fix: add lowpass filter at 3000Hz before agate in audio extraction

extract.py

@@ -323,7 +323,7 @@ def extract_audio_loudnorm(input_file: str, duration: int, start_sec: float,
     map_args = ["-map", f"0:a:{stream_idx}"] if stream_idx is not None else []
     base = ["ffmpeg", "-y", *ss_args, "-i", input_file, "-t", str(duration), "-vn", *map_args]
     measure_af = f"loudnorm=I={TARGET_LUFS}:TP={TARGET_TP}:LRA={TARGET_LRA}:print_format=json"
-    gate_filter = ",agate=threshold=0.02:ratio=20:attack=5:release=50" if loud_gate else ""
+    gate_filter = ",lowpass=f=3000,agate=threshold=0.02:ratio=20:attack=5:release=50" if loud_gate else ""
 
     # ─── Pass 1: measure ───
     print(f"[loudness] {stem}: pass 1/2 measuring…")
@@ -332,7 +332,7 @@ def extract_audio_loudnorm(input_file: str, duration: int, start_sec: float,
     if r.returncode != 0:
         print(f"[loudness] {stem}: pass 1 failed ({r.stderr.strip()[-80:]}), falling back to simple extract", file=sys.stderr)
         if loud_gate:
-            cmd_fallback = base + ["-af", f"agate=threshold=0.02:ratio=20:attack=5:release=50", *AUDIO_CODEC, out_path]
+            cmd_fallback = base + ["-af", "lowpass=f=3000,agate=threshold=0.02:ratio=20:attack=5:release=50", *AUDIO_CODEC, out_path]
         else:
             cmd_fallback = base + [*AUDIO_CODEC, out_path]
         r2 = subprocess.run(cmd_fallback, capture_output=True, text=True)
@@ -344,7 +344,7 @@ def extract_audio_loudnorm(input_file: str, duration: int, start_sec: float,
     if measured is None:
         print(f"[loudness] {stem}: could not parse measurement, falling back to simple extract", file=sys.stderr)
         if loud_gate:
-            cmd_fallback = base + ["-af", f"agate=threshold=0.02:ratio=20:attack=5:release=50", *AUDIO_CODEC, out_path]
+            cmd_fallback = base + ["-af", "lowpass=f=3000,agate=threshold=0.02:ratio=20:attack=5:release=50", *AUDIO_CODEC, out_path]
         else:
             cmd_fallback = base + [*AUDIO_CODEC, out_path]
         r2 = subprocess.run(cmd_fallback, capture_output=True, text=True)
@@ -495,7 +495,7 @@ for _file_idx, INPUT_FILE in enumerate(INPUT_FILES):
             cmd_audio = [
                 "ffmpeg", "-y", *ss_args, "-i", INPUT_FILE, "-t", str(DURATION),
                 "-vn", *map_args, "-af",
-                "agate=threshold=0.02:ratio=20:attack=5:release=50",
+                "lowpass=f=3000,agate=threshold=0.02:ratio=20:attack=5:release=50",
                 *AUDIO_CODEC, audio_out
             ]
         else: