SingMai Electronics
PT9 User Manual Revision 0.3
Page 17 of 38
Figure 11 Zone plate image after frame comb decoding.
Unfortunately it is not always possible to ‘afford’ a frame comb decoder, but it is possible to
improve the decoded image by processing the encoded image. If the luma and chroma are comb
filtered prior to being summed to form composite video, we can remove the frequencies that
cause the problems for the line comb decoder. Ideally the comb filter architectures for the
encoder and decoder should be matched, which is the case for the SingMai PT9 encoder and the
PT4 line comb decoder.
The line comb decoded image using this technique is shown in Figure 12.
As you can see, the cross-colour is almost completely removed compared with the image in Figure
10, yet, because only the high frequency luma is combed, we still preserve as much picture detail
as possible compared with, for example, a simple notch filter. Of course this solution is not a
panacea as there is some loss of resolution, but the image detail would have been masked by the
cross-colour.
Figure 13 shows the comparative images using an NTSC zone plate.
For those instances where a line comb decoder is the only option, the PT9 can make substantial
improvements in the perceived video image.
