Doom9's Forum - View Single Post - Can CCE encode top field first interlaced video?

SomeJoe · 23rd January 2003, 23:18

How can any program actually swap the display order of fields and get a correct result? All fields are both spatially and temporally different.

For instance, say I capture and encode an NTSC DV source. DV is lower/bottom field first, temporally. (Meaning that the bottom field was actually recorded 1/60th of a second before the top field).

The correct way to encode this is to store the upper and lower fields in the MPEG stream, and clear the top field first flag for that frame. Thus the MPEG-2 playback device will decode the frame, see that TFF=0, and should then first convert the bottom field to analog and display it in the bottom field of the display device. Following this, it would convert the top field to analog and display it in the top field of the display device.

CCE seems to assume that reversing the stored order of the field data and then setting the TFF flag to 1 is equivalent. But how can this be?

This would mean that the MPEG-2 playback device would decode the frame, see that TFF=1, and would then convert the top field data to analog and display it in the top field of the display device. This is temporally correct, as the now-swapped data in the top field of the MPEG-2 frame was indeed captured first, but this data was originally captured out of the bottom field, and is now being displayed in the top field of the display. The two fields will be displayed spatially incorrectly, as if every two lines of the frame were swapped top for bottom.

The crux of the matter is that the MPEG-2 to analog conversion process has no facility for flagging whether the data from the first encoded field of a frame is supposed to be displayed in the top field or the bottom field of the display device. The analog conversion must simply assume that the first encoded field of the frame is supposed to be displayed in the top field, and the second in the bottom. The TFF flag can control the temporal order, but not the spatial order.

Please correct me here if there is something I have missed ... technical data on the MPEG-2 playback process is somewhat slim.

23rd January 2003, 23:18	#9 \| Link
SomeJoe Registered User Join Date: Jan 2003 Posts: 315	How can any program actually swap the display order of fields and get a correct result? All fields are both spatially and temporally different. For instance, say I capture and encode an NTSC DV source. DV is lower/bottom field first, temporally. (Meaning that the bottom field was actually recorded 1/60th of a second before the top field). The correct way to encode this is to store the upper and lower fields in the MPEG stream, and clear the top field first flag for that frame. Thus the MPEG-2 playback device will decode the frame, see that TFF=0, and should then first convert the bottom field to analog and display it in the bottom field of the display device. Following this, it would convert the top field to analog and display it in the top field of the display device. CCE seems to assume that reversing the stored order of the field data and then setting the TFF flag to 1 is equivalent. But how can this be? This would mean that the MPEG-2 playback device would decode the frame, see that TFF=1, and would then convert the top field data to analog and display it in the top field of the display device. This is temporally correct, as the now-swapped data in the top field of the MPEG-2 frame was indeed captured first, but this data was originally captured out of the bottom field, and is now being displayed in the top field of the display. The two fields will be displayed spatially incorrectly, as if every two lines of the frame were swapped top for bottom. The crux of the matter is that the MPEG-2 to analog conversion process has no facility for flagging whether the data from the first encoded field of a frame is supposed to be displayed in the top field or the bottom field of the display device. The analog conversion must simply assume that the first encoded field of the frame is supposed to be displayed in the top field, and the second in the bottom. The TFF flag can control the temporal order, but not the spatial order. Please correct me here if there is something I have missed ... technical data on the MPEG-2 playback process is somewhat slim. __________________ - SomeJoe