"TMS320C55x DSP Library DSPLIB Programmer's Reference"

Q3.12 Format / Q.15 Format / Q.31 FormatA.1 Q3.12 FormatTable A–1. Q3.12 Bit FieldsQ.3.12 format places the sign bit after the fourth binary digit from the right, andthe next 12 bits contain the two’s complement fractional component. Theapproximate allowable range of numbers in Q.3.12 representation is (–8,8)and the finest fractional resolution is 2 12 2.441 10 4 .Bit 15 14 13 12 11 10 9 … 0Value S I3 I2 I1 Q11 Q10 Q9 … Q0A.2 Q.15 FormatQ.15 format places the sign bit at the leftmost binary digit, and the next 15 leftmostbits contain the two’s complement fractional component. The approximate allowablerange of numbers in Q.15 representation is (–1,1) and the finest fractional resolutionis 2 15 3.05 10 5 .Table A–2. Q.15 Bit FieldsBit 15 14 13 12 11 10 9 … 0Value S Q14 Q13 Q12 Q11 Q10 Q9 … Q0A.3 Q.31 FormatQ.31 format spans two 16-bit memory words. The 16-bit word stored in the lowermemory location contains the 16 least-significant bits, and the higher memorylocation contains the most-significant 15 bits and the sign bit. The approximateallowable range of numbers in Q.31 representation is (–1,1) and the finest fractionalresolution is 2 31 4.66 10 10 .Table A–3. Q.31 Low Memory Location Bit FieldsBit 15 14 13 12 … 3 2 1 0Value Q15 Q14 Q13 Q12 … Q3 Q2 Q1 Q0Table A–4. Q.31 High Memory Location Bit FieldsBit 15 14 13 12 … 3 2 1 0Value S Q30 Q29 Q28 … Q19 Q18 Q17 Q16A-2