On 26 Oct, 16:25, Randy Yates <ya...@ieee.org> wrote:
> thunder <ja...@hotmail.com> writes:
> > Hi
>
> > I have a question regarding Fixed point Arithmetic addition.
>
> > For example, i have two fixed point numbers:
>
> > a = unsigned Q7.8 format (7-bit integer, 8 bit factional).
> > b = unsigned Q7.8 format ( * *" * * * * * * * * * * " * * * * * *).
>
> > Now a + b = c, where c is an unsigned Q8.8 result.
>
> > Qs: How do I transform c into d, where d is a unsigned Q7.9 result ??
>
> There is no way in general to do this conversion and avoid some kind of
> nonlinear effect since the range of Q7.9 is smaller than Q8.8. *The most
> obvious method would be to saturate the Q8.8 result to Q7.9.
>
> It would be good to know the reason why you're trying to rescale in this
> manner - there may be a better way to do things from a higher level
> point-of-view.
> --
> Randy Yates * * * * * * * * * * *% "The dreamer, the unwoken fool -
> Digital Signal Labs * * * * * * *% *in dreams, no pain will kiss the brow..."
> mailto://ya...@ieee.org * * * * *% *http://www.digitalsignallabs.com% 'Eldorado Overture', *Eldorado*, ELO

Hello

Thanks for all your answers.

This is not a homeowrk question, but some actual compute engine that i
am trying to design.

It turns out that there was a misunderstanding between what i thought
the C-model for this compute engine was modeling and the actual
implementation in the C-model.

My initial understanding was that the C-model was adding two signed
Q7.8 numbers (result being a signed Q8.8 number) and then somehow
quantising it to signed Q7.9 value. After having posted the question
to the newsgroup, the soultions i had mentioned in my post did not
seem to be instinctively correct. Thus my next approach was to turn
the signed Q8.8 into a saturated Q7.8 value and then add a '0' to the
LSB to make it into a signed Q7.9 value.

As it turns out, after discussion with the s/w person buliding the C-
model, they are not doing the signed Q8.8 to signed Q7.9
transformation. The C-model is adding two signed Q7.9 values to
generate a signed Q8.9 value and then adding a third signed Q7.9 value
to get a final result of signed Q9.9 value.

Thankfully thus i don't have to worry about the transformation.


Thanks all once again for all your helpful answers.


J