FOCUS Enhancements FS453 - Manual (Page 10)

FS453/4 AND FS455/6 DATA SHEET: HARDWARE REFERENCE
3.2 Flicker Reduction
Computer images are displayed progressively. That is, for a given frame of video, each line of video is
scanned onto the monitor sequentially. SDTV images, however, are interlaced. Each SDTV frame of
video is broken into two fields (one composed of odd lines and the other of even lines). First the odd
lines are scanned onto the TV, and then the even lines are scanned onto the TV.
The energy decay rate of the phosphors on a TV screen is fast enough that the older field of video will
appear somewhat dimmer than the newer field of video. As the fields are constantly changing, this can
result in a visible flicker between the two fields of data on the TV screen. This flicker is especially visible
when one field contains a long dark line, while an adjacent line (in the other field) contains a long white
line. The higher energy line will decay in brightness much faster than the low energy line, and in turn will
appear to flicker heavily.
Most scan converters simply average the pixel data between lines. This removes the Black-or-White
relationships between lines that viewers recognize as video flicker. The problem with this solution is that
data becomes blurred. Single black or white lines are reduced to grays. Detailed areas of video (such as
the gap in the letter ‘e’) lose their distinction.
3.2.1 Flicker Filter Challenges
The goal is to completely remove flicker from the image without blurring detailed video. To preserve the
video details, the flicker filter should have a flat frequency response (+/- 1dB) between pixels in the
horizontal, and diagonal directions. It must also avoid introducing new artifacts into the digital video
stream. Artifacts include repeating pixels, losing pixels; and creating colors that are not interpolations of
original pixel colors.
3.2.2 FS453 Solution
The FS453 uses a patented flicker filter that calculates output pixel values as a function of both vertical
(line averaging) and horizontal (pixel averaging) pixel relationships. In effect the FS453 can decide
where and how to reduce flicker within the image.
Figure 3 (below) shows a normalized plot of the frequency response of pixels along diagonal after being
processed by the FS453's flicker filter. The response is flat for the majority of the frequency space. This
maintains pixel sharpness while providing excellent flicker suppression.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
1
0.75
0.5
0.25
0
0.25
0.5
0.75
1
Horizontal Direction
Diagonal at 27 degrees
Diagonal at 45 degrees
Normalized Frequency
Ga
in
in
De
ci
b
els
Figure 3: FS453 Flicker Filter Diagonal Response
JANUARY, 2005, VERSION 3.0 10 COPYRIGHT ©2003-4 FOCUS ENHANCEMENTS, INC.
FOCUS Enhancements Semiconductor