ADV7181 AD [Analog Devices], ADV7181 Datasheet - Page 49

• Manufacturer Part Number: ADV7181
• Description: Multiformat SDTV Video Decoder
• Manufacturer: AD [Analog Devices]

Letterbox Detection
Incoming video signals may 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 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 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 2-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.
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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 60. LB_LCx Access Information
Signal Name
LB_LCT[7:0]
LB_LCM[7:0]
LB_LCB[7:0]
LB_TH[4:0] Letterbox Threshold Control, Address 0xDC [4:0]
Table 61. LB_TH Function
LB_TH[4:0]
01100 (default)
01101 to
10000
00000 to
01011
LB_SL[3:0] Letterbox Start Line, Address 0xDD [7:4]
The LB_SL[3:0] bits are set at 0100b by default. This means that
letterbox detection window starts after the EDTV VBI data line.
For an NTSC signal, this window is from Line 23 to Line 286.
Changing the bits to 0101, the detection window starts on
Line 24 and ends on Line 287.
LB_EL[3:0] Letterbox End Line, Address 0xDD [3:0]
The LB_EL[3:0] bits are set at 1101b by default. This means that
letterbox detection window ends with the last active video line.
For an NTSC signal, this window is from Line 262 to Line 525.
Changing the bits to 1100, the detection window starts on
Line 261 and ends on Line 254.
Gemstar Data Recovery
The Gemstar-compatible data recovery block (GSCD) supports
1× and 2× data transmissions. In addition, it can also serve as a
closed caption decoder. Gemstar-compatible data transmissions
can only occur in NTSC. Closed caption data can be decoded in
both PAL and NTSC.
The block is configured via I
•
•
•
The recovered data is not available through I
into the horizontal blanking period of an ITU-R. BT656-com-
patible data stream. The data format is intended to comply with
GDECEL[15:0] allow data recovery on selected video lines
on even fields to be enabled and disabled.
GDECOL[15:0] enable the data recovery on selected lines
for odd fields.
GDECAD configures the way in which data is embedded
in the video data stream.
Address
0x9B
0x9C
0x9D
Description
Default threshold for detection of black lines.
Increase threshold (need larger active video
content before identifying non-black lines).
Decrease threshold (even small noise levels
can cause the detection of non-black lines).
2
C in the following way:
Register Default Value
Readback only
Readback only
Readback only
2
C, but is inserted
ADV7181B