Re: [Lame-dev] calc_noise potential issue

[Guillaume L. <gle...@tf...>]

--On Dimanche 28 mars 2004 18:16 +0200 Gabriel Bouvigne 
<bou...@mp...> wrote:

> Guillaume Lessard wrote:
>
>>> In calc_noise, we have this:
>>>
>>> noise = *distort++ = noise / *l3_xmin++;
>>>
>>> /* multiplying here is adding in dB, but can overflow */
>>> noise = FAST_LOG10(Max(noise,1E-20));
>>>
>
>>> In those 2 cases, we are considering the noise of the sfb to be -20, and
>>> even worst we are adding -20 to total noise instead of 0.
>
>> This is just a question of size. As long as 1e-20 is much smaller than a
>> normal value, everything is fine. All we can do is use the log of a
>> vanishingly small number instead of log(0); 10^(-20) seems, at first
>> blush, good enough.
>
> The (potential) problem is that:
> 1) we are then considering the sfb to be distorted, and are increasing
> the "over_count" value
> b) we are using a log of the value, so 10^(-20) is becoming -20, which is
> not minor at all when added to tot_noise.

It is minor. The code replaces 0 with 0.00000000000000000001. Remember that 
log(0) is minus infinity, an unrepresentable value. We are replacing -NaN 
with -20, thus avoiding an error.

For kicks, I added the line "printf("%f ", noise);" just after the log 
calculation. Here's a sampling of the results:

 -0.529911 -0.539173 -0.741660 -0.704893 -1.057069 -1.230736 -1.895052 
-2.570281

(and it goes on and on).

The logs of the noise values are mostly between -3 and 0, which is to say 
between 0.001 and 1. This shows that the value chosen to avoid NaN errors 
is correct and well below any typically calculated noise.

--
Guillaume Lessard
gle...@tf...