[vdr] Re: Linear volume response in VDR

[Klaus Schmidinger <Klaus.Schmidinger@cadsoft.de>]

Udo Richter wrote:
> 
> Emil Naepflein wrote:
> > +     for (i = 1, Vol = 0; i < Volume; i++)
> > +        Vol += (256 - i) * (256 - i);
> > +
> > +     am.volume_left = am.volume_right = Vol / 21718;
> 
> A little math never hurts: This formula gives exactly the same result,
> but without a loop:
> 
>    Vol = (393216 -1537*Volume +2*Volume*Volume) * (Volume-1) / 130308;
> 
>    am.volume_left = am.volume_right = Vol;
> 
> This is a cubic equation satisfying the relation 1->1 and nearly
> 256->256, plus a gradient of 0 close to 256.
> 
> An alternate proposal would be:
> 
>    Vol = Volume + (255*Volume - Volume*Volume)* q / 65025
> 
> Q is a free parameter in this quadratic variant. For q=0, this is the
> linear identity as before, and for q=255 this has a gradient of 0 at
> 255. All q=0..255 should work, and theres always 0->0 and 255->255.
> 
> Of course, if anyone knows what kind of non-linearity is behind the
> volume control, this would be even better.

Thanks for the explanation.
I think I'll go with the last formula, with q = 255.
However, shouldn't the rightmost contant be 65535 (0xFFFF)?
It doesn't make much difference, actually, but I'm having a hard
time understanding 65025, while 65535 would seem natural...

Klaus