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Analog video decoder revisited: Brush up on comb filters

15 Aug 2012  | Daniel Ogilvie

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As mentioned, one of the issues with these artifacts is that they are not static and cannot be removed easily with further post processing such as noise reduction; because the artifacts are moving they interfere with the motion adaptation of the television de-interlacer.

A similar issue arises if the output of the video decoder is to be compressed as all MPEG compression methods effectively send only the motion of an image. Unable to discriminate between artifacts, video source noise and 'real' image motion it can be shown that up 20% of satellite and cable digital broadcast bandwidth is utilized to send unnecessary information. This is extremely useful bandwidth that is especially useful given the high compression ratios used by today's broadcasters and is the difference between the viewer seeing the highly visible MPEG artifacts such as blocking, or not.

One large improvement to the video decoder that has been made by some manufacturers is to add a 3D comb filter. The filter utilizes the same phase relationship in the subcarrier but a pixel accurate frame delay ensures the pixels are exactly spatially aligned. Even on the most complex images near perfect, artifact free decoding is the result: (I say near perfect because frame combs are very sensitive to clock jitter and clock jitter as little as 1ns over the frame delay period can result is residual subcarrier. PAL also has an addition subcarrier offset which means perfect cancellation cannot occur even with a frame comb).

Figure 2 shows the artifact free image achievable with a well designed frame comb.

Figure 2: The left image shows a zone plate with a line comb filter. The colors in the image are caused by 'failures' in the line comb. The right image shows the 'clean' image from a frame comb filter.

The zone plate image in figure 2 may seem rather esoteric, but you may be more familiar with the shimmering colors created by a newsreader's check shirt, something they seem to have a peculiar penchant for wearing.

But of course, the frame comb does not answer all our problems. We are now looking across comb filter taps of one frame (for NTSC) and 2 frames for PAL, a delay of 80ms in the latter case. Whereas any difference spatially across the 2 or 4 lines caused the line comb filter to fail, now any difference temporally across 33ms or 80ms will cause the frame comb to fail, with similar artifacts being created. The only recourse open to most video decoders is to choose the line comb, or if that is also failing to go to the notch filter mode, a clean but low bandwidth fall back.

For PAL in particular it can be shown that for a large amount of video material the frame comb cannot be chosen because of image motion; it becomes an expensive luxury good only for those static demonstration zone plate images. However we have yet one more, under-utilized comb option and that is the field comb. The field comb has an aperture of 262 lines for NTSC and 312 lines for PAL and is therefore much closer spatially than the line comb and closer temporally than the frame comb (especially for PAL).

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