Initial release

engine/mp3/mp3enc-filter.h

@@ -0,0 +1,69 @@
+#pragma once
+// mp3enc-filter.h
+// Polyphase analysis filterbank: 32 new PCM samples in, 32 subband samples out.
+// ISO 11172-3 clause 2.4.3.2 (encoder side, Annex C).
+// Part of mp3enc. MIT license.
+
+#include <cmath>
+#include <cstring>
+
+#ifndef M_PI
+#    define M_PI 3.14159265358979323846
+#endif
+// The analysis filterbank state for one channel.
+// Holds the 512 sample FIFO and produces 32 subband outputs per call.
+struct mp3enc_filter {
+    float buf[512];  // circular buffer of past PCM samples
+    int   off;       // current write offset into buf
+
+    void init() {
+        memset(buf, 0, sizeof(buf));
+        off = 0;
+    }
+
+    // Feed 32 new PCM samples, produce 32 subband samples.
+    // pcm: pointer to 32 float samples (mono, this channel only)
+    // sb_out: output array of 32 subband values
+    //
+    // Algorithm (ISO 11172-3 Annex C, encoding process):
+    // 1. Shift 32 new samples into the 512 sample FIFO
+    // 2. Window: multiply by 512 Ci coefficients (mp3enc_enwindow)
+    // 3. Partial sum: 512 windowed values -> 64 values
+    // 4. Matrixing: 64 values -> 32 subband samples via DCT
+    void process(const float * pcm, float * sb_out) {
+        // Step 1: shift new samples into the buffer.
+        // The newest sample goes at buf[off], oldest at buf[off+31].
+        // We store them in reverse order so windowing is a straight multiply.
+        off = (off - 32) & 511;
+        for (int i = 0; i < 32; i++) {
+            buf[(off + i) & 511] = pcm[31 - i];
+        }
+
+        // Step 2 + 3: window and partial sum.
+        // z[i] = sum over j=0..7 of: buf[(off + i + 64*j) & 511] * Ci[i + 64*j]
+        // This produces 64 values from the 512 windowed samples.
+        float z[64];
+        for (int i = 0; i < 64; i++) {
+            float sum = 0.0f;
+            for (int j = 0; j < 8; j++) {
+                int buf_idx = (off + i + 64 * j) & 511;
+                int win_idx = i + 64 * j;
+                sum += buf[buf_idx] * mp3enc_enwindow[win_idx];
+            }
+            z[i] = sum;
+        }
+
+        // Step 4: matrixing (ISO 11172-3, Annex C, analysis filter).
+        // sb_out[k] = sum over i=0..63 of: z[i] * cos((2*k + 1) * (16 - i) * PI / 64)
+        // for k = 0..31 (subbands)
+        // Formula verified against ISO 11172-3 Table C.1
+        for (int k = 0; k < 32; k++) {
+            float sum = 0.0f;
+            for (int i = 0; i < 64; i++) {
+                float angle = (float) M_PI * (float) (2 * k + 1) * (float) (16 - i) / 64.0f;
+                sum += z[i] * cosf(angle);
+            }
+            sb_out[k] = sum;
+        }
+    }
+};