  This paper describes the concept of an FPGA-based digital camera trigger for
imaging atmospheric Cherenkov telescopes, developed for the future Cherenkov
Telescope Array (CTA). The proposed camera trigger is designed to select images
initiated by the Cherenkov emission of extended air showers from very-high
energy (VHE, E>20 GeV) photons and charged particles while suppressing
signatures from background light. The trigger comprises three stages. A first
stage employs programmable discriminators to digitize the signals arriving from
the camera channels (pixels). At the second stage, a grid of low-cost FPGAs is
used to process the digitized signals for camera regions with 37 pixels. At the
third stage, trigger conditions found independently in any of the overlapping
37-pixel regions are combined into a global camera trigger by few central
FPGAs. Trigger prototype boards based on Xilinx FPGAs have been designed, built
and tested and were shown to function properly. Using these components a full
camera trigger with a power consumption and price per channel of about 0.5 W
and 19 Euro, respectively, can be built. With the described design the camera
trigger algorithm can take advantage of pixel information in both the space and
the time domain allowing, for example, the creation of triggers sensitive to
the time-gradient of a shower image; the time information could also be
exploited to online adjust the time window of the acquisition system for pixel
data. Combining the results of the parallel execution of different trigger
algorithms (optimized, for example, for the lowest and highest energies,
respectively) on each FPGA can result in a better response over all photons
energies (as demonstrated by Monte Carlo simulation in this work).
