TMC22x5yA
PRODUCT SPECIFICATION
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.
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 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.
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
Picture Correlation
Hue and Saturation Error Signals
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.
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.
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
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
OHL
1HL
2HL
UYE
Luma
Comparison
LYE
YESG
YWBY
OHH
UPE
LPE
Hue
Comparison
Chroma
Demodulation
&
Rectangular
to Polar
1HH
Conversion
UME
LME
Saturation
Comparison
2HH
CESG
CSETBY
65-22x57-59
CEST[1:0]
Figure 18. Generation of Upper and Lower Comb Fail Signals
50
REV. 1.0.0 2/4/03
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
