How is a distress call recognized as a distress call?

In countless episodes, a Starfleet ship receives a distress call from a vessel or outpost of an unknown species. By the time of TNG, the Universal Translator can instantly lock onto a language from the first word of a hail. But in several episodes of ENT, it's shown that the UT requires at least several seconds of conversation to begin picking up a language. Even in ENT, however, a distress call is always immediately recognized as a distress call.

What characteristics of a transmission would make it universally recognizable as a distress call? I'm interested in both in-universe and out-of-universe answers.