DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

2:1 downsampling
Color video originates with R’G’B’ components.
Transcoding to Y’CBCR is necessary if signals are to be
used in the studio. The conversion involves matrixing
(to Y’CBCR in 4:4:4 form), then chroma subsampling to
4:2:2. Chroma subsampling requires a 2:1 downsampler.
If this downsampling is attempted by simply dropping
alternate samples, any signal content between the
original 0.25 fS and 0.5 fS will cause aliasing in the
result. Rejection of signal content at and above 0.25 fS is required. The required filter is usually implemented as
an FIR lowpass filter having its corner frequency somewhat
less than one-quarter of the (original) sampling
frequency. After filtering, alternate result samples can
be dropped. There is no need to calculate values that
will subsequently be discarded, however! Efficient
chroma subsamplers take advantage of that fact, interleaving
the CB and CR components into a single filter.
In Figure 16.12, on page 153, I presented a very simple
lowpass filter that simply averages two adjacent
samples. That filter has a corner frequency of 0.25f S.
However, it makes a slow transition from passband to
stopband, and it has very poor attenuation in the stopband
(above 0.25f S). It makes a poor resampling filter.
More than two taps are required to give adequate
performance in studio video subsampling.
In 4:2:2 video, chroma is cosited: Each chroma sample
must be located at the site of a luma sample. A symmetrical
filter having an even number of (nonzero) taps
does not have this property. A downsampling filter for
cosited chroma must have an odd number of taps.
Oversampling
I have explained the importance of prefiltering prior to
A-to-D conversion, and of postfiltering following
D-to-A conversion. Historically, these filters were
implemented in the analog domain, using inductors and
capacitors. In discrete form, these components are
bulky and expensive. It is extremely difficult to incorporate
inductive and capacitive elements with suitable
values and precision onto integrated circuits. However,
A-to-D and D-to-A converters are operating at higher
174 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES