[Someone]

We could bootstrap the format. First repeatedly send a p × q bit stream (with p and q prime) that teaches the aliens about a compressed format F1, then send a larger F1-encoded description of format F2, send a F2-encoded description of F3, …, send a Fn-encoded description of mp4, and then send a mp4-encoded video.

For a simple version of that, see https://rosettaproject.org/, which uses large print to make it much more likely intelligent life will discover the small print.

I don’t see video as high on the list of things we would like to send, though. There’s lots and lots of stuff we should send earlier, such as data on physical constants and chemical elements (things we expect any aliens to know, and that they could use to decipher our language)

[gricardo99]

on the list? how about close to pointless? i don’t see how bootstrapping the decoding gets you to anything other than another inscrutable format of data.

without context about the point and use of the data i can’t imagine it would have any use. you need to know about the display devices and human audio, visual sensory and perception capabilities to make any use of the data. it would be like receiving some complex data format meant to generate a symphony of seismic waves that the observer enjoys through various specialized sensors, without knowing anything about this purpose of the data to begin with, let alone the many details specific to the use of the data by whatever devices “play” the data and how it’s perceived by the observer.

[Someone]

If you manage to tell the other party how compression format X works, you can use it after that, and send information faster and using less energy (if we’re talking interstellar communication, broadcasting likely takes way more energy than any message preparation does)

If you intend to send lots of data (which, as you point out, you will have to, to give the receiver any chance of making sense of it), that may be a net win, even if you spend lots of time getting there.

You could, for example, teach that the bit pattern of ‘=‘ signals equality by sending simple math expressions

  .. = ..
  . + . = ..

and later try to convey how run-length encoding works by reusing = in that context

   aaaabbc = 4a2bc

Easy for the receiver to figure out? Absolutely not, but if you give them a chance, they may figure it out, and if the alternative is that you can only send a third of the data, that may be the better choice.

[Retric]

You don’t need to decode video signals as RGB values to be meaningful.

The value is simply the abundance of data and the patterns that show up within it because most images are close to the ones before them. Suppose you sent every book ever written in English as ASCII data and someone tried to decode it as an audio signal. There is nothing in the data that’s going to show someone is on the right track or not. However with uncompressed video you can find patterns and the closer you get the more obvious it becomes.