diff --git a/libavcodec/mpeg4video.h b/libavcodec/mpeg4video.h
index fdb16be07d..60857cce11 100644
--- a/libavcodec/mpeg4video.h
+++ b/libavcodec/mpeg4video.h
@@ -70,6 +70,8 @@ typedef struct Mpeg4DecContext {
     int rvlc;
     ///< could this stream contain resync markers
     int resync_marker;
+    ///< time distance of first I -> B, used for interlaced b frames
+    int t_frame;
 
     /* bug workarounds */
     int divx_version;
diff --git a/libavcodec/mpeg4videodec.c b/libavcodec/mpeg4videodec.c
index 95abad0ccd..4d045e263b 100644
--- a/libavcodec/mpeg4videodec.c
+++ b/libavcodec/mpeg4videodec.c
@@ -1736,7 +1736,7 @@ static int decode_vol_header(Mpeg4DecContext *ctx, GetBitContext *gb)
     else
         s->avctx->time_base.num = 1;
 
-    s->t_frame = 0;
+    ctx->t_frame = 0;
 
     if (ctx->shape != BIN_ONLY_SHAPE) {
         if (ctx->shape == RECT_SHAPE) {
@@ -2123,14 +2123,14 @@ static int decode_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb)
         }
         ff_mpeg4_init_direct_mv(s);
 
-        if (s->t_frame == 0)
-            s->t_frame = s->pb_time;
-        if (s->t_frame == 0)
-            s->t_frame = 1;  // 1/0 protection
-        s->pp_field_time = (ROUNDED_DIV(s->last_non_b_time, s->t_frame) -
-                            ROUNDED_DIV(s->last_non_b_time - s->pp_time, s->t_frame)) * 2;
-        s->pb_field_time = (ROUNDED_DIV(s->time, s->t_frame) -
-                            ROUNDED_DIV(s->last_non_b_time - s->pp_time, s->t_frame)) * 2;
+        if (ctx->t_frame == 0)
+            ctx->t_frame = s->pb_time;
+        if (ctx->t_frame == 0)
+            ctx->t_frame = 1;  // 1/0 protection
+        s->pp_field_time = (ROUNDED_DIV(s->last_non_b_time, ctx->t_frame) -
+                            ROUNDED_DIV(s->last_non_b_time - s->pp_time, ctx->t_frame)) * 2;
+        s->pb_field_time = (ROUNDED_DIV(s->time, ctx->t_frame) -
+                            ROUNDED_DIV(s->last_non_b_time - s->pp_time, ctx->t_frame)) * 2;
         if (!s->progressive_sequence) {
             if (s->pp_field_time <= s->pb_field_time || s->pb_field_time <= 1)
                 return FRAME_SKIPPED;
diff --git a/libavcodec/mpegvideo.h b/libavcodec/mpegvideo.h
index 5620791682..30a726f53d 100644
--- a/libavcodec/mpegvideo.h
+++ b/libavcodec/mpegvideo.h
@@ -607,7 +607,6 @@ typedef struct MpegEncContext {
     PutBitContext tex_pb;            ///< used for data partitioned VOPs
     PutBitContext pb2;               ///< used for data partitioned VOPs
     int mpeg_quant;
-    int t_frame;                       ///< time distance of first I -> B, used for interlaced b frames
     int padding_bug_score;             ///< used to detect the VERY common padding bug in MPEG4
 
     /* divx specific, used to workaround (many) bugs in divx5 */