Question

I need to be able to convert a C SInt32 integer to a float in the range [-1, 1] and back. I’ve seen discussions of this question regarding 24 bit integers:

C/C++ – Convert 24-bit signed integer to float

And I’ve tried something similar:

 // Convert int - float
 SInt32 integer = 1;
 Float32 factor = 1;
 Float32 f = integer / (0x7FFFFFF + 0.5);

 // Perform some processing on the float
 Process(f);

 // Scale the float
 f = f * factor;

 // Convert float - int
 integer = f * (0x7FFFFFF + 0.5);

However this doesn’t work. I know it doesn’t work because the work I’m doing involves audio programming and the conversion causes a hissing sound.

I’m pretty sure it is a conversion problem because when I make the float smaller by setting the factor to 0.0001 the crackling disappears. Maybe the back conversion is putting the int out of it’s limits and is causing it to be truncated.

Any advice would be greatly appreciated.

Answer 1

const float recip = 1.0 / (32768.0*65536.0);
// hope that compiler will calculate this in advance
// From the expression an semi-advanced programmer can also immediately spot
// where the value comes from
float value = int_value * recip;
int value2 = value * (32768.0*65536.0);

The process is not reversible: one can lose up to 7 bits of accuracy.