diff --git a/src/defs.h b/src/defs.h
index 3874baa9..ffaf9c10 100644
--- a/src/defs.h
+++ b/src/defs.h
@@ -561,7 +561,8 @@ typedef struct {
 #define  VSYNC_NONINTERLACED_DEJITTER  4
 #define  VSYNC_BLANKING                5
 #define  VSYNC_POLARITY                6
-#define  NUM_VSYNC                     7
+#define  VSYNC_FORCE_INTERLACE         7
+#define  NUM_VSYNC                     8
 
 #define  VIDEO_PROGRESSIVE   0
 #define  VIDEO_INTERLACED    1
diff --git a/src/geometry.c b/src/geometry.c
index fcb0daa1..d5352389 100644
--- a/src/geometry.c
+++ b/src/geometry.c
@@ -33,7 +33,8 @@ static const char *vsync_names[] = {
    "Non Interlaced",
    "Flywheel",
    "Blanking",
-   "Polarity"
+   "Polarity",
+   "Force Interlaced"
 };
 
 static const char *setup_names[] = {
@@ -376,6 +377,10 @@ void geometry_get_fb_params(capture_info_t *capinfo) {
         capinfo->video_type = VIDEO_PROGRESSIVE;
     }
 
+    if (capinfo->vsync_type == VSYNC_FORCE_INTERLACE) {
+        capinfo->vsync_type = VSYNC_INTERLACED;
+    }
+
     capinfo->sizex2 = geometry->fb_sizex2;
     switch(geometry->fb_bpp) {
         case BPP_4:
diff --git a/src/rgb_to_hdmi.c b/src/rgb_to_hdmi.c
index 55aa3e83..d83d802c 100644
--- a/src/rgb_to_hdmi.c
+++ b/src/rgb_to_hdmi.c
@@ -1043,11 +1043,14 @@ int calibrate_sampling_clock(int profile_changed) {
 
    // Instead, calculate the number of lines per frame
    double lines_per_2_vsyncs_double = ((double) vsync_time_ns) / (((double) nlines_time_ns) / ((double) nlines));
-
    one_line_time_ns = nlines_time_ns / nlines;
 
-   // If number of lines is odd, then we must be interlaced
-   interlaced = ((int)(lines_per_2_vsyncs_double + 0.5)) % 2;
+   if (geometry_get_value(VSYNC_TYPE) ==  VSYNC_FORCE_INTERLACE) {
+       interlaced = 1;
+   } else {
+       // If number of lines is odd, then we must be interlaced
+       interlaced = ((int)(lines_per_2_vsyncs_double + 0.5)) % 2;
+   }
    one_vsync_time_ns = vsync_time_ns >> 1;
    lines_per_vsync = ((int) (lines_per_2_vsyncs_double + 0.5) >> 1);
    lines_per_2_vsyncs = (int) (lines_per_2_vsyncs_double + 0.5);