22051AKHC View Datasheet(PDF) - Fairchild Semiconductor

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22051AKHC

Multistandard Digital Video Decoder Three-Line Adaptive Comb Decoder Family, 8 & 10 bit

Fairchild Semiconductor 

TMC22x5yA

PRODUCT SPECIFICATION

Generation of the Comb Fail Signals

Luma Error Signals

The signals from the 3 low pass filters, 0HL, 1HL, and 2HL

are subtracted from one another to produce an error signal

proportional to the luma comb fail. The resulting signals

(0HL - 1HL), produces LYE, and either (1HL - 2HL) in

NTSC or (0HL - 2HL) in PAL produces UYE. The LYE and

UYE luma error signals are rectified if negative. In cases

where the luminance component is constant, the error will be

zero. Where the luminance goes from black to white over 2

lines, the error signal will go to its maximum value.

The luma error signals can be doubled to facilitate inputs

with low picture levels by setting the YESG register bit

HIGH. The resulting signal is clipped to ensure no overflow

occurs

Hue and Saturation Error Signals

In the past, comb decoders have relied upon comparing the

difference in chroma magnitude between two lines to deter-

mine a comb fail. In fact, this chroma signal is normally the

output of the high-pass or band-pass filter, and therefore con-

tains all the high frequency luminance information as well.

As this signal was never demodulated, the sign bit was

immaterial and was used only to rectify the chroma signal.

This allowed chroma signals which where equal in magni-

tude but opposite in phase, and high frequency luminance

signals, to fool the comb fail circuit.

The TMC22x5yA uses a new, innovative approach to over-

come this problem. To detect comb failures in the high-

frequency portion of the video signal the outputs from the

three high-pass filters, 0HH, 1HH, and 2HH, are passed

through simple demodulators. The outputs from which

provide the phase and magnitude of the in-phase and quadra-

ture components of the high frequency data. These compo-

nents are compared to determine the difference in phase and

magnitude between 0H & 1H in all configurations, LME and

LPE, and between 1H & 2H in NTSC or 0H & 2H in PAL,

UME and UPE. The magnitude error signals can be doubled

to facilitate inputs with low picture levels by setting the

CESG register bit HIGH. The doubled magnitude error

signals are limited to ensure no overflow occurs.

The algorithm used to separate the quadrature components

depends upon the relationship between the normalized sub-

carrier frequency and the number of pixels per line. This

algorithm is preset for either a NTSC/M or PAL/I subcarrier

frequency and a pixel data rate of 13.5MHz. It is therefore

necessary to compensate for other pixel data rates by select-

ing the appropriate default using the CEST[1:0] register bits.

Picture Correlation

The degree of picture correlation depends upon the differ-

ences between the UYE, UME, and UPE upper error signals

and the LYE, LME, and LPE lower error signals, and is mea-

sured as a percentage of full scale error. In flat fields of color

you would have 0% error in picture correlation, however in

sharp vertical transitions say between yellow and blue you

would have large % errors between UYE and LYE and

between UPE and LPE, while there would be 0% error

between UME and LME.

Adapting the Comb Filter

In NTSC it is possible to switch from a 3 line comb to a 2

line comb, and then to a simple decoder output. The 3 line

comb to 2 line comb switch can be disabled, forcing the 3

line comb to switch directly to simple. The switching

between these two comb architectures is independent of the

OHL

1HL

2HL

Luma

Comparison

YESG YWBY

UYE

LYE

OHH

1HH

2HH

Chroma

Demodulation

&

Rectangular

to Polar

Conversion

Hue

Comparison

Saturation

Comparison

UPE

LPE

UME

LME

CEST[1:0]

CESG CSETBY

65-22x57-59

Figure 18. Generation of Upper and Lower Comb Fail Signals

50

REV. 1.0.0 2/4/03

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