diff -Naur mythtv-0.21-20088-old/libs/libmythtv/videoout_xv.cpp mythtv-0.21-20088-new/libs/libmythtv/videoout_xv.cpp
--- mythtv-0.21-20088-old/libs/libmythtv/videoout_xv.cpp	2009-03-22 14:50:29.000000000 +0000
+++ mythtv-0.21-20088-new/libs/libmythtv/videoout_xv.cpp	2009-03-22 18:47:17.000000000 +0000
@@ -72,6 +72,10 @@
 #define round(x) ((int) ((x) + 0.5))
 #endif
 
+#ifndef V_INTERLACE
+#define V_INTERLACE (0x010)
+#endif
+
 #ifndef XVMC_CHROMA_FORMAT_420
 #define XVMC_CHROMA_FORMAT_420 0x00000001
 #endif
@@ -395,6 +399,19 @@
 
     rate = (dot_clock * 1000.0) / rate;
 
+    // For an interlaced display mode, we want the refresh rate to be the field
+    // rate, so that we can use an x2 deinterlacer. It may be the case that some
+    // drivers (possibly openchrome) use false mode lines, with a doubled clock
+    // rate, so as to make the frame rate calculation produce the field rate.
+    // We check for 25 or 30 to avoid twice doubling in that case.
+    if ((mode_line.flags & V_INTERLACE) != 0
+        && rate == 25 || rate == 30)
+    {
+        VERBOSE(VB_GENERAL, "Doubling refresh rate for interlaced display mode");
+
+        rate *= 2;
+    }
+
     // Assume 60Hz if rate isn't good:
     if (rate < 20 || rate > 200)
     {