GXF time base is always based on "fields" per second even for

non-interlaced video.
Should fix issue 1766.

Originally committed as revision 22256 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/gxf.c

@@ -313,7 +313,7 @@ static int gxf_header(AVFormatContext *s, AVFormatParameters *ap) {
         st = s->streams[idx];
         if (!main_timebase.num || !main_timebase.den) {
             main_timebase.num = si.frames_per_second.den;
-            main_timebase.den = si.frames_per_second.num * si.fields_per_frame;
+            main_timebase.den = si.frames_per_second.num * 2;
         }
         st->start_time = si.first_field;
         if (si.first_field != AV_NOPTS_VALUE && si.last_field != AV_NOPTS_VALUE)
@@ -344,7 +344,7 @@ static int gxf_header(AVFormatContext *s, AVFormatParameters *ap) {
             if (!main_timebase.num || !main_timebase.den) {
                 // this may not always be correct, but simply the best we can get
                 main_timebase.num = fps.den;
-                main_timebase.den = fps.num;
+                main_timebase.den = fps.num * 2;
             }
         } else
             av_log(s, AV_LOG_INFO, "UMF packet too short\n");