Reply To: This is impossible!

February 14, 2023 at 6:32 pm #4905

Bob, it doesn’t make any difference for addition and subtraction, but for multiplication, it does: Where does the fixed-point DSP put its binary point in the 32-bit word?  I would hope at least a few bits in from the left.  You should be able to multiply (without any additional shifting) from, say, -32.000000 to +31.999999.  That would put the implied binary point 6 bits in from the left.

If you do things right (and the 56K did just a couple of things wrong), doing audio in a fixed-point DSP where you have access to the entire double-wide word after mult or mac, and if they do it right, it makes linear interpolation in table lookup easy.  The 56K had it off by one bit.  You had to do an LSR because the Mot guys weren’t thinking when they defined where the binary point was going and how the double-wide accumulator was split up.