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TMC22x5yA
PRODUCT SPECIFICATION
40
REV. 1.0.0 2/4/03
Decoder Introduction
All composite video decoders perform fundamentally the
same operation. The first stage is to separate the luminance
and chrominance. The second stage is to lock the internally
generated sine and cosine waveforms to the burst on the
decoded chrominance signal, demodulate, and then filter the
chrominance signal to produce the color difference signals.
The last stage either scales the luminance and color differ-
ence signals, or converts them into red, green, and blue
component video signals. These three stages are shown in
Figure 3.
The complete separation of composite video signals into
pure luminance (luma) and chrominance (chroma) signals is
practically impossible, especially when the input source
contains intraframe motion. Therefore, the luminance (luma)
signal will generally contain some high frequency chromi-
nance, termed
cross luma
, and the chroma signal will
contains some of the high frequency luma signal, centered
around the subcarrier frequency, termed
cross color
.
The degree of cross luma and cross color is directly propor-
tional to the filter used for the YC separation, the picture con-
tent, and the complexity of any post processing of the
decoded signals.
Figure 3. Fundamental Decoder Block Diagram
65-22x5y-44
YC Filter
Y
Y
U
V
B
R
G
C
Composite
Chrominance
sin(wt)
cos(wt+
φ
)
Luminance
–
–
–
–
–
–
–
–
Matrix
Demodulation
Green
Red
Blue
Burst Locked
Loop
YC Separation
The relationship between the chrominance and luminance
bandwidths is shown for both PAL and NTSC in Figure 4,
wherein the shaded area denotes the part of the composite
video frequency spectrum shared by both the chrominance
and high frequency luminance signals.
The
Luma Notch and Chroma Bandpass
Technique for
YC Separation
The simplest method of separating these chrominance and
luminance signals, is to assume the chroma bandwidth is
limited to a few hundred kilohertz around the subcarrier
frequency. In this case a notch filter designed to remove just
these frequencies from the composite video frequency
spectrum provides the luma signal, while a bandpass filter
Figure 4. Comparison of the Frequency Spectrum of NTSC and PAL Composite Video Signals
Chrominance
Subcarrier
PAL
NTSC
Chrominance
(& High Frequency
Luminance)
Chrominance
(& High Frequency
Luminance)
Sound Carrier
Center Frequency
Chrominance
SubcarrierSound Carrier
Center Frequency
Frequency (MHz)
Frequency (MHz)
Amplitude
(dB)
Amplitude
(dB)
Luminance
1
1
2
2
3
3
4
4
4.5
5
6
-20
-3
0
-20
-3
0
Luminance