spk is the recommended extension for files generated by BinarySpikeMonitor. It's constructor syntax is the same as SpikeMonitor so that one can transparently switch between the two. The binary
spk format can be decoded using provided Python code under
tools/python/auryn_binary_tools.py (see Python tools) or directly decoded to the human readable ras format using the command line tool Auryn Binary Extract which is compiled by default (versions >0.6.1 and can be found at
Binary spike raster files provide a significant speed advantage over regular ras files because the output does not need to be formatted to ASCII output. The files can be later converted to the standard ras format using aube or by any other custom written analysis software. From an analysis point of view binary raster files are advantageous because they allow quick access (bisect) to any time point or range within the file. ASCII based ras files need to be read sequentially from the beginning which quickly becomes arduous for large files when one is only interested in the activity in a small part towards the end of the file. Finally, binary spike raster files are smaller in size and can be compressed even more efficiently than regular ras files.
See also this discussion here http://fzenke.net/auryn/forum/viewtopic.php?f=6&t=14.