TMC22x5yA
PRODUCT SPECIFICATION
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 theYC separation, the picture con-
tent, and the complexity of any post processing of the
decoded signals.
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.
–
G
Luminance
Y
Y
Green
Red
–
–
–
Matrix
YC Filter
R
B
–
Composite
–
–
U
V
C
Chrominance
Blue
Demodulation
–
sin(wt)
Burst Locked
Loop
cos(wt+φ)
65-22x5y-44
Figure 3. Fundamental Decoder Block Diagram
The Luma Notch and Chroma Bandpass Technique for
YC Separation
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 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
PAL
NTSC
Amplitude
Amplitude
(dB)
(dB)
Chrominance
Chrominance
Subcarrier
Subcarrier
Sound Carrier
Center Frequency
Sound Carrier
0
0
Center Frequency
-3
-3
Luminance
Luminance
Chrominance
(& High Frequency
Luminance)
Chrominance
(& High Frequency
Luminance)
-20
-20
Frequency (MHz)
Frequency (MHz)
1
2
3
4
5
6
1
2
3
4
4.5
Figure 4. Comparison of the Frequency Spectrum of NTSC and PAL Composite Video Signals
40
REV. 1.0.0 2/4/03
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
