[PlanetCCRMA] Pentium-4 and denormal numbers on planetccrma

[Fernando Lopez-Lezcano]

On Fri, 2005-01-28 at 15:35, Steve Harris wrote:
> On Fri, Jan 28, 2005 at 03:17:41PM -0800, Fernando Pablo Lopez-Lezcano wrote:
> > On Fri, 2005-01-28 at 14:51, Fernando Lopez-Lezcano wrote:
> > > On Wed, 2005-01-12 at 03:40, Steve Harris wrote:
> > > > On Mon, Jan 10, 2005 at 11:07:55 +0100, andersvi@extern.uio.no wrote:
> > > > NB, using the SSE instruction set uses a more efficient denormal handler
> > > > (about 40x slower than processing a normal number), but it still doesnt
> > > > zero them, if you also call this function I hacked up when the program
> > > > starts:
> > > 
> > > Hi Steve, I'm trying to use this to see if I can get freeverb back from 
> > > denormal hell, but I can't compile it as position independent code as in
> > > that mode the compiler uses the bx register (I did some searches but
> > > could not find a solution that both compiled and did not segfault :-)
> > 
> > I tried adding noise to the inputs (from
> > http://www.musicdsp.org/files/denormal.pdf):
> > 
> > unsigned int rand_state = 1;
> > 
> > inline add_white_noise (float &val) {
> >   rand_state = rand_state * 1234567UL + 890123UL;
> >   int mantissa = rand_state & 0x807F0000;
> >   int flt_rnd = mantissa | 0x1e999999;
> >   val += *reinterpret_cast <const flat*> (&flt_rnd);
> > }
> > 
> > And while it is a hack it did work, surprisingly. 
> > Perhaps not the best solution.....
> 
> Its a perfectly reasonable solution, sorry, I should have suggested it.
> The thing to test would be to turn up the RT60 time as high as posible and
> measure the amplitude of the output to see how much it affects the SNR.

I just tried this: I connected hydrogen to jack-rack running the hacked
freeverb (max wet output, no dry output, max room size, 0 damping, 0.5
width) and jack-rack to bitscope. After stopping hydrogen the "smallest
non-zero sample" shown on bitscope slowly falls down to a noisy -400db
(the "largest sample" field hovers around -336db). Bitscope is handy :-)
As much as I hate to introduce "noise" in the signal chain I think most
probably -300db is quiet enough. I can't hear it, but maybe its just my
aging ears :-)

The best thing to do, obviously, would be to figure out why the
undenormal routines in freeverb are not doing their job. I look at it
and it seems they are inserted in the right places. 

For now I'll probably release this, at least it brings freeverb back
from the ladspa dead. 

Maybe this is the way to "cure" (as in "don't think too much")
freqtweak, it also has denormal issues in its current release. 

-- Fernando