R.L. -

First, congratulations! So where are you now? Geographically, as well as employment...

Second: I briefly worked as a network TV tech almost 3 decades ago, and what I know is both rudimentary and dimly remembered, but: the video portion of a television signal has a pretty fat bandwidth and is coded in AM. Besides the light intensity parameters for each pixel (translated as beam intensity for each of the red/blue/green spots) there are synchronisation signals to instruct the receiver to reset at the end of each "frame". If I recall correctly, TV runs at 24 frames/sec. [The audio signal is on a sideband, and is FM]

The hypothetical ETs may have invented something like TV themselves, so they might (if we're lucky) recognize the nature of our signal - for instance, by the periodicity of the 'synch' signal imbedded in it. Even if not, that repetition might give a clue to the overall format: that it, that the data is framed in discrete packages.

The trickiest part may be deducing that the varying intensities are to be 'transponded' from electrons to visible light by way of a cathode ray tube - there's nothing inherent in the signal that gives any indication of what sort of receiver is meant to decode it. Chances are (I'm thinking of Contact while I type this) they'll listen to it first, and decide that it's not all meant to be audio.

Hope that helps,

E'