English term (edited): digital [television] adapter (DTA),

décodeur TNT = décodeur Télévision Numérique terrestre


A digital television adapter (DTA), or digital-to-analog converter [set-top box], or commonly known as a converter box, is a television tuner that receives a digital television (DTV) transmission, and converts the digital signal into an analog signal that can be received and displayed on an analog television set. It may refer to over-the-air broadcast television signals received by an television antenna, or to cable TV systems which switched to digital cable (a Cable converter box). It normally does not refer to satellite TV, which has always required a set-top box either to operate the big satellite dish, or to be the integrated receiver/decoder (IRD) in the case of direct-broadcast satellites (DBS).
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https://en.wikipedia.org/wiki/Digital_television_adapter

Adaptateur TNT pour téléviseur

Le principe de l’adaptateur TNT pour téléviseur est de transformer le signal numérique, d’adapter son ancien matériel pour recevoir la TNT gratuitement avec un canal hertzien. On l’appelle aussi décodeur TNT et récepteur TNT.

http://www.commentcamarche.net/faq/15283-tnt-et-adaptateur-t...