Re: [Discuss-gnuradio] Half-Band Filter Module Output Clipping?

[Bruce McGuffin]

Matt Ettus wrote:
> Bruce McGuffin wrote:
>   
> > Hi
> >
> > I've been looking at the USRP2 filter characteristics, to see if it will 
> > work for me. After going through the
> > half-band filter code (hb_dec.v from gnu-radio 3.2) I understand what 
> > the filter coefficients are, but I have
> > a question about the output from this module.
> >
> > Near the end of the module the filtered data is clipped and rounded in 
> > lines 152-153:
> >  ...

> > It looks to me like final_sum_unrounded is 25-bits long, so the input to 
> > final_round is the
> > 20 LSBs of final_sum_unrounded (final_sum_unrounded[19:0]), and the 
> > output final_sum is those 20
> > bits rounded to 18 bits in the usual way. But this data is two's 
> > complement, so clipping the 5 MSBs from
> > final_sum_unrounded can drasticaly change the data value, for example 
> > changing both the sign and
> > magnitude, even when the magnitude of final_sum_unrounded  <  2^17.
> >
> > What am I missing here?
> >     
>
> Based on the coefficients, we can guarantee that the final sum will have 
> a magnitude which fits in 20 bits, including the sign bit.  We can just 
> drop those extra sign bits, since they'll all be the same.
>
> Matt
>   
So I'm curious. If the final sum fit into 20 bits, why were the 
accumulation and
following sum carried out with 24 or 25 bits?

Bruce