The Engineer's Guide to Standards Conversion - Snell

to remove from the lower luminance frequencies, and thus there is no need to
continue the comb filter response in that region.
The simple filter of Fig 2.9.1 has a comb response from DC upwards. The
vertical resolution loss of such a filter can be largely restored by running the comb
filter only in a passband centred around subcarrier. Within the passband, combing
is used to remove luminance from the chroma. This chroma is then subtracted from
the composite input signal to leave luminance. Below the passband the entire input
spectrum is passed as luminance and the vertical resolution loss is restored. The line
comb gives quite good results in NTSC, as horizontal and vertical resolution are
good, but the loss of vertical resolution at high frequency means that diagonal
resolution is poor. A line comb filter is at a disadvantage in PAL because of the
spreading between U and V components. What is needed is a comb filter having
delays of two lines, but this will have an even more severe effect on diagonal
frequencies, so PAL comb filters are often found with only single line delays, a
choice influenced by commonality with an NTSC product. Although the three
dimensional spectrum of PAL is complicated, it is possible to combine elements of
both vertical and temporal types of filter to obtain a spatio-temporal response
which is closely matched to the characteristics of PAL.
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