completely set aside human assumptions.
How will the aliens even know they are looking at signals encoding video and audio? Do aliens have technology and knowhow in the RF regime? Will they even be able to receive, store, or analyze the TV waveforms? Do they have the notion of amplitude and frequency modulation? Also, how will they know that the signal encodes video? Absent ground truth, low entropy (or equivalent measures like bigrams, Markov models, etc.) is the only way one can determine that a sequence is a language. Even if they are able to convert the RF signal to some low-entropy sequence of symbols, there is a very real possibility of them going off on a tangent: mapping the information to a completely different language.
A real world example is the symbols of Indus valley civilization. Several decades into the information age, and about a century since the seals were discovered, researchers haven't even conclusively established that the symbols encode a language.
This is what drives me nuts about Star Trek shows where they encounter completely new aliens and they all seem to have compatible video-chat protocols.
