Essentially what it boils down to is that char on arm systems, in my case the raspberry pi, are treated as unsigned characters.  So right now we have a fix for the hackrf firmware update that only works on x86 systems.   A simple change to the source and sink typecasting from char to explicit int8_t on the lookup table on the source.cc should fix this issue for all platforms.  In addition, I believe a typecast in the convert function for the sink.cc would do it as well but I have not tested it and I don't believe my current PR reflects that either.