ADV7180
Letterbox Detection
There is no letterbox detected bit. Read the LB_LCT[7:0] and
LB_LCB[7:0] register values to determine whether or not the
letterbox-type video is present in software.
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
that WSS contains.
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]
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, this may indicate that the
currently shown picture is in wide-screen format.
Table 94. LB_LCx Access Information
Signal Name
LB_LCT[7:0]
LB_LCM[7:0]
LB_LCB[7:0]
Address
0x9B
0x9C
0x9D
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].
LB_TH[4:0], Letterbox Threshold Control,
Address 0xDC [4:0]
Table 95. LB_TH Function
LB_TH[4:0]
Description
01100 (default)
01101 to 10000
Default threshold for detection of black lines.
Increase threshold (need larger active video
content before identifying nonblack lines).
Decrease threshold (even small noise levels
can cause the detection of nonblack lines).
Detection at the Start of a Field
The ADV7180 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 the number of black lines that
were actually found. By default, the ADV7180 starts looking for
those black lines in sync with the beginning of active video, for
example, immediately 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.
00000 to 01011
LB_SL[3:0], Letterbox Start Line, Address 0xDD [7:4]
The LB_SL[3:0] bits are set at 0100 by default. For an NTSC
signal, this window is from Line 23 to Line 286.
By changing the bits to 0101, the detection window starts on
Line 24 and ends on Line 287.
Detection at the End of a Field
LB_EL[3:0], Letterbox End Line, Address 0xDD [3:0]
The ADV7180 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].
The LB_EL[3:0] bits are set at 1101 by default. This means that the
letterbox detection window ends with the last active video line.
For an NTSC signal, this window is from Line 262 to Line 525.
By changing the bits to 1100, the detection window starts on
Line 261 and ends on Line 254.
Detection at the Midrange
Some transmissions of wide-screen video include subtitles
within the lower black box. If the ADV7180 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 reporting any line count parameter.
Rev. A | Page 71 of 112
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