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Reducing trim at low bitrate

Some informal tests seem to confirm that reducing the trim at 32-64 kbps
improves quality (better HF). It's not clear whether it's also the case
at 96 kb/s and above, so we're leaving it as is for those rates.

This corresponds to buildC in this thread:
https://hydrogenaud.io/index.php/topic,113985.0.html

Also see:
https://hydrogenaud.io/index.php/topic,111798.0.html

--- a/celt/celt_encoder.c

+++ b/celt/celt_encoder.c

@@ -780,7 +780,7 @@

 static int alloc_trim_analysis(const CELTMode *m, const celt_norm *X,

       const opus_val16 *bandLogE, int end, int LM, int C, int N0,

       AnalysisInfo *analysis, opus_val16 *stereo_saving, opus_val16 tf_estimate,

-      int intensity, opus_val16 surround_trim, int arch)

+      int intensity, opus_val16 surround_trim, opus_int32 equiv_rate, int arch)

 {

    int i;

    opus_val32 diff=0;

@@ -788,6 +788,14 @@

    int trim_index;

    opus_val16 trim = QCONST16(5.f, 8);

    opus_val16 logXC, logXC2;

+   /* At low bitrate, reducing the trim seems to help. At higher bitrates, it's less

+      clear what's best, so we're keeping it as it was before, at least for now. */

+   if (equiv_rate < 64000) {

+      trim = QCONST16(4.f, 8);

+   } else if (equiv_rate < 80000) {

+      opus_int32 frac = (equiv_rate-64000) >> 10;

+      trim = QCONST16(4.f, 8) + QCONST16(1.f/16., 8)*frac;

+   }

    if (C==2)

    {

       opus_val16 sum = 0; /* Q10 */

@@ -1954,7 +1962,7 @@

       } else {

          alloc_trim = alloc_trim_analysis(mode, X, bandLogE,

             end, LM, C, N, &st->analysis, &st->stereo_saving, tf_estimate,

-            st->intensity, surround_trim, st->arch);

+            st->intensity, surround_trim, equiv_rate, st->arch);

       }

       ec_enc_icdf(enc, alloc_trim, trim_icdf, 7);

       tell = ec_tell_frac(enc);