module twos_to_offset(twos,shifted);

   // this module converts a two's complement number to a positive integer
   // offset around the middle value (ie N/2).  So for example, for an
   // 8-bit number, the offset is 128.

   parameter NBITS = 8;
   parameter MEAN = (1<<(NBITS-1));

   input [NBITS-1:0] twos;
   output [NBITS-1:0] shifted;      

   wire 	      sign = twos[NBITS-1];
   wire [NBITS-1:0]   mag = sign ? {0,~twos[NBITS-2:0]}+1 
		                 : {0, twos[NBITS-2:0]};
   wire [NBITS-1:0]   shifted = sign ? MEAN-mag : MEAN+mag;

endmodule 

module offset_to_twos(shifted,twos);

   // this module converts a positive integer offset around its middle 
   // value (ie N/2) to a two's complement number, centered around zero.

   parameter NBITS = 8;
   parameter MEAN = (1<<(NBITS-1));

   input [NBITS-1:0]  shifted;      
   output [NBITS-1:0] twos;

   wire 	      sign = (shifted <= MEAN);
   wire [NBITS-1:0]   twos = sign ? (~(MEAN-shifted))+1 : shifted-MEAN;

endmodule