NEW YORK (AP) — Twitter and the TV ratings firm Nielsen are joining forces to create a way to measure TV-related conversation on the micro-blogging site.

The companies say the new Nielsen Twitter TV Rating will be available at the start of the fall 2013 season.

This new service will be designed to complement Nielsen’s traditional TV ratings, giving TV networks and advertisers the real-time metrics required to chart TV audience social activity. It’s the first-ever measurement of the total audience for social TV activity — both those participating in the conversation and those who are exposed to it.

Increasingly, viewers discuss TV on Twitter, resulting in a new dynamic between audiences and programming. The companies say Twitter’s more than 140 million active users send 1 billion tweets every 2 1/2 days.