vp520s Mitel, vp520s Datasheet - Page 10

• Manufacturer Part Number: vp520s
• Description: Pal/ntsc To Cif/qcif Converter
• Manufacturer: Mitel

VP520S
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Fig 9 : Interpolating from QCIF to NTSC
LUM
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QCIF LINES
CHROM
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EVEN
NTSC LINES
ODD
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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
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.
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.
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.
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.
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.
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 )
One field of NTSC video consists of 240 chrominance and
The line repeat requires an additional FIFO line delay
Chrominance data also passes through the input FIFO
A simplified approach is used when decimating down to
When interpolating from CIF up to NTSC resolutions, it is
Three chrominance lines are derived from the same