  The High Energy Stereoscopic System (H.E.S.S.) is one of the three arrays of
imaging atmospheric Cherenkov telescopes (IACTs) currently in operation. It is
composed of four 12-meter telescopes and a 28-meter one, and is sensitive to
gamma rays in the energy range ~30 GeV - 100 TeV. The cameras of the 12-m
telescopes recently underwent a substantial upgrade, with the goal of improving
their performance and robustness. The upgrade involved replacing all camera
components except for the photomultiplier tubes (PMTs). This meant developing
new hardware for the trigger, readout, power, cooling and mechanical systems,
and new software for camera control and data acquisition. Several novel
technologies were employed in the cameras: the readout is built around the new
NECTAr digitizer chip, developed for the next generation of IACTs; the camera
electronics is fully controlled and read out via Ethernet using a combination
of FPGA and embedded ARM computers; the software uses modern libraries such as
Apache Thrift, ZMQ and Protocol buffers. This work describes in detail the
design and the performance of the upgraded cameras.
