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_postproc.go

@@ -0,0 +1,67 @@
+package amrnb
+
+// Output post-processing building blocks, ported from opencore-amrnb
+// weight_a.cpp (Weight_Ai), preemph.cpp (preemphasis) and post_pro.cpp
+// (Post_Process). weightAi applies gamma bandwidth expansion to an LP filter;
+// preemphasis is the 1st-order y[n]=x[n]-g·x[n-1]; postProcess is the decoder's
+// output high-pass biquad with the ×2 output scaling. Bit-exact with the
+// reference.
+
+// weightAi computes the bandwidth-expanded coefficients a_exp[i] = a[i]·fac[i-1]
+// (with a_exp[0]=a[0]). Mirrors Weight_Ai.
+func weightAi(a []int16, fac []int16, aExp []int16) {
+	aExp[0] = a[0]
+	for i := 1; i <= cM; i++ {
+		aExp[i] = int16((int32(a[i])*int32(fac[i-1]) + 0x4000) >> 15)
+	}
+}
+
+// preemphasis applies y[n] = x[n] - g·x[n-1] in place over L samples, carrying
+// one sample of memory. Mirrors preemphasis (preemph.cpp).
+func preemphasis(memPre *int16, signal []int16, g int16, L int) {
+	temp := signal[L-1]
+	for i := L - 1; i >= 1; i-- {
+		signal[i] = sub(signal[i], mult(g, signal[i-1]))
+	}
+	signal[0] = sub(signal[0], mult(g, *memPre))
+	*memPre = temp
+}
+
+// post-process output high-pass biquad coefficients (post_pro.cpp).
+var postB = [3]int16{7699, -15398, 7699}
+var postA = [3]int16{8192, 15836, -7667}
+
+// postProcessState holds the biquad's double-precision input/output memory.
+type postProcessState struct {
+	x0, x1                 int16
+	y1hi, y1lo, y2hi, y2lo int16
+}
+
+// process applies the output high-pass filter (with ×2 gain) in place over lg
+// samples. The feedback memory is kept as (hi,lo) double precision. Mirrors
+// Post_Process.
+func (st *postProcessState) process(signal []int16, lg int) {
+	a1, a2 := int32(postA[1]), int32(postA[2])
+	b0, b1, b2 := int32(postB[0]), int32(postB[1]), int32(postB[2])
+	for i := 0; i < lg; i++ {
+		x2 := st.x1
+		st.x1 = st.x0
+		st.x0 = signal[i]
+
+		L := int32(st.y1hi) * a1
+		L += (int32(st.y1lo) * a1) >> 15
+		L += int32(st.y2hi) * a2
+		L += (int32(st.y2lo) * a2) >> 15
+		L += int32(st.x0) * b0
+		L += int32(st.x1) * b1
+		L += int32(x2) * b2
+
+		L = L_shl(L, 3)
+		signal[i] = round_(L_shl(L, 1))
+
+		st.y2hi = st.y1hi
+		st.y2lo = st.y1lo
+		st.y1hi = int16(L >> 16)
+		st.y1lo = int16((L >> 1) - (int32(st.y1hi) << 15))
+	}
+}