TMC22X5YA datasheet(49/84 Pages) CADEKA

49 / 84 page

PRODUCT SPECIFICATION

TMC22x5yA

REV. 1.0.0 2/4/03

In either of these methods, the “K” signal can be used to cross

fade between the YCOMB and the SIMPLE bandsplit signals.

The resulting comb ﬁlter equation can be expressed as:

Combed Luma = Simple

+ (K * Combed High

Frequency Luma)

Combed Chroma = Simple

- (K * Combed High

Frequency Luma)

In the case of the chroma comb, the weighted combed high

frequency luma is subtracted from the SIMPLE high pass ﬁl-

ter output to produce the combed chroma signal, and for

luma comb ﬁlters the weighted combed high frequency luma

is added to the SIMPLE low pass ﬁlter output to provide the

combed luminance signal.

Comb Fails

The inputs to the comb ﬁlter are monitored to detect discon-

tinuities that would cause the comb ﬁlter operation to fail.

Whenever a signiﬁcant failure is predicted, the comb ﬁlter

architecture is modiﬁed and an error signal proportional to

the discontinuity is produced. For ﬂat areas of color, it is a

relatively simple to produce an error signal that switches

between the outputs of the comb ﬁlter and the simple band

split ﬁlter without visibly softening the picture horizontally

or vertically. However, as horizontal frequencies increase

during vertical transitions, so the decision for switching

between the comb and simple bandsplit decoder becomes

more complex.

A line based comb ﬁlter can separate the luma and chroma

signals from line repetitive composite video signals, with no

loss of luma or chroma bandwidth. However, if there is a ver-

tical transition, i.e. a change from one scan line to the next,

as shown for a NTSC two line comb in Figure 17, a comb

fail occurs. The comb fail shown in Figure 17, clearly illus-

trates the resulting vertical smearing of the luma and chroma

signals.

In addition to the smearing, the resulting phase of the

chrominance signal with respect to the burst can cause hue

errors in the demodulated picture. In this example, the

chrominance signal would be demodulated with a 180

degree phase error. Unlike the “simple” decoder technique

any errors in the comb ﬁlter decoding produce components

that if re-encoded will never reproduce the original compos-

ite video waveform. It is therefore imperative that the num-

ber and magnitude of comb fails be kept to its absolute

minimum. This is not possible with non-adaptive comb ﬁlter

architectures, and all vertical and diagonal transitions in the

picture will cause irreversible picture degradation. For this

reason, all the TMC22x5yA comb ﬁlter decoders implement

an adaptive comb ﬁlter architecture.

To aid in this decision making process, comprehensive comb

fail signals are generated and fed to a user-programmable

lookup table (XLUT). The output of the lookup table pro-

vides the control for the cross fade between the comb and

simple bandsplit decoder.

Comb Fail Detection

The traditional approach of using the low frequency data to

look for vertical luma transitions, and rectifying the high

frequency data to estimate vertical transitions in the chroma

provides adequate comb fail detection. However, chroma

signals that are equal in magnitude but 180 degrees apart in

phase, which can also have a small difference in luma level,

for example green and magenta, can produce undetected

comb fails in the comb ﬁlter output.

To overcome problems with simpler comb fail measurement

techniques, the TMC22x5yA generates an array of patented

comb fail and comb ﬁlter control signals. To produce these

signals each input to the comb ﬁlter is passed through a sim-

ple bandsplit decoder. This provides a luma signal from the

low frequency portion of the comb ﬁlter input, and the hue

(phase) and saturation (magnitude) from the high frequency

portion of the comb ﬁlter input. These signals are compared

and the differences in luma, hue, and saturation are used to

determine the type of comb ﬁlter used to generate the

YCOMB signal and to provide the cross fade control signal

“K”. The “K” signal can be weighted within the XLUT

lookup table, allowing the user to tailor the comb

ﬁlter response to their system requirements.

Figure 17. Example of a Comb Fail Using a NSTC Two Line Comb Filter

65-22x5y-58

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