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use scale factor instead of sum for downmix level adjustment. avoids division 

inside downmix loop. 7% faster 5.1-to-stereo downmixing.

Originally committed as revision 12287 to svn://svn.ffmpeg.org/ffmpeg/trunk
This commit is contained in:
Justin Ruggles 2008-03-01 16:10:19 +00:00
parent 9d10e6e6c0
commit d8870f120e
1 changed files with 9 additions and 6 deletions
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15
libavcodec/ac3dec.c
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@ -154,7 +154,7 @@ typedef struct {
int center_mix_level; ///< Center mix level index
int surround_mix_level; ///< Surround mix level index
float downmix_coeffs[AC3_MAX_CHANNELS][2]; ///< stereo downmix coefficients
float downmix_coeff_sum[2]; ///< sum of downmix coeffs for each output channel
float downmix_coeff_adjust[2]; ///< adjustment needed for each output channel when downmixing
float dynamic_range[2]; ///< dynamic range
int cpl_coords[AC3_MAX_CHANNELS][18]; ///< coupling coordinates
int num_cpl_bands; ///< number of coupling bands
@ -404,11 +404,14 @@ static void set_downmix_coeffs(AC3DecodeContext *s)
s->downmix_coeffs[nf][0] = s->downmix_coeffs[nf+1][1] = smix;
}
s->downmix_coeff_sum[0] = s->downmix_coeff_sum[1] = 0.0f;
/* calculate adjustment needed for each channel to avoid clipping */
s->downmix_coeff_adjust[0] = s->downmix_coeff_adjust[1] = 0.0f;
for(i=0; i<s->fbw_channels; i++) {
s->downmix_coeff_sum[0] += s->downmix_coeffs[i][0];
s->downmix_coeff_sum[1] += s->downmix_coeffs[i][1];
s->downmix_coeff_adjust[0] += s->downmix_coeffs[i][0];
s->downmix_coeff_adjust[1] += s->downmix_coeffs[i][1];
}
s->downmix_coeff_adjust[0] = 1.0f / s->downmix_coeff_adjust[0];
s->downmix_coeff_adjust[1] = 1.0f / s->downmix_coeff_adjust[1];
}
/**
@ -747,8 +750,8 @@ static void ac3_downmix(AC3DecodeContext *s)
v0 += s->output[j][i] * s->downmix_coeffs[j][0];
v1 += s->output[j][i] * s->downmix_coeffs[j][1];
}
v0 /= s->downmix_coeff_sum[0];
v1 /= s->downmix_coeff_sum[1];
v0 *= s->downmix_coeff_adjust[0];
v1 *= s->downmix_coeff_adjust[1];
if(s->output_mode == AC3_CHMODE_MONO) {
s->output[0][i] = (v0 + v1) * LEVEL_MINUS_3DB;
} else if(s->output_mode == AC3_CHMODE_STEREO) {