Brusque --

> some months ago Olin posted the formula that changed DSP world: :)
>     Filt <-- Filt + (New - Filt)*FFrac
>
> Saddly, my 8 bit implementation have a problem that I can't surpass:
>     filt = filt + ((sig16)new - (sig16)filt) / 2;
>
> The value never goes to 255 because of integer roundings:
>     filt = 254 + (255 - 254) / 2;
>     filt = 254 + (1)/2;
>     filt = 254;
>
> There's a way to change this? I've tried to raise the resolution making:
>     filt = filt + ((sig16)new*2 - (sig16)filt*2 + 1) / 4;
>
> But got same results because:
>     filt = 254 + ((sig16)255*2 - (sig16)254*2 + 1) / 4;
>     filt = 254 + (510 - 508 + 1) / 4;
>     filt = 254 + (510 - 508 + 1) / 4;
>     filt = 254 + (1) / 4;

I think you correctly recognized that you need to round rather than
truncate the result, but you only rounded one term instead of the total:

Original:
      filt = filt + (new - filt) / 2;

Rounded:
      filt = ((filt*2 + (new*2 - filt*2) / 2) + 1) / 2;
      filt = ((254*2 + (255*2 - 254*2) / 2) + 1) / 2;
      filt = ((508 + (510 - 508) / 2) + 1) / 2;
      filt = ((508 + 2 / 2) + 1) / 2;
      filt = (509 + 1) / 2;
      filt = 510 / 2;
      filt = 255;

I hope this helps.

-- Dave Tweed

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