diff --git a/src/i830_sdvo.c b/src/i830_sdvo.c
index d9b76d4..b30ae1c 100644
--- a/src/i830_sdvo.c
+++ b/src/i830_sdvo.c
@@ -122,10 +122,10 @@ static void i830_sdvo_write_sdvox(xf86OutputPtr output, uint32_t val)
     uint32_t		    bval = val, cval = val;
     int			    i;
 
-    if (dev_priv->output_device == SDVOB)
-	cval = INREG(SDVOC);
+    if (dev_priv->output_device == SDVOC)
+	cval |= INREG(SDVOC);
     else
-	bval = INREG(SDVOB);
+	bval |= INREG(SDVOB);
     
     /*
      * Write the registers twice for luck. Sometimes,