The abovequoted line is the main idea of your post.
My response is, I’m not sure why you thought anyone in this thread needed to be reminded of this.
-FrL-
Ditto Sailor and others. I am SURE aliens who could receive the signal and were interested in understanding it would make a great deal of sense of it pretty early on. There are plenty of patterns nested within other patterns that would guide them in how to structure things. The fact that they can extract variables that have various degrees of autocorrelation on what they will quickly read as two degrees of freedom will tell them they are getting some kind of map of something. They will recognize that there are three somewhat related degrees of freedom. I would think it would be easy to decide this thing they are studying is an image, or at least images would quickly be one of the leading contenders for what it means.
As far as color goes, well, you need three degrees of freedom to represent color as humans perceive it. You can’t generate all the colors we can see with three real colored lights, because we have mixed sensitivities to colors; the three lights you would have to use to create all the colors we can see would either have to be impossible colors (that is, much more pure than spectral colors), or you would have to be allowed to use negative amounts of light. But, you can create a large chunk of our color range with three real lights, which is what television does.
I think they would take some time to puzzle out what the three colors we paid attention to were. If they spent decades working on it, they could certainly come up with some good theories and gradually get it. Plus, there must have been TV shows about how TV works, and about color vision, and so forth.
As an astronomy student I read an article (by Shapely, I think, or maybe Gamow) about 23 separate things you can measure about a star by studying its light. This was at the time an old article - maybe it was from the 40’s or 50’s. We aren’t talking about images of stars, just featureless pinpoints. It’s only the light you get to study, not how it is distributed. Twenty three separate things. And we’ve gotten a great deal better since then.
They’d get it.
If their life, biology, and life circumstances are much like ours, IMHO they will able to decode it to a fairly high fidelity.
The less they are like us, the less likely they are to to get it “right” as either we or
they would “see” it.
If they are totally unlike us, they are unlikely to get it right…and even if they get it right, it will always be pretty alien to them.
So, if it is meaningful for them to get it right, they will, and if they can’t it probably doesnt matter anyway IMHO.
Cryptographers at the NSA or any other similar place would decode it so fast it would make you head spin. They make short work of much more complex problems.
The aliens do not have to be anything like us to figure out the luminance signal is an array of lines which slowly change over time and to see adjacent lines are related and how succesive frames and fields are related. All this is trivial. That they may not have eyes like ours is unimportant. The only important thing is that they be able to somehow appreciate a moving graphic in some way. And if they cannot appreciate a moving graphic in any way then they are not a very advanced form of life and I doubt they would have any intelligence.
They would easily be able to see moving pictures.
Same thing with the audio. They could easily appreciate it one way or another. It does not have to be sound waves to them. Now, the problem is the language but they could learn the language easily by watching enough TV. Archealogists have decyphered old inscriptions with much less.
Audio from radio would be more difficult because they have nothing to give them context.
Older analog signals like FM, AM and NTSC would be relatively easy to decypher.
However, digital signals are much, much harder. Since the EM spectrum is a finite resource it’s useful to try to cram as much data as possible into each band. That means over time any technological society that uses EM waves for communication will use more and more advanced compression algorithms. The better your compression algorithm, the more closely your signal resembles random noise. There’s a reason why compression and encryption are closely related fields in computer science.
Every technological society may only have a tiny window where their signals are decipherable by others. The same story may be repeated over and over again throughout the galaxy: Radio is invented on some planet and within two hundred years every bit of available spectrum is filled with what appears to be white noise.