diff --git a/docs/module-internals.md b/docs/module-internals.md
index b640e00..9c401e7 100644
--- a/docs/module-internals.md
+++ b/docs/module-internals.md
@@ -171,6 +171,10 @@ failure regressions cover bounded mux diagnostics and atomic cleanup.
 and preserves the supported properties that survive the 8-bit BGR boundary:
 `yuv420p`/`yuv422p`/`yuv444p` chroma sampling, recognized color tags, encoder
 time base, MP4/MOV track timescale, source pixel format, and component depth.
+Both raw-CFR and timestamped-NUT inputs use ffmpeg's passthrough FPS mode. This
+keeps one encoded frame per supplied frame when an older ffmpeg receives a
+fine-grained source encoder time base such as `1/90000`; implicit synchronization
+can otherwise synthesize thousands of duplicate frames between CFR timestamps.
 HDR transfer functions and component depths above 8 bits are rejected before
 encoding so the OpenCV boundary cannot silently reduce them to SDR 8-bit.
 `probe_video_timestamps` reads authoritative per-frame display PTS through
diff --git a/src/remove_ai_watermarks/video_encoding.py b/src/remove_ai_watermarks/video_encoding.py
index 36ce507..e89da14 100644
--- a/src/remove_ai_watermarks/video_encoding.py
+++ b/src/remove_ai_watermarks/video_encoding.py
@@ -378,8 +378,7 @@ def raw_video_command(
         "-map_chapters",
         "-1",
     ]
-    if timestamped_input:
-        command.extend(["-fps_mode", "passthrough"])
+    command.extend(["-fps_mode", "passthrough"])
     if copy_input_timestamps:
         command.extend(["-avoid_negative_ts", "disabled"])
     if output.suffix.lower() in {".mp4", ".mov", ".m4v"} and profile.time_base is not None:
diff --git a/tests/test_video_invisible.py b/tests/test_video_invisible.py
index abc030d..60e520a 100644
--- a/tests/test_video_invisible.py
+++ b/tests/test_video_invisible.py
@@ -127,6 +127,7 @@ def test_encoder_and_mux_commands_separate_streaming_frames_from_source_audio(
     assert "pipe:0" in command
     assert str(source) not in command
     assert command[command.index("-map") + 1] == "0:v:0"
+    assert command[command.index("-fps_mode") + 1] == "passthrough"
     assert "-shortest" not in command
 
     calls: list[list[str]] = []