TMC22X5YA datasheet(41/84 Pages) CADEKA

41 / 84 page

PRODUCT SPECIFICATION

TMC22x5yA

REV. 1.0.0 2/4/03

centered at the subcarrier frequency produces the chroma

signal. This simple technique works well in pictures contain-

ing large ﬂat areas of color, however this is rarely the case.

If, as is generally true, the picture contains high frequency

luma and chroma transitions, for example herring bone suit

jackets, branches of trees, text, etc., cross color and cross

luma artifacts are evident.

The presence of cross color or cross luma is generally

acceptable when viewing the decoded picture on a monitor

from several feet, as would be the case in most homes on

commercial television sets. However, these artifacts become

increasingly difﬁcult to process, or ignore, when the image is

to be compressed or manipulated. In these cases more

sophisticated methods of separating the luma and chroma

signals, such as frame, ﬁeld, or line based comb ﬁlter decod-

ers, are required.

Another important disadvantage of the “luma notch ﬁlter and

bandpass chroma” technique is that once a notch ﬁlter has

been used on the luminance channel this portion of the lumi-

nance frequency spectrum is lost. This effect becomes

increasingly objectionable if the decoder component outputs

are subsequently re-encoded into a composite video signal.

Comb Filter Architectures for YC Separation

A comb ﬁlter uses the relationship between the number of

subcarrier cycles per line period, to cancel the chrominance

signal over multiple line periods. This is shown for an NTSC

two line comb ﬁlter in Figure 6. In NTSC there a 227.5 sub-

carrier cycles per line period, therefore the subcarrier can be

canceled by simply adding two consecutive ﬁeld scan lines.

In PAL(B/I/ etc.) there are 283.7516 subcarrier cycles per

line period, ignoring the 0.0016 cycle advance caused by the

25Hz offset, the PAL subcarrier can be canceled by adding

the ﬁrst and third line of three consecutive ﬁeld scan lines.

Due to the 270 degree advance, it is not possible to use infor-

mation from consecutive ﬁeld lines without adding a PAL

modiﬁer. A PAL modiﬁer produces a 90 degree phase shift in

the chrominance signal by multiplying the chrominance

signal by a signal at two times the subcarrier frequency that

is phased locked to the subcarrier burst reference in the com-

posite video waveform. In addition the PAL modiﬁer inverts

the V component of the chrominance signal. This document

refers to line based comb decoders when discussing decoders

that use inputs from sequential scan lines, i.e. lines from the

same ﬁeld, ﬁeld based comb decoders when describing

decoders that use inputs from sequential ﬁelds, and ﬁnally

frame based comb decoders when examining decoders that

use inputs from sequential frames.

Figure 6.

Composite Line-Based Comb Decoders

The phase relationship of the quadrature modulated chromi-

nance signal can also be represented as in Figure 7. The three

line comb based decoder is clearly biased towards 1H which

illustrates the inherent one line delay through a 3 line comb,

while a two line comb based decoder is biased towards 0H.

In the following discussions a ﬂat color represents video of

constant luma and chroma magnitude and phase.

In NTSC, adding two adjacent lines of ﬂat color will cancel

the chroma and leave the luma whereas subtracting two lines

of ﬂat color will cancel the luma and leave the chroma. In a 3

line comb ﬁlter the ﬂat color on 0H and 2H is added to pro-

vide the ﬂat color average before adding or subtracting from

1H.

In PAL, adding the ﬂat color from 0H and 2H will cancel the

chroma and leave the luma while subtracting the ﬂat color

from 0H and 2H will cancel the luma and leave the chroma.

However, chroma generated in this manner has no simple

Delay = 1/T

1/2

1/2T

3/2T

5/2T

3T 7/2T

9/2T

5T 11/2T 6T

Frequency

Amplitude

1.0

Figure 5. Examples of Notch and Bandpass Filters

Chrominance

Subcarrier

Notch Filter

Bandpass Filter

Chrominance

Subcarrier

Frequency

FSC

Frequency

Amplitude

(dB)

Amplitude

(dB)

Luminance

-20

-3

-20

-3

Chrominance

(& High Frequency

Luminance)

Chrominance

(& High Frequency

Luminance)

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