M4 field disk
From Halopedia, the Halo wiki
M4 Field Disks (also known as recorder chips or helmet chips) are used in helmet recorders to store visual and audio files created by the recorder's video and audio sensors. They can be plugged into nearly any information terminal or display unit that can play optical and audio data. It displays the information about the user and his current state.
During the Battle of Sigma Octanus IV, the Spartan-IIs employed these, and John-117's recorder chip contained the record of his mission to Côte d'Azur, as well as intelligence on Covenant, Huragok and Mgalekgolo. This was much appreciated by HIGHCOM.
Private Jenkins' helmet was equipped with one, and SPARTAN-117 used a playback system in his helmet to view the squad's progress as he tried to find Captain Keyes. It was using this chip's recording that he first saw the Flood. The recording was later saved in a report John-117 had made.
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