Modified 8bit FFT in c

Hello, deif.
You use int8_t ie 8bit char here

const prog_int8_t Sinewave[N_WAVE-N_WAVE/4] PROGMEM = {

And why do you read words into the fix_fft() function?

wr = pgm_read_word_near(Sinewave + j+N_WAVE/4);

Thanks.