feat(capability): 转码能力包（vendor 纯 Go AMR-NB 解码器 + 黄金向量逐样本对照 + 8 哨兵 + 注册面）

internal/capability/transcode/amrnb/dsp_agc.go

@@ -0,0 +1,77 @@
+package amrnb
+
+// Adaptive gain control for the post-filter, ported from opencore-amrnb agc.cpp.
+// agc scales the post-filtered signal so its energy tracks the synthesis
+// energy, with a per-sample gain smoothed by agc_fac across the subframe. The
+// running gain is carried in agcState. Bit-exact with the reference.
+
+const cAGC_FAC = 29491 // 0.9, the post-filter AGC factor
+
+// agcState holds the smoothed per-sample gain across subframes.
+type agcState struct {
+	pastGain int16
+}
+
+// energyOld accumulates the energy of a prescaled signal (used when energyNew
+// would overflow). Mirrors energy_old.
+func energyOld(in []int16, lTrm int) int32 {
+	var s int32
+	for i := 0; i < lTrm; i++ {
+		temp := in[i] >> 2
+		s = L_mac(s, temp, temp)
+	}
+	return s
+}
+
+// energyNew returns the energy of a signal (>>4 scaled), falling back to
+// energyOld on saturation. Mirrors energy_new.
+func energyNew(in []int16, lTrm int) int32 {
+	var s int32
+	for i := 0; i < lTrm; i++ {
+		s = L_mac(s, in[i], in[i])
+	}
+	if s != maxInt32 {
+		return s >> 4
+	}
+	return energyOld(in, lTrm)
+}
+
+// agc scales sigOut in place so its energy approaches sigIn's, applying a gain
+// smoothed across the subframe by agcFac and carried in the state. Mirrors agc.
+func (st *agcState) agc(sigIn, sigOut []int16, agcFac int16, lTrm int) {
+	s := energyNew(sigOut, lTrm)
+	if s == 0 {
+		st.pastGain = 0
+		return
+	}
+	exp := norm_l(s) - 1
+	gainOut := round_(L_shl(s, exp))
+
+	s = energyNew(sigIn, lTrm)
+	var g0 int16
+	if s == 0 {
+		g0 = 0
+	} else {
+		i := norm_l(s)
+		gainIn := round_(s << i)
+		exp -= i
+
+		temp := div_s(gainOut, gainIn)
+		ss := int32(temp) << 7
+		ss = L_shr(ss, exp)
+		ss = invSqrt(ss)
+		lTemp := ss << 9
+		ii := int16((lTemp + 0x8000) >> 16)
+		temp = 32767 - agcFac
+		g0 = int16((int32(ii) * int32(temp)) >> 15)
+	}
+
+	gain := st.pastGain
+	for i := 0; i < lTrm; i++ {
+		gain = int16((int32(gain) * int32(agcFac)) >> 15)
+		gain += g0
+		lTemp := (int32(sigOut[i]) * int32(gain)) << 1
+		sigOut[i] = int16(lTemp >> 13)
+	}
+	st.pastGain = gain
+}