ADV7181B
Rev. B | Page 49 of 100
Table 59. CGMS Access Information
Signal Name
Register Location
Address
Register Default Value
CGMS1[7:0]
CGMS 1[7:0]
150d
0x96
Readback Only
CGMS2[7:0]
CGMS 2[7:0]
151d
0x97
Readback Only
CGMS3[3:0]
CGMS 3[3:0]
152d
0x98
Readback Only
0
REFERENCE COLOR BURST
(9 CYCLES)
FREQUENCY = F
SC
= 3.579545MHz
AMPLITUDE = 40 IRE
1
CCAP1[7:0]
7 CYCLES
OF 0.5035MHz
(CLOCK RUN-IN)
CCAP2[7:0]
2 3 4 5 6 7 0 1 2 3 4 5 6 7
P
A
R
I
T
Y
S
T
A
R
T
P
A
R
I
T
Y
BYTE 1
BYTE 0
33.764
μ
s
10.003
μ
s
10.5
±
0.25
μ
s
12.91
μ
s
27.382
μ
s
50 IRE
40 IRE
04984-033
Figure 33. Closed Caption Data Extraction
Table 60. CCAP Access Information
Signal Name
Register Location
Address
Register Default Value
CCAP1[7:0]
CCAP 1[7:0]
153d
0x99
Readback only
CCAP2[7:0]
CCAP 2[7:0]
154d
0x9A
Readback only
Letterbox Detection
Incoming video signals can conform to different aspect ratios
(16:9 wide screen of 4:3 standard). For certain transmissions in
the wide screen format, a digital sequence (WSS) is transmitted
with the video signal. If a WSS sequence is provided, the aspect
ratio of the video can be derived from the digitally decoded bits
WSS contains.
In the absence of a WSS sequence, letterbox detection can be
used to find wide screen signals. The detection algorithm
examines the active video content of lines at the start and end
of a field. If black lines are detected, it indicates the picture
currently displayed is in wide screen format.
The active video content (luminance magnitude) over a line of
video is summed together. At the end of a line, this accumulated
value is compared with a threshold, and a decision is made as to
whether or not a particular line is black. The threshold value
needed can depend on the type of input signal; some control is
provided via LB_TH[4:0].
Detection at the Start of a Field
The ADV7181B expects a section of at least six consecutive
black lines of video at the top of a field. Once those lines are
detected, Register LB_LCT[7:0] reports back the number of
black lines that were actually found. By default, the ADV7181B
starts looking for those black lines in sync with the beginning of
active video, for example, straight after the last VBI video line.
LB_SL[3:0] allows the user to set the start of letterbox detection
from the beginning of a frame on a line-by-line basis. The
detection window closes in the middle of the field.
Detection at the End of a Field
The ADV7181B expects at least six continuous lines of black
video at the bottom of a field before reporting back the number
of lines actually found via the LB_LCB[7:0] value. The activity
window for letterbox detection (end of field) starts in the mid-
dle of an active field. Its end is programmable via LB_EL[3:0].
Detection at the Midrange
Some transmissions of wide screen video include subtitles
within the lower black box. If the ADV7181B finds at least two
black lines followed by some more nonblack video, for example,
the subtitle, and is then followed by the remainder of the
bottom black block, it reports back a midcount via LB_LCM[7:0].
If no subtitles are found, LB_LCM[7:0] reports the same number
as LB_LCB[7:0].
There is a two-field delay in the reporting of any line count
parameters.
There is no letterbox detected bit. The user is asked to read the
LB_LCT[7:0] and LB_LCB[7:0] register values and to conclude
whether or not the letterbox-type video is present in software.
