VP520S

9

fact needs some interpolation - see the relevant section ).

Chrominance, however, is decimated by two. When produc-

ing QCIF data the luminance channel is decimated by two, and

the chrominance by four.

When the VP520S is used to derive interlaced CCIR601

video, the internal address generator will read the CIF/QCIF

frame store twice in order to produce the two fields. Each field

has its own set of coefficients.

Internal RAM is provided which will support four CIF line

delays for both chrominance and luminance. Five tap filters

are thus possible for CIF conversions. With a QCIF system the

internal RAM could theoretically be used to provide eight QCIF

line delays. In practice, however, little benefit is obtained by

using vertical filters with more than seven taps, and thus only

six line delays are used.

Polyphase filters are used to support the spatial conver-

sions. PAL conversion is relatively simple and only requires a

set of coefficients for each mode. NTSC conversion requires

several sets of coefficients since the 240 lines in a field must

be converted to 288 lines of CIF. One line is repeated in every

five to produce six lines which are then filtered with their own

coefficients.

The generation of interpolated outputs requires CIF / QCIF

data to be repeatedly read from the frame store at various line

intervals. This is all handled by the internal address generator,

and is transparent to the user. The device then produces

coincident luminance and chrominance data which has been

interpolated from data in the frame store. The first line will be

produced to match the delay from the VREF input which has

been pre-defined. This delay must be greater than the internal

pipeline delay, which itself is mode dependent ( delay yet to be

determined ).

The device introduces black lines at the top and bottom of

the fields. Thus the first and last lines in the interpolated field

will be filtered with varying amounts of black information.

PAL VERTICAL FILTERING

When producing CIF data the five tap filters

provide

outputs for every line at the 6.75 MHz decimated line rate.

Every filtered luminance line is used but every other filtered

chrominance line will be discarded. Filter outputs correspond-

ing to odd numbered PAL chrominance lines in any field being

at the centre are used to provide the CIF chrominance lines.

This is shown by the arrows in Figure 7.

When decimating down to QCIF seven tap filters are used,

which provide outputs for every line at the 3.375 MHz line rate.

Only every other filtered luminance line, and every fourth

chrominance line are actually stored in the frame store.

Different PAL lines are used to produce the offset luminance

and chrominance lines as indicated by the arrows in Figure 7.

When interpolating from CIF the luminance channel con-

ceptually uses a 10 tap filter, with every other input line

containing only zero's. Thus only five coefficients are actually

used when producing interpolated lines for the even field, and

five different coefficients are used when producing the odd

field. The device thus stores two sets of five coefficients; one

set for each field produced by reading the CIF frame store

twice.

The chrominance filter is conceptually a 20 tap filter with

three lines of zero's for every actual input. Thus each chromi-

nance channel needs four sets of five coefficients; two sets are

needed to produce one field, and two sets are needed for the

other field. The same chrominance data is read twice for a

given pair of luminance lines, in order to provide inputs for the

filter. Thus the internal line delays contain the same set of

chrominance data on two consecutive lines supplying data to

the filters.

When interpolating from QCIF, seven coefficients can be

0

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QCIF LINES

LUM

CHROM

PAL LINES

EVEN

ODD

NTSC LINES

EVEN

ODD

Fig 8 : QCIF Spatial Relationships