Scan Doubling Primer, Line Doubling Techniques

SCAN DOUBLING PRIMER

Garden variety composite video or S-Video images are interlaced, meaning that the picture information from each video frame is split into two fields. Field one contains all of the odd horizontal lines and Field two contains all of the even horizontal lines. Each field is displayed for only 1/60 of a second, so the human eye actually merges all the odd and even lines from the two fields into a single image which is refreshed completely 30 times a second (timings are for NTSC). Unfortunately, this merging process is not perfect and we can easily perceive the video scan lines, especially on large screen displays.

Scan doublers increase the number of scan lines resulting in a new type of video image, IDTV which stands for Improved Definition Television. The images are no longer in their original NTSC, PAL, or SECAM format since they have been decoded to an RGB format, the number of scan lines has been doubled, and the horizontal scan rate has been doubled. Scan doublers output a non-interlaced image, and employ progressive scanning, where the image is completely drawn in a single pass from the top to the bottom of the screen before moving to the next frame. Scan doubled images are virtually free from scan lines and have a solid, film-like appearance. Various methods are employed by scan doublers to achieve a non-interlaced, scan doubled image and two of these techniques are discussed in the following section.

Scan doublers double the horizontal scan rate, but the vertical refresh rate is not changed. If the input video signal is 50 Hz the output signal will still be 50 Hz.

LINE DOUBLING TECHNIQUES

The IN1222 and IN1422 will allow users to select and store a different line doubling technique for each input as required by the type of input source. The IN1222 / IN1422 offer three choices: Line Doubling, Frame Mode, and Disable Doubling.

Line Doubling - is the default mode for INLINE scan doublers and is best for displaying motion video sources with minimal motion artifacts. This method stores a single field and displays each line within the field twice (the first and last lines of the frame are only displayed once). Looking from top to bottom on the display would be:

Field One - Line 1, Line 3, Line 3, Line 5, Line 5, etc. for the first 1/60th of a second and Field Two - Line 2, Line 2, Line 4, Line 4, Line 6, Line 6, etc. for the second 1/60 of a second.

Frame Mode - is the preferred mode for still video, offering the greatest image detail on input sources such as document cameras and slide-to-video converters. While frame mode offers the best detail for still sources, it should not be used with motion video sources, as extreme motion artifacts will occur. In Frame Mode (also known as Field Overlay) the scan doubler stores both fields of one frame in memory and then displays both fields on the screen at the same time. The display would appear from top to bottom as follows:

Line 1, Line 2, Line 3, Line 4, Line 5, Line 6, etc. for 1/60th of a second.

The same image is actually displayed twice, for a total time of 1/30th of a second.

Disable Doubling - Low quality video images with excessive noise or poor detail may not benefit from scan doubling, therefore the IN1222 and IN1422 provide a way to disable the scan doubling on selected inputs. The output video is still decoded from composite video or S-Video to RGBHV (or other selected output format), but the signal remains at its original horizontal scan rate and is still interlaced.

IIN1222 / IN1422 / IN1510 / IN1710 OPERATIONS MANUAL - REV. 2 12/04/99