What device transforms signals back into a message?

The device that transforms signals back into a message is a decoder. In telecommunications and data communication, a decoder serves a critical function by converting coded messages—from either analog or digital signals—back into a format that can be understood by humans or other systems.

Decoders work by interpreting the input signals that have been encoded by a transmitter or encoder and reversing that process to yield the original information. For instance, during audio or video transmission, decoders decompress the data to restore it to its original quality—ensuring clarity in communication. This makes decoders essential in systems like television broadcast, where signals transmitted over the air are received and converted back to visual and auditory outputs.

In contrast, an encoder performs the initial transformation of a message into a signal, while a modulator is involved in converting digital data into an analog signal for transmission. A transmitter amplifies and sends the modulated signals through various communication mediums, but it does not convert back to the original message. Therefore, decoder is the most accurate choice for this question.