
TMC22x5yA
PRODUCT SPECIFICATION
40
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+
f
)
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