[Ffmpeg-devel] [PATCH] flacenc - lpc and options

[Michael Niedermayer]

Hi

On Fri, Jun 30, 2006 at 01:40:48AM -0400, Justin Ruggles wrote:
[...]
> 2) added 8 compression options to AVCodecContext which are used by the
> encoder.  most of them are set to -1 by default, which indicates for the
> encoder to use its default setting.  this is so that other codecs can
> reuse these fields if they wish, and can have different default values.
> 
> 3) modified flacenc.c to first check for the compression_level and set
> all the options based on that value, then read the other AVCodecContext
> values and override the defaults if they are explicitly set by the user.
> 
> If any of this should be changed, let me know.  Michael's idea of a
> 'preset' field in AVCodecContext would work, but it was unnecessary in
> this case.  I guess it could still be added for shortcuts like "best" or
> "fast".
> 
> Since I have not yet added any more extensive order search options, the
> compression level only goes from 0 to 5 for now.  Below are the results
> of a few tests comparing ffmpeg to the reference encoder.  The faster
> decode times for the ffmpeg-generated files are mostly due to the fact
> that MD5 sums are not implemented yet, so the decoder doesn't do any
> verification.

[...]
> +    if(avctx->min_prediction_order >= 0) {
> +        if(s->options.use_lpc) {
> +            if(avctx->min_prediction_order == 0 ||
> +                    avctx->min_prediction_order > 32) {

this could be MAX_LPC_ORDER


[...]
> +/**
> + * Apply Welch window function to audio block
> + */
> +static void apply_welch_window(const int32_t *data, int len, double *w_data)
> +{
> +    int i, n2;
> +    double w;
> +    double c;
> +
> +    n2 = (len >> 1);
> +    c = 2.0 / (len - 1.0);
> +	for(i=0; i<n2; i++) {
> +		w = c - i - 1.0;
> +        w = 1.0 - (w * w);
> +        w_data[i] = data[i] * w;
> +        w_data[len-1-i] = data[len-1-i] * w;
> +	}
> +}
> +
> +static double *welch_window;
> +static int window_size;
> +
> +static void window_init(int size)
> +{
> +    int i, n2;
> +    double w;
> +    double c;
> +
> +    window_size = size;
> +    welch_window = av_malloc(size * sizeof(double));
> +
> +    n2 = (size >> 1);
> +    c = 2.0 / (size - 1.0);
> +	for(i=0; i<n2; i++) {
> +		w = c - i - 1.0;

there are tabs in the line above
the code is duplicated from apply_welch_window
and this is not thread safe, window_size, ... are non constant static
variables

[...]

> +
> +/**
> + * Levinson-Durbin recursion.
> + * Produces LPC coefficients from autocorrelation data.
> + */
> +static void compute_lpc_coefs(const double *autoc, int max_order,
> +                              double lpc[][MAX_LPC_ORDER], double *ref)
> +{
> +   int i, j, i2;
> +   double r, err, tmp;
> +   double lpc_tmp[MAX_LPC_ORDER];
> +
> +   for(i=0; i<max_order; i++) lpc_tmp[i] = 0;
> +   err = autoc[0];
> +
> +   for(i=0; i<max_order; i++) {
> +      r = -autoc[i+1];
> +      for(j=0; j<i; j++) {
> +          r -= lpc_tmp[j] * autoc[i-j];
> +      }
> +      r /= err;
> +      ref[i] = fabs(r);
> +
> +      err *= 1.0 - (r * r);
> +
> +      i2 = (i >> 1);
> +      lpc_tmp[i] = r;
> +      for(j=0; j<i2; j++) {
> +         tmp = lpc_tmp[j];
> +         lpc_tmp[j] += r * lpc_tmp[i-1-j];
> +         lpc_tmp[i-1-j] += r * tmp;
> +      }
> +      if(i % 2) {

i&1 is faster then i%2


[...]

> +    for(i=0; i<sub->order; i++) {
> +        sub->coefs[i] = coefs[sub->order-1][i];
>      }
> -    return bits[sub->order];
> +    porder = get_max_p_order(max_porder, n, sub->order);
> +    encode_residual_lpc(res, smp, n, sub->order, sub->coefs, sub->shift);

hmm, coeffs is copied into sub->coefs and then used, why isnt it used withot
copying?


[...]
> +    /**
> +     * sets audio frame size using frame duration in milliseconds
> +     * - encoding: set by user.
> +     * - decoding: unused.
> +     */
> +    int block_time_ms;

iam a little unsure about this, maybe you could leave this one out for now
and set it just based on compression_level

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

In the past you could go to a library and read, borrow or copy any book
Today you'd get arrested for mere telling someone where the library is