EVAL-ADV7180LFEBZ Analog Devices Inc, EVAL-ADV7180LFEBZ Datasheet - Page 73

• Manufacturer Part Number: EVAL-ADV7180LFEBZ
• Description: BOARD EVAL FOR ADV7180 LFCSP
• Manufacturer: Analog Devices Inc

Specifications of EVAL-ADV7180LFEBZ

• Main Purpose: Video, SDTV Video Decoder - NTSC, PAL, SECAM
• Embedded: No
• Utilized Ic / Part: ADV7180
• Primary Attributes: CVBS (Composite), Y/C (S-video), and YPrPb (Component) Inputs
• Secondary Attributes: 8-bit ITU-R BT.656 YCrCb 4:2:2 Output
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
that 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, 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 ADV7180 expects a section of at least six consecutive black
lines of video at the top of a field. After those lines are detected,
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.
Detection at the End of a Field
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].
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].
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There is a two-field delay in reporting any line count parameter.
There is no letterbox detected bit. Read the LB_LCT[7:0] and
LB_LCB[7:0] register values to determine whether the letterbox-
type video is present in the 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 98. 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 99. 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 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.
LB_EL[3:0], Letterbox End Line, Address 0xDD[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.
Description
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)
Address
0x9B
0x9C
0x9D
ADV7180