sandflow · palemieux · Dec 17, 2025 · Dec 13, 2025 · Dec 13, 2025 · Dec 13, 2025
diff --git a/README.md b/README.md
@@ -275,6 +275,15 @@ If `frame_rate` is:
 
 Default: `"2997DF"`
 
+#### start_tc
+
+`"start_tc" : null | "HH:MM:SS:FF" | "HH;MM;SS;FF"`
+
+If not `null`, specifies the starting timecode for the SCC file. The timecode
+must be consistent with the value of the `frame_rate` parameter.
+
+Default: `null`
+
 ### LCD filter configuration (`"lcd"`)
 
 #### Description

diff --git a/src/main/python/ttconv/scc/config.py b/src/main/python/ttconv/scc/config.py
@@ -34,6 +34,7 @@
 
 from ttconv.config import ModuleConfiguration
 from ttconv.style_properties import TextAlignType
+from ttconv.time_code import SmpteTimeCode
 
 
 class TextAlignment(Enum):
@@ -130,6 +131,11 @@ class SccWriterConfiguration(ModuleConfiguration):
     metadata={"decoder": SCCFrameRate.from_value}
   )
 
+  start_tc: typing.Optional[str] = field(
+    default=None,
+    metadata={"decoder": lambda y: str(y) if y is not None else None}
+  )
+
   @classmethod
   def name(cls):
     return "scc_writer"
diff --git a/src/main/python/ttconv/scc/writer.py b/src/main/python/ttconv/scc/writer.py
@@ -213,10 +213,10 @@ def insert(self, other: _Chunk):
   def __len__(self):
     return len(self._octet_buffer)
 
-  def to_string(self, fps : Fraction, is_df: bool):
+  def to_string(self, fps : Fraction, is_df: bool, start_offset: int) -> str:
     if fps not in (FPS_29_97, FPS_30):
       raise ValueError(f"Frame rate {fps} out-of-range")
-    
+
     if fps != FPS_29_97 and is_df:
       raise ValueError("Frame rate must be fractional if drop frame is true")
 
@@ -229,7 +229,7 @@ def _octet2hex(octet):
     if len(self._octet_buffer) % 2 == 1:
       packets.append(_octet2hex(self._octet_buffer[-1]) + _octet2hex(0))
 
-    return str(SmpteTimeCode.from_frames(self.get_begin(), fps, is_df)) + "\t" + " ".join(packets)
+    return str(SmpteTimeCode.from_frames(self.get_begin() + start_offset, fps, is_df)) + "\t" + " ".join(packets)
 
 MAX_LINEWIDTH = 32
 
@@ -409,4 +409,15 @@ def _isd_progress(progress: float):
         edm_chunk.set_begin(int(caption.get_end() * config.frame_rate.fps))
         chunks.append(edm_chunk)
 
-  return "Scenarist_SCC V1.0\n\n" + "\n\n".join(map(lambda e: e.to_string(config.frame_rate.fps, config.frame_rate.df), chunks))
+  start_offset = 0
+  if config.start_tc is not None:
+    start_tc = SmpteTimeCode.parse(config.start_tc, config.frame_rate.fps)
+    if start_tc.is_drop_frame() != config.frame_rate.df:
+      raise RuntimeError("The drop-frame status of the specified start_timecode does not match the drop-frame status of the specified frame_rate")
+    start_offset = start_tc.to_frames()
+
+  for chunk in chunks:
+    if start_offset + chunk.get_begin() < 0:
+      raise RuntimeError("The SCC stream would start earlier than the specified start timecode")
+
+  return "Scenarist_SCC V1.0\n\n" + "\n\n".join(map(lambda e: e.to_string(config.frame_rate.fps, config.frame_rate.df, start_offset), chunks))
diff --git a/src/main/python/ttconv/time_code.py b/src/main/python/ttconv/time_code.py
@@ -254,6 +254,9 @@ def from_frames(nb_frames: Union[int, Fraction], frame_rate: Fraction, is_df: Op
     if frame_rate is None:
       raise ValueError("Cannot compute time code from frames without frame rate")
 
+    if nb_frames is None or nb_frames < 0:
+      raise ValueError("Number of frames must not None and must not be less than zero")
+
     if is_df is None:
        drop_frame: bool = frame_rate.denominator == 1001
     else:

diff --git a/src/test/python/test_scc_writer.py b/src/test/python/test_scc_writer.py
@@ -266,6 +266,39 @@ def test_basic_30FPS(self):
     scc_from_model = scc_writer.from_model(model, config)
     self.assertEqual(scc_from_model, expected_scc)
 
+  def test_zero_start(self):
+    ttml_doc_str = """<?xml version="1.0" encoding="UTF-8"?>
+<tt xml:lang="en" xmlns="http://www.w3.org/ns/ttml"
+    xmlns:ttp="http://www.w3.org/ns/ttml#parameter"
+    ttp:frameRate="30">
+  <body>
+    <div>
+      <p begin="0f" end="90f">Hello</p>
+     </div>
+  </body>
+</tt>"""
+
+    model = imsc_reader.to_model(et.ElementTree(et.fromstring(ttml_doc_str)))
+
+    with self.assertRaises(RuntimeError):
+      scc_writer.from_model(model)
+
+    scc_from_model = scc_writer.from_model(model, SccWriterConfiguration(start_tc="01:00:00;00"))
+    expected_scc="""Scenarist_SCC V1.0
+
+00:59:59;21	9420 9420 94ae 94ae 9440 9440 c8e5 ecec ef80 942f 942f
+
+01:00:02;29	942c 942c"""
+    self.assertEqual(scc_from_model, expected_scc)
+
+    scc_from_model = scc_writer.from_model(model, SccWriterConfiguration(frame_rate=SCCFrameRate.FPS_30_NDF, start_tc="01:00:00:00"))
+    expected_scc="""Scenarist_SCC V1.0
+
+00:59:59:21	9420 9420 94ae 94ae 9440 9440 c8e5 ecec ef80 942f 942f
+
+01:00:03:00	942c 942c"""
+    self.assertEqual(scc_from_model, expected_scc)
+
   def test_multi_regions(self):
     expected_scc="""Scenarist_SCC V1.0