[FFmpeg-devel] [PATCH 1/6] avcodec/vorbisenc: Add pre-echo detection

[Tomas Härdin]

On 2017-08-22 03:23, Tyler Jones wrote:
> +
> +/**
> + * Calculate the variance of a block of samples
> + *
> + * @param in     Array of input samples
> + * @param length Number of input samples being analyzed
> + * @return       The variance for the current block
> + */
> +static float variance(const float *in, int length, AVFloatDSPContext *fdsp)
> +{
> +    int i;
> +    float mean = 0.0f, square_sum = 0.0f;
> +
> +    for (i = 0; i < length; i++) {
> +        mean += in[i];
> +    }
> +
> +    square_sum = fdsp->scalarproduct_float(in, in, length);
> +
> +    mean /= length;
> +    return (square_sum - length * mean * mean) / (length - 1);
> +}

Isn't this method much more numerically unstable compared to the naïve 
method? Might not matter too much when the source data is 16-bit, but 
throwing it out there anyway

DSP methods for computing mean and variance could be a good project for 
someone wanting to learn

/Tomas