Why does fixed-point signal multiplication give different output depending on the datatype?

I have a Simulink model where a constant value of "-500" (stored as a "fixdt(1,16,0.1,-400)" ) is multiplied by "-1" via a product block in 3 different ways:

1. By a constant value of "-1" stored as a fixed-point integer "fixdt(1,16,0.1,-400)".
2. By a constant value of "-1" stored as an int8.
3. By a constant value of "-1" stored as a single.

For each of these 3 cases I obtain a slightly different precision in the result.

Why does the datatype of "-1" affect the result of the product block?

Answer 1

[MathWorks Support Team]

The designed behavior for product and other operations applied to* fixed-point signals *with non-zero bias is to do a conversion to a more "operation friendly" type to do the operation. This results in more efficient code (generally), but as a side effect it could also lead to less accurate results.

The developers are working on improving the behavior of non-zero bias multiplications in future releases.

For the time being, please consider a few tips to obtain more accurate results:

• Ensure that the slope of the fixed-point datatype can be represented as a power of 2 (e.g. 0.125=2^(-3)).
• Try to use a bias equal to 0 in the fixed-point datatype.