Irregular Webcomic! #4547

Even if you can figure out a common transmission frequency for radio communication with an alien species, you still need to figure out how they use that frequency to encode their transmission.

You can't just transmit a radio wave of a fixed frequency and strength, because that carries no information^[1]. It's the equivalent of singing a single fixed note at a constant volume, as opposed to speaking. To convey information you need to modulate the radio (or sound) wave, which means changing the either the frequency or strength or both in an organised way.

Amplitude modulation is a simple method of encoding an audio signal into radio waves. Since the time period of audio frequencies ranges from 0.05 to 5×10^-4 seconds, while radio signals have periods more like 10^-6 seconds, you can simply multiply the radio wave by the audio wave, to produce a radio wave that varies in amplitude, or strength, with the same shape as the audio signal. This, also known as AM radio, was the earliest method of encoding audio into radio waves, and is still in use.

Honestly, if you were trying to design a radio signal to carry audio to an alien, AM would probably be your go-to, as it's going to be pretty obvious how to decode it for any being intelligent enough to receive radio in the first place. However AM has several disadvantages which mean that other types of modulation are much better practical choices, at least for non-alien communication purposes. It's very inefficient, using most of the radio power simply for the carrier signal rather than the encoded audio, and it requires a relatively high bandwidth, or range of nearby frequencies that need to be clear to avoid interference. So AM radio stations need to be separated by large gaps in the radio spectrum, meaning you can't have very many different signals (or stations) all being used at once. There's also the issue that there's no obvious way to encode anything but audio - you can encode a video signal into AM, but it's not going to be anywhere near as obvious to an alien how to decode and display such a signal.

Frequency modulation (or FM) is usually a better choice for many terrestrial applications. Rather than varying the amplitude of the radio signal in synch with the audio signal, it varies the frequency of the radio signal. Think of monitoring the radio frequency on which the signal is being transmitted. With AM, the frequency is steady, but with FM the frequency wobbles around, and the speed and distance of that wobble is the audio signal.

Again, if you were an alien radio engineer, you could probably figure out what was happening and recover some semblance of the original audio signal. It's not quite as straightforward as AM though, as there are a range of multipliers and offsets that you can choose to use, so your resulting audio would be somewhat distorted if you don't know those. And again, there's no obvious way to encode video or anything more complicated.

AM and FM are analogue modulation methods, which apply the full variation in the audio signal to the radio wave . There are also some other analogue modulations that do slightly more complicated things. But fundamentally different to these are digital modulation methods. These take your audio (or video, or text or whatever) and convert it into bits like a computer file - for example an MP3 for audio, or and AVI file for video. Then you modulate your radio signal to carry the bits of information, rather than an analogue audio signal. There are various ways to do this, encoding the bits into discrete variations in either amplitude, frequency, phase, or some other aspect of the radio wave.

This is a great way to send information - if your receiver knows how to decode the signal. Because distortion below a certain level is completely ignorable and has no effect on the quality of the signal, and you can also include error correction to account for higher levels of distortion. And if you send an AVI file, your recipient can just save the result to an AVI file and play the video.

But this is never going to work for communicating with an alien. They have no way of knowing what your digital signal corresponds to in terms of a stream of bits. They have no way of knowing if you're encoding audio as an MP3, a WAV, an AIFF, an OGG, AAC, FLAC, PPM, or whatever! In fact, they almost certainly don't use any of those formats.

And video, just forget it. You don't even know how many colours they might be trying to encode. A digitally encoded signal is essentially a ridiculously complicated code that you have no hope of ever figuring out, unless you know the format beforehand.

The idea of unencountered alien species casually transmitting video signals to one another and being able to decode and display them is basically implausible.

Except, you can engineer your signal to be deliberately easier to decode by some alien species. In fact we've done it. The Voyager Golden Record, included on the deep space probes Voyager 1 and 2 was a deliberate attempt to produce encoded data that a technological alien civilisation could decode without too much difficulty. In this case an "instruction manual" was included, which contains graphics illustrating that the aliens should expect a series of images, and giving the pixel dimensions, signal timing data, and other information to enable them to assemble the images.

Figuring this out, however, is likely to take significant thought and time. So although you can construct a signal designed to be easily decoded, there's still no guarantee that an alien species would be able to do so, or would be able to do so quickly. Especially quickly enough to establish a two-way real time video stream as often depicted in Star Trek and other science fiction.

[1] In the information theoretic sense. Detecting a single unmodulated radio signal implicitly carries the information "something is transmitting this signal". But it doesn't carry any form of message from the sender.