Primarily used in broadcast engineering to link remote broadcast locations to the host studio, a hardware codec, implemented with digital signal processing, is used to compress the audio data enough to travel through a pair of a 33.6k modems.
POTS codecs have the disadvantages of being restricted to relatively low bit rates and being susceptible to variable line quality.
ISDN and IP codecs have the advantage of being natively digital, and operate at much higher bitrates, which results in fewer compression artifacts.
Special lines must be run to a location, however, and must be ordered well in advance of the event so that there is ample time for installation of equipment.
Audio can be sent in either direction, and most can also pass low-speed non-audio data, allowing the remote DJ to control broadcast automation or other studio equipment via RS-232.