dsp: support dynamic bs412 chunk duration updates

internal/dsp/bs412.go

@@ -72,6 +72,35 @@ func NewBS412Limiter(thresholdDBr, pilotLevel, rdsInjection, chunkDurationSec fl
 	}
 }
 
+// UpdateChunkDuration reconfigures the limiter for a new chunk size.
+// Call this from GenerateFrame when the actual chunk duration is known
+// (computed as samples/sampleRate) to avoid calibration errors if the
+// engine's chunk duration differs from the value passed to NewBS412Limiter.
+// Safe to call on every chunk; no-ops when duration has not changed.
+func (l *BS412Limiter) UpdateChunkDuration(chunkSec float64) {
+	if chunkSec <= 0 {
+		return
+	}
+	windowSec := 60.0
+	newBufLen := int(math.Ceil(windowSec / chunkSec))
+	if newBufLen < 10 {
+		newBufLen = 10
+	}
+	if newBufLen == len(l.powerBuf) {
+		return // no change
+	}
+	// Resize buffer — drop history to avoid stale power readings from the
+	// old window size distorting the rolling average.
+	l.powerBuf = make([]float64, newBufLen)
+	l.bufIdx = 0
+	l.bufFull = false
+	l.powerSum = 0
+	attackTC := 2.0 / chunkSec
+	releaseTC := 5.0 / chunkSec
+	l.attackCoeff = 1.0 - math.Exp(-1.0/attackTC)
+	l.releaseCoeff = 1.0 - math.Exp(-1.0/releaseTC)
+}
+
 // ProcessChunk measures the audio power of a chunk and returns the
 // gain factor to apply to the audio composite for BS.412 compliance.
 // Call once per chunk with the average audio power of that chunk.