Jean-Marc Valin

OK, so I thought the fixed-point code in 1.2-beta1 was getting pretty good. But that was until a user ((wouldn't things be simple without them!) was able to make it fail horribly by feeding it totally clipped speech. It turns out that the file manages to trigger at least a half-dozen overflows all around the code, some of them easily fixed, some not.

So here's the deal with fixed-point. Some CPUs/DSPs support saturating versions of add/sub/mul/... and some don't. Most G.72x codecs are usually implemented assuming that they exist, so they don't need to worry about overflows. For Speex, I decided to do it without assuming hardware saturation, so it can run on ARM and other chips (including x86) that don't support saturation. And that's how everything suddenly becomes more complicated. If once in a while 0.5 + 0.6= 1.0, you usually don't care too much. On the other hand, if 0.5 + 0.6 = -0.9, then suddenly you do care.

So the fundamental question here is how much overflows on corrupted input can be tolerated (based on the "garbage in, garbage out" principle) and how much needs to be avoided regardless of the input? Answer when I get to the bottom of this. To be continued...