FOCUS Enhancements FS453, FS454, FS455, FS456 3.2 Flicker Reduction

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.

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.

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

0.75

0.5

0.25

0.5

0.75

Horizontal Direction

Diagonal at 27 degrees

Diagonal at 45 degrees

Normalized Frequency

Gain in Decibels

Figure 3: FS453 Flicker Filter Diagonal Response

FOCUS Enhancements Semiconductor