Vorbis dev - Sep 2002 - Libvorbis suggestion & floating point exception

Hello,

I'm not quite sure if this is the right list for my questions, please excuse
if it isn't.

1. While using vorbislib (Win2K/Borland C++ Builder 5), I'm getting
reproducible floating point exceptions at lots of sample rate/bit rate
combinations. For example, the original encoder_example.c, with just the
initialization line changed to

  vorbis_encode_init(&vi,2,22050,48000,48000,48000);

generates a "floating point exception 0xc0000090" in psy.c /
bark_noise_hybridmp (line 642).
The code reads:

    D = tN * tXX - tX * tX;
    R = (A + fi * B) / D;

Before the crash, tXX and tX are both 0, thus D == 0. This causes a "divide
by zero".
As said, it is reproducible every single time. At higher sample rates it
works at 48kbps, but crashes at higher bit rates. I tested the 1.0 build
(static as well as the pre-built DLL from the distribution) and the CVS
snapshot, same result.

However, at 44.1kHz all bit rates seem to work.

Any ideas?

<p>2. In vorbisenc.c / vorbis_encode_setup_managed, there's a piece
that reads:

  if(nominal_bitrate<=0.){
    if(max_bitrate>0.){
      nominal_bitrate=max_bitrate*.875;
    }else{  ...

If only min_bitrate and max_bitrate are passed, and nominal_bitrate set to
zero for "auto" mode, nominal_bitrate might actually end lower than
min_bitrate (e.g. if min_bitrate == max_bitrate for "CBR" emulation).

I think it should be something like:

  if(nominal_bitrate<=0.){
    if(max_bitrate>0.){
      nominal_bitrate=max(min_bitrate, max_bitrate*.875);
    }else{  ...

Greetings,
  Jan

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Since nobody likes to reply, I'll do it myself :)

The floating point exception is a "divide by zero", which happens
quite
often (and in several places) in libvorbis, the most prominent place being
fit_line called with one or zero points to fit.

I figured this does not affect some (most) people because of the floating
point exception handling turned off by their compilers/on their systems, so
I tried to just disable it and it actually seems to work.

So for anyone having the same problem, I inserted these two functions:

---snip---

unsigned short FPU_Disable_Exceptions()
{
  unsigned short cw, cwnew;
  asm { FSTCW cw };
  cwnew = cw | 0x0005; // Disable divide by zero/invalid operation
exceptions
  asm { FLDCW cwnew };
  return cw;
}

void FPU_Restore_Exceptions(unsigned short cw)
{
  asm { FCLEX; FLDCW cw };
}

--- snip ---

Then just put

  unsigned short fpucw = FPU_Disable_Exceptions();

at the beginning and

  FPU_Restore_Exceptions(fpucw);

at the end of the function calling vorbis_analysis(). This will disable
exception handling and just return NaN when dividing by zero. The encoded
streams seem to be correct and flawless, though.

Note this is just a workaround and in no way portable code, I tested it only
under W2K, with Borland C++ Builder 5.

Greetings,
  Jan

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On 24 Sep 2002 at 0:50, Jan Lellmann wrote:
> I'm not quite sure if this is the right list for my questions, please
> excuse if it isn't.
> 
> 1. While using vorbislib (Win2K/Borland C++ Builder 5), I'm getting
> reproducible floating point exceptions at lots of sample rate/bit rate
> combinations. 
Hello, Jan!

I use BCB4, and shortly before the 1.0 release, I was having divide by 
zero exceptions in libvorbis.  However, my problems were fixed by the 
1.0 release, and I haven't had any trouble since.  I'd be happy to test 
something for you, if you'd like.


--
-:-:- David K. Gasaway
-:-:- XNS  : =David K Gasaway
-:-:- Email: dave@gasaway.org
-:-:- Web  : dave.gasaway.org

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