Detection At Start Of Field; Detection At End Of Field; Detection At Mid Range - Analog Devices ADV7850 Hardware Manual

sdp_lbox_thr[4:0], Addr 90 (SDP), Address 0xDC[4:0]
This control is used to set the threshold for black line detection in letterbox detection. A larger value increases the possibility of
detecting a line as black.
Function
sdp_lbox_thr[4:0]
00010 
sdp_lbox_blk_lvl[2:0], Addr 90 (SDP), Address 0xDC[7:5]
This control is used to set the expected blank level in the lbox detection block. A larger value corresponds to a higher blank level.
Function
sdp_lbox_blk_lvl[2:0]
000 
001
010
011
100
101
110
111

Detection at Start of Field

6.17.1
The ADV7850 expects a section of at least six consecutive black lines of video at the top of a field. Once those lines are detected,
sdp_lbox_blk_top[7:0] reports back the number of black lines actually found. By default, the ADV7850 starts looking for those black lines
in synchronization with the beginning of active video, for example, straight after the last VBI video line.
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 End of Field

6.17.2
The ADV7850 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 sdp_lbox_blk_bot[7:0] value. The activity window for the letterbox detection (end of field) starts in the middle of
the active field. Its end is programmable via sdp_lbox_end_del[3:0].

Detection at Mid Range

6.17.3
Some transmissions of wide screen video include subtitles within the lower black box. If the ADV7850 finds at least two black lines,
followed by some more non black video (for example, the subtitle), and finally followed by the remainder of the bottom black block, it
reports back a mid count via sdp_lbox_blk_sub_bot[7:0].
Note: There is a two field delay in the reporting of any line count parameters. There is no letterbox detected bit. The user is requested to
read the
sdp_lbox_blk_top[7:0]
video in the software.
sdp_lbox_blk_top[7:0], SDP Map, Address 0x4C, [7:0]
sdp_lbox_blk_bot[7:0], SDP Map, Address 0x4D, [7:0]
sdp_lbox_blk_sub_bot[7:0], SDP Map, Address 0x4E, [7:0]
Access Information
Signal Name
sdp_lbox_blk_top[7:0]
sdp_lbox_blk_bot[7:0]
sdp_lbox_blk_sub_bot[7:0]
Rev. A May 2012
Description
Default threshold for detection of black lines
Description
None
None
None
None
None
None
None
None
and sdp_lbox_blk_sub_bot[7:0] values, and come to a conclusion about the presence of letterbox type
Address Default
0x4C Read back only
0x4D Read back only
0x4E Read back only
Description
Number of black lines detected at top of field
Number of black lines detected at bottom of field
Number of black lines detected at bottom of field
(including subtitle lines)
124
ADV7850
sdp_lbox_beg_del[3:0] allows the