diff --git a/videocr/video.py b/videocr/video.py
index 8535b87..86d4826 100644
--- a/videocr/video.py
+++ b/videocr/video.py
@@ -14,32 +14,45 @@ class Video:
     use_fullframe: bool
     num_frames: int
     fps: float
-    ocr_frame_start: int
-    num_ocr_frames: int
     pred_frames: List[PredictedFrame]
     pred_subs: List[PredictedSubtitle]
 
-    def __init__(self, path: str, lang: str, use_fullframe=False,
-                 time_start='0:00', time_end=''):
+    def __init__(self, path: str):
         self.path = path
-        self.lang = lang
-        self.use_fullframe = use_fullframe
         v = cv2.VideoCapture(path)
         self.num_frames = int(v.get(cv2.CAP_PROP_FRAME_COUNT))
         self.fps = v.get(cv2.CAP_PROP_FPS)
         v.release()
 
-        self.ocr_frame_start = self._frame_index(time_start)
+    def run_ocr(self, lang: str, use_fullframe=False,
+                time_start='0:00', time_end='') -> None:
+        self.lang = lang
+        self.use_fullframe = use_fullframe
+
+        ocr_start = self._frame_index(time_start)
 
         if time_end:
             ocr_end = self._frame_index(time_end)
+            if ocr_end < ocr_start:
+                raise ValueError('time_start is later than time_end')
         else:
             ocr_end = self.num_frames
-        self.num_ocr_frames = ocr_end - self.ocr_frame_start
+        num_ocr_frames = ocr_end - ocr_start
 
-        if self.num_ocr_frames < 0:
-            raise ValueError('time_start is later than time_end')
+        # get frames from ocr_start to ocr_end
+        v = cv2.VideoCapture(self.path)
+        v.set(cv2.CAP_PROP_POS_FRAMES, ocr_start)
+        frames = (v.read()[1] for _ in range(num_ocr_frames))
 
+        # perform ocr to frames in parallel
+        with futures.ProcessPoolExecutor() as pool:
+            ocr_map = pool.map(self._single_frame_ocr, frames, chunksize=10)
+            self.pred_frames = [PredictedFrame(i + ocr_start, data) 
+                                for i, data in enumerate(ocr_map)]
+
+        v.release()
+
+    # convert time str to frame index
     def _frame_index(self, time: str) -> int:
         t = time.split(':')
         t = list(map(int, t))
@@ -58,19 +71,6 @@ class Video:
 
         return index
 
-    def run_ocr(self) -> None:
-        v = cv2.VideoCapture(self.path)
-        v.set(cv2.CAP_PROP_POS_FRAMES, self.ocr_frame_start)
-        frames = (v.read()[1] for _ in range(self.num_ocr_frames))
-
-        # perform ocr to all frames in parallel
-        with futures.ProcessPoolExecutor() as pool:
-            ocr_map = pool.map(self._single_frame_ocr, frames, chunksize=10)
-            self.pred_frames = [PredictedFrame(i + self.ocr_frame_start, data) 
-                                for i, data in enumerate(ocr_map)]
-
-        v.release()
-
     def _single_frame_ocr(self, img) -> str:
         if not self.use_fullframe:
             # only use bottom half of the frame by default
@@ -99,7 +99,7 @@ class Video:
         bound = WIN_BOUND
         i = 0
         j = 1
-        while j < self.num_ocr_frames:
+        while j < len(self.pred_frames):
             fi, fj = self.pred_frames[i], self.pred_frames[j]
 
             if fi.is_similar_to(fj):
@@ -118,7 +118,7 @@ class Video:
             j += 1
 
         # also handle the last remaining frames
-        if i < self.num_ocr_frames - 1:
+        if i < len(self.pred_frames) - 1:
             self._append_sub(PredictedSubtitle(self.pred_frames[i:]))
 
     def _append_sub(self, sub: PredictedSubtitle) -> None: