
PRODUCT SPECIFICATION
TMC22091/TMC22191
43
Hardware Keying
The KEY input switches the COMPOSITE D/A converter 
input from the luminance and chrominance combiner output 
to the CVBS data bus on a pixel-by-pixel basis. This is a 
"soft" switch, executed over four PXCK periods to minimize 
out-of-band spurious signals. The video signal from the 
CVBS bus can only present on the COMPOSITE output. The 
CHROMA and LUMA outputs continue to present encoded 
PD port data when CVBS is active.
Hardware keying is enabled by the Key Control Register bit 
6. Normally, keying is only effective during the Active Video 
portion of the waveform as determined by the VA registers 
15 and 18. The Horizontal Blanking interval is generated by 
the encoder state machine even if the KEY signal is held 
HIGH through Horizontal Blanking. However, it is possible 
to allow digital Horizontal Blanking to be passed through 
from the CVBS bus to the COMPOSITE output by setting 
Key Control Register bit 5 HIGH. In this mode, KEY is 
always active, and may be exercised at will.
The KEY input is registered into the encoder just like Pixel 
Data is clocked into the PD port. It may be considered a 25th 
Pixel Data bit. It is internally pipelined, so the midpoint of 
the key transition occurs at the output of the pixel that was 
input at the same time as the KEY signal.
Data Keying
Data Keying internally generates a Key signal that acts 
exactly as the external KEY signal. There are three Key 
Value Registers 05, 06, and 07 that are matched against the 
input data to the three tables in the CLUT. These tables are 
designated D, E, and F. They contain different information 
depending on the input mode selected as shown in Table 16.
Figure 22. Hardware Keying
24359A
*
*
VN+2
VN+3
VN+4
VN+5
VN+6
VN+28
VN+29
VN+30
VN+27
VN+26
VN+25
PN
PN+1
PN+2
PN+3
KEY MIDPOINT
KEY is advanced five PXCK cycles when
Control Register OE bit 4 is HIGH (TMC22191).
P
P
P
P
P
P
P
P
V
PN+4
PN+26
PN+27
PN+28
PN+25
PN+24
PN+23
PXCK
PD
COMPOSITE
OUTPUT
CVBS
KEY
0
1
2
3
4
5
6
7
8
9
38
39
40
41
42
43
44
45
46
47
48
49
The key registers may be individually enabled using bits 
3,2,1 of the Key Control Register. Bit 4 of the same register 
enables/disables Data Keying in its entirety. Data Keying and 
Hardware Keying are logically ORed: when both are 
enabled, either one will result in a key switch to the CVBS 
channel.
The key comparison is based on the input data to the tables 
in the CLUT. When operating in color-index mode, all three 
tables receive the same input value, so any one of the three 
registers is sufficient to identify a key value. The outputs of 
all enabled key registers are ANDed to produce the KEY sig-
nal. If more than one key register are enabled and their key 
values are not identical, no key will be generated.
Table 16. Table D, E, F Contents
Mode
GBR
RGB
YC
B
C
R
CI
Table D
Green
Red
Y
CI
Table E
Blue
Green
C
B
CI
Table F
Red
Blue
C
R
CI