On Thu, 11 Jul 2002, Abramo Bagnara wrote:
> Jaroslav Kysela wrote:
> >
> > On Tue, 9 Jul 2002, Abramo Bagnara wrote:
> >
> > > > > > > And the summing/normalization code in pcm_route.c treats the sample as
> > > > > > > if it were unsigned.
> > > > >
> > > > > Be careful here. Use of signed vs. unsigned was an hard decision.
> > > > > Can you point me exactly where you see a problem?
> > > >
> > > > In the following code snippet
> > > >
> > > > add_int64_att:
> > > > sum.as_sint64 += (u_int64_t) sample * ttp->as_int;
> > > >
> > > > each of the three variables has a signed integer type. Converting
> > > > the sample from int32_t to u_int64_t would give wrong results for
> > > > negative values, e.g., the sample -1 (0xffffffff) would be converted
> > > > to 4294967295 (0x00000000ffffffff).
> > > >
> > > > The following code
> > > >
> > > > norm_int:
> > > > if (sum.as_sint64 > (u_int32_t)0xffffffff)
> > > > sample = (u_int32_t)0xffffffff;
> > > > else
> > > > sample = sum.as_sint64;
> > > >
> > > > doesn't check for negative values of sum.as_sint64. Additionally,
> > > > using 0xffffffff in case of overflow results in sample == -1.
> > > >
> > > > These code snippets would be correct if the sample would be unsigned,
> > > > but my point was that this isn't the case.
> > >
> > > I've just checked: this has been broken by Jaroslav with CVS version
> > > 1.55. I'd like to restore the original behaviour I wrote (that used
> > > unsigned for efficiency reason), but I don't understand what Jaroslav
> > > tried to solve.
> > >
> > > Jaroslav?
> >
> > You cannot use unsigned values, because we need to sum samples (it means:
> > add positive and substract negative ones). If we are in positive range, we
> > can only add samples, so the final sum result is wrong.
>
> Wow, I've never realized this...
> So mixing can only be done with signed values, otherwise we have bogus
> value for overranges. Right?
>
> Taken this in account, I'm tempted to consider unsigned formats as
> second class citizen (i.e. format hard to handle and with some added
> difficulties wrt manipulation). I'm wrong?
Yes, unsigned formats are dangerous in the sense of arithmetics.
Jaroslav
-----
Jaroslav Kysela <[EMAIL PROTECTED]>
Linux Kernel Sound Maintainer
ALSA Project http://www.alsa-project.org
SuSE Linux http://www.suse.com
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