On Thu, Jul 07, 2005 at 02:11:28PM -0300, Jose Marcelo Lima Duarte wrote: > I'm trying to build a costas loop to demodulate a BPSK signal from a > non-geostationary satellite. I need to make it work at real > time; since our computer can't store all the data during the transmission > period. The base band signal has a frequency of 70 kHz.
> In order to make a fast costas loop, I made a fast_atan2() function. I think > it would be interesting to include it into the GNU Radio library. > I create the function following the instructions of the site below. > http://www.dspguru.com/comp.dsp/tricks/alg/fxdatan2.htm Good. Just out of curiosity, did you benchmark your demodulator code to determine if atan2 was anywhere near the critical path? > I would like to know if someone has developed a nco that use table technique > to calculate the sin and -cos? It would be nice to make an gr_fast_nco that > find the values of sin and cos in a table instead of calculate them at each > nco step. See gr_fxpt_nco.h It uses a table lookup plus interpolation strategy to compute sine and cosine to 16-bits. BTW, the code below divides by zero when: x >= 0 and x == -y or x < 0 and x == y <soap_box_mode> "Premature optimization is the root of all evil" http://en.wikipedia.org/wiki/Talk:C._A._R._Hoare Rules for software success: * Write test cases first * Ensure it computes the right answer for all cases * MEASURE PERFORMANCE in your application (oprofile is handy). * Then, if it's really a problem (top one or two cycle burners) consider making it faster. </soap_box_mode> > float fast_atan2( float y, float x){ > float r, theta; > if ( x>=0 ){ // quadrante 1 ou 4 > r = (x-y) / (x+y); > theta = PI4 - PI4 * r; > if ( y>=0 ) > return theta; //quadrante 1 > else > return -theta; //quadrante 4 > } > else { // quadrante 2 ou 3 > r = (x+y) / (x-y); > theta = 3*PI4 - PI4 * r; > if ( y>=0 ) > return theta; // quadrante 2 > else > return -theta; // quadrante 3 > } > } _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
