
ADV7188
Table 91. PAL Line Enable Bits and
Corresponding Line Numbering
Line Number
(ITU-R BT.470)
12
8
13
9
14
10
15
11
0
12
1
13
2
14
3
15
4
16
5
17
6
18
7
19
8
20
9
21
10
22
11
23
12
321 (8)
13
322 (9)
14
323 (10)
15
324 (11)
0
325 (12)
1
326 (13)
2
327 (14)
3
328 (15)
4
329 (16)
5
330 (17)
6
331 (18)
7
332 (19)
8
333 (20)
9
334 (21)
10
335 (22)
11
336 (23)
Letterbox Detection
Incoming video signals may conform to different aspect ratios
(16:9 wide screen or 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.
Rev. 0 | Page 69 of 112
Line[3:0]
Enable Bit
GDECOL[0]
GDECOL[1]
GDECOL[2]
GDECOL[3]
GDECOL[4]
GDECOL[5]
GDECOL[6]
GDECOL[7]
GDECOL[8]
GDECOL[9]
GDECOL[10]
GDECOL[11]
GDECOL[12]
GDECOL[13]
GDECOL[14]
GDECOL[15]
GDECEL[0]
GDECEL[1]
GDECEL[2]
GDECEL[3]
GDECEL[4]
GDECEL[5]
GDECEL[6]
GDECEL[7]
GDECEL[8]
GDECEL[9]
GDECEL[10]
GDECEL[11]
GDECEL[12]
GDECEL[13]
GDECEL[14]
GDECEL[15]
Comment
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Closed caption
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Not valid
Closed caption
Not valid
In the absence of a WSS sequence, letterbox detection may 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, this may indicate that the
currently shown picture 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 may depend on the type of input signal; some control is
provided via LB_TH[4:0].
Detection at the Start of a Field
The ADV7188 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 ADV7188 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 ADV7188 expects at least six continuous lines of black
video at the bottom of a field before reporting 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 middle
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 ADV7188 finds at least two
black lines followed by some more nonblack video, for example,
the subtitle, followed by the remainder of the bottom black
block, it reports 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 2-field delay in the reporting of any line count
parameters.
There is no letterbox detected bit. Read the LB_LCT[7:0] and
LB_LCB[7:0] register values to conclude whether or not the
letterbox-type video is present in software.
LB_LCT[7:0] Letterbox Line Count Top, Address 0x9B [7:0];
LB_LCM[7:0] Letterbox Line Count Mid, Address 0x9C [7:0];
LB_LCB[7:0] Letterbox Line Count Bottom, Address 0x9D [7:0]
Table 92. LB_LCx Access Information
1
Signal Name
LB_LCT[7:0]
LB_LCM[7:0]
LB_LCB[7:0]
Address
0x9B
0x9C
0x9D
1
This register is a readback register; default value does not apply.