STV0118 datasheet(17/42 Pages) STMICROELECTRONICS

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IV - FUNCTIONAL DESCRIPTION (continued)

In addition, the luminance that is added to the

chrominance to create the composite CVBS signal

can be trap-filtered at 3.58MHz (NTSC) or

4.43MHz (PAL). This allows to cope with applica-

tion oriented towards low-end TV sets which are

subject to cross-color if the digital source has a

wide luminance bandwidt h (e.g. some DVD

sources). Note that the trap filter does not affect the

S-VHS luminance output nor the RGB outputs.

A 7.5 IRE pedestal can be programmed if needed

with all standards (see Reg1, bit setup). This allows

in particular to encode Argentinian and non-Ar-

gentinian PAL-N, or Japanese NTSC (NTSC with

no set-up).

A programmable delay can be inserted on the

luminance path to compensate any chroma/luma

delay introduced by off-chip filtering (chroma and

luma transitionsbeing coincident at the DAC output

with default delay) (Reg3, bits del[2:0]).

IV.10 - Chrominance Encoding

U and V chroma components are computed from

demultiplexed Cb, Cr samples. Before modulating

the subcarrier, these are band-limited and interpo-

lated at CKREF clock rate. This processing eases

the filtering following D/A conversion and allows a

more accurate encoding. A set of 4 different filters

is available for chroma filtering to fit a wide variety

of applications in the different standards and in-

clude filters recommended by ITU-R Rec624-4 and

SMPTE170-M. The available 3dB bandwidths are

1.1, 1.3, 1.6 or 1.9MHz, refer to Figures 18 to 22

for the various frequency responses (Reg1, bits

flt[1:0]).

The narrower bandwidths are useful against cross-

luminance artefacts, the wider bandwidths allow to

keep higher chroma contents and then an im-

proved image quality.

IV.11 - Composite Video Signal Generation

The composite video signal is created by adding

the luminance (after optional trap filtering, Reg 3

bits entrap and trap_pal) and the chrominance

components.A saturationfunction is included in the

adder to avoid overflow errors should extreme

luminance levels be modulated with highly satu-

rated colors (this does not correspond to natural

colors but may be generated by computers or

graphic engines).

A ‘color killing’ function is available (Reg 1, bit coki)

whereby the composite signal contains no chromi-

nance, i.e. replicates the trap-filtered luminance.

Note that this function does not suppress the

chrominance on the S-VHS outputs (nevertheless

suppressing the S-VHS chrominance is possible

using bit “bkg_c” in Reg 5).

0.5

1.5

2.5

3.5

Frequency (x106) (Hz)

-1

-2

-3

-4

-5

-6

-7

-8

-9

3=1.9

RGB

3=1.6

3=1.3

3=1.1

3=2.45

Figure 18 : Various Chroma Filters Available

+ RGB Filter

-5

-10

-15

-20

-25

-30

-35

-40

Frequency (x106) (Hz)

Figure 19 : 1.1MHz Chroma Filter (flt = 00)

-5

-10

-15

-20

-25

-30

-35

-40

Frequency (x106) (Hz)

Figure 20 : 1.3MHz Chroma Filter (flt = 01)

STV0118

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