VP520S

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QCIF LINES

LUM

CHROM

NTSC LINES

EVEN

ODD

Fig 9 : Interpolating from QCIF to NTSC

used in each set since six line delays are provided. The

luminance filter conceptually contains 28 taps ( four sets of

seven coefficients with two sets used to produce each field ).

Similarly the chrominance filter consists of 56 taps arranged

as eight sets of seven coefficients with four sets needed for

each field. In order to provide data for the filters each lumi-

nance line is read twice, and each chrominance line is read

four times to produce each field.

NTSC VERTICAL FILTERING

One field of NTSC video consists of 240 chrominance and

luminance lines, which must be converted to 288 lines of CIF

luminance and 144 lines of CIF chrominance. The luminance

increase is mechanized by repeating the first line in every five

to produce six lines, which are then applied to the vertical

filters. A different set of coefficients is used for each line,

requiring a total of 30 to be stored within the device. The line

repeat causes one set of line data to be used twice, but each

time different coefficients are used by the filter. This technique

is equivalent to interpolating the data by six, and then decimat-

ing by five. The required coefficients for each of the six sets

can be derived by conceptually using this approach.

The line repeat requires an additional FIFO line delay

before the four delays used by the filters. By reducing the

horizontal blanking time it is possible to read six lines ( one is

repeated ) from the FIFO in the time taken to acquire five lines

of video with blanking.

Chrominance data also passes through the input FIFO

and one line in every five is repeated. This is done in order to

avoid differential delays with the luminance data. Three chromi-

nance lines are only needed, however, for every five original

lines. They are produced by using three sets of five coeffi-

cients and discarding two filtered lines in every five. The three

selected filter outputs are chosen such that the centre line of

the filter is closest to the CIF line number needed. The centre

lines which are actually used are shown in Figure 8, and result

in a sequence of two chosen outputs then a gap followed by

one output then a gap. Simply using every other output would

not give the best fit.

A simplified approach is used when decimating down to

QCIF resolution, and the input FIFO is not used. Six luminance

lines are derived from ten NTSC lines by choosing the six

outputs produced when the centre line in the filter is closest to

the QCIF line that is needed. Overall this results in a luminance

sequence consisting of two outputs then a gap, followed by

one output then a gap and is shown in Figure 8.

Three chrominance lines are derived from the same

inputs by using three sets of seven coefficients. The chromi-

nance sequence is also shown in Figure 7, and consists of an

output then three gaps, followed by an output and two gaps.

When interpolating from CIF up to NTSC resolutions, it is

necessary to read lines of data from the CIF frame store with

reduced blanking periods. The timing is calculated such that

six lines are read in the time that five lines would have been

read if they had the correct blanking period. These fast lines

are continuously filtered using all the available information,

and the results are written to an output FIFO. This FIFO is then

read with the correct blanking period inserted in order to

provide NTSC data at the output pins. Thus five lines are read

out in the time taken to load six lines ( one of which need not

actually be written since it is never used )