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diff --git a/tools/spectral_editor/dct.js b/tools/spectral_editor/dct.js
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+const dctSize = 512; // Default DCT size, read from header
+
+// --- Utility Functions for Audio Processing ---
+// Fast O(N log N) IDCT using FFT
+// JavaScript equivalent of C++ idct_512
+function javascript_idct_512(input) {
+    return javascript_idct_512_fft(input);
+}
+
+// Hanning window for smooth audio transitions (JavaScript equivalent)
+function hanningWindow(size) {
+    const window = new Float32Array(size);
+    const PI = Math.PI;
+    for (let i = 0; i < size; i++) {
+        window[i] = 0.5 * (1 - Math.cos((2 * PI * i) / (size - 1)));
+    }
+    return window;
+}
+
+const hanningWindowArray = hanningWindow(dctSize); // Pre-calculate window
+
+// ============================================================================
+// FFT-based DCT/IDCT Implementation
+// ============================================================================
+// Fast Fourier Transform using Radix-2 Cooley-Tukey algorithm
+// This implementation MUST match the C++ version in src/audio/fft.cc exactly
+
+// Bit-reversal permutation (in-place)
+// Reorders array elements by reversing their binary indices
+function bitReversePermute(real, imag, N) {
+    // Calculate number of bits needed
+    let temp_bits = N;
+    let num_bits = 0;
+    while (temp_bits > 1) {
+        temp_bits >>= 1;
+        num_bits++;
+    }
+
+    for (let i = 0; i < N; i++) {
+        // Compute bit-reversed index
+        let j = 0;
+        let temp = i;
+        for (let b = 0; b < num_bits; b++) {
+            j = (j << 1) | (temp & 1);
+            temp >>= 1;
+        }
+
+        // Swap if j > i (to avoid swapping twice)
+        if (j > i) {
+            const tmp_real = real[i];
+            const tmp_imag = imag[i];
+            real[i] = real[j];
+            imag[i] = imag[j];
+            real[j] = tmp_real;
+            imag[j] = tmp_imag;
+        }
+    }
+}
+
+// In-place radix-2 FFT (after bit-reversal)
+// direction: +1 for forward FFT, -1 for inverse FFT
+function fftRadix2(real, imag, N, direction) {
+    const PI = Math.PI;
+
+    // Butterfly operations
+    for (let stage_size = 2; stage_size <= N; stage_size *= 2) {
+        const half_stage = stage_size / 2;
+        const angle = direction * 2.0 * PI / stage_size;
+
+        // Precompute twiddle factors for this stage
+        let wr = 1.0;
+        let wi = 0.0;
+        const wr_delta = Math.cos(angle);
+        const wi_delta = Math.sin(angle);
+
+        for (let k = 0; k < half_stage; k++) {
+            // Apply butterfly to all groups at this stage
+            for (let group_start = k; group_start < N; group_start += stage_size) {
+                const i = group_start;
+                const j = group_start + half_stage;
+
+                // Complex multiplication: (real[j] + i*imag[j]) * (wr + i*wi)
+                const temp_real = real[j] * wr - imag[j] * wi;
+                const temp_imag = real[j] * wi + imag[j] * wr;
+
+                // Butterfly operation
+                real[j] = real[i] - temp_real;
+                imag[j] = imag[i] - temp_imag;
+                real[i] = real[i] + temp_real;
+                imag[i] = imag[i] + temp_imag;
+            }
+
+            // Update twiddle factor for next k (rotation)
+            const wr_old = wr;
+            wr = wr_old * wr_delta - wi * wi_delta;
+            wi = wr_old * wi_delta + wi * wr_delta;
+        }
+    }
+}
+
+// Forward FFT: Time domain → Frequency domain
+function fftForward(real, imag, N) {
+    bitReversePermute(real, imag, N);
+    fftRadix2(real, imag, N, +1);
+}
+
+// Inverse FFT: Frequency domain → Time domain
+function fftInverse(real, imag, N) {
+    bitReversePermute(real, imag, N);
+    fftRadix2(real, imag, N, -1);
+
+    // Scale by 1/N
+    const scale = 1.0 / N;
+    for (let i = 0; i < N; i++) {
+        real[i] *= scale;
+        imag[i] *= scale;
+    }
+}
+
+// DCT-II via FFT using reordering method (matches C++ dct_fft)
+// Reference: Numerical Recipes Chapter 12.3
+function javascript_dct_fft(input, N) {
+    const PI = Math.PI;
+
+    // Allocate arrays for N-point FFT
+    const real = new Float32Array(N);
+    const imag = new Float32Array(N);
+
+    // Reorder input: even indices first, then odd indices reversed
+    // [x[0], x[2], x[4], ...] followed by [x[N-1], x[N-3], x[N-5], ...]
+    for (let i = 0; i < N / 2; i++) {
+        real[i] = input[2 * i];           // Even indices: 0, 2, 4, ...
+        real[N - 1 - i] = input[2 * i + 1]; // Odd indices reversed: N-1, N-3, ...
+    }
+    // imag is already zeros (Float32Array default)
+
+    // Apply N-point FFT
+    fftForward(real, imag, N);
+
+    // Extract DCT coefficients with phase correction
+    // DCT[k] = Re{FFT[k] * exp(-j*pi*k/(2*N))} * normalization
+    const output = new Float32Array(N);
+    for (let k = 0; k < N; k++) {
+        const angle = -PI * k / (2.0 * N);
+        const wr = Math.cos(angle);
+        const wi = Math.sin(angle);
+
+        // Complex multiplication: (real[k] + j*imag[k]) * (wr + j*wi)
+        // Real part: real*wr - imag*wi
+        const dct_value = real[k] * wr - imag[k] * wi;
+
+        // Apply DCT-II normalization
+        if (k === 0) {
+            output[k] = dct_value * Math.sqrt(1.0 / N);
+        } else {
+            output[k] = dct_value * Math.sqrt(2.0 / N);
+        }
+    }
+
+    return output;
+}
+
+// IDCT (DCT-III) via FFT using reordering method (matches C++ idct_fft)
+// Reference: Numerical Recipes Chapter 12.3
+function javascript_idct_fft(input, N) {
+    const PI = Math.PI;
+
+    // Allocate arrays for N-point FFT
+    const real = new Float32Array(N);
+    const imag = new Float32Array(N);
+
+    // Prepare FFT input with inverse phase correction
+    // FFT[k] = DCT[k] * exp(+j*pi*k/(2*N)) / normalization
+    // Note: DCT-III needs factor of 2 for AC terms
+    for (let k = 0; k < N; k++) {
+        const angle = PI * k / (2.0 * N);  // Positive angle for inverse
+        const wr = Math.cos(angle);
+        const wi = Math.sin(angle);
+
+        // Inverse of DCT-II normalization with correct DCT-III scaling
+        let scaled;
+        if (k === 0) {
+            scaled = input[k] / Math.sqrt(1.0 / N);
+        } else {
+            // DCT-III needs factor of 2 for AC terms
+            scaled = input[k] / Math.sqrt(2.0 / N) * 2.0;
+        }
+
+        // Complex multiplication: scaled * (wr + j*wi)
+        real[k] = scaled * wr;
+        imag[k] = scaled * wi;
+    }
+
+    // Apply inverse FFT
+    fftInverse(real, imag, N);
+
+    // Unpack: reverse the reordering from DCT
+    // Even output indices come from first half of FFT output
+    // Odd output indices come from second half (reversed)
+    const output = new Float32Array(N);
+    for (let i = 0; i < N / 2; i++) {
+        output[2 * i] = real[i];              // Even positions
+        output[2 * i + 1] = real[N - 1 - i];  // Odd positions (reversed)
+    }
+
+    return output;
+}
+
+// Convenience wrappers for dctSize = 512 (backward compatible)
+function javascript_dct_512_fft(input) {
+    return javascript_dct_fft(input, dctSize);
+}
+
+function javascript_idct_512_fft(input) {
+    return javascript_idct_fft(input, dctSize);
+}
+