Datasheet

ADV7188
Rev. A | Page 76 of 112
GDECOL [15:0], Gemstar Decoding Odd Lines,
Address 0x4A [7:0], Address 0x4B [7:0]
The 16 bits of the GDECOL [15:0] form a collection of 16 indi-
vidual line decode enable signals. See
Table 94 and Table 95.
To retrieve closed caption data services on NTSC (Line 21),
GDECOL [11] must be set.
To retrieve closed caption data services on PAL (Line 22),
GDECOL [14] must be set.
The default value of GDECOL [15:0] is 0x0000. This setting
instructs the decoder not to attempt to decode Gemstar or CC
data from any line in the odd field. The user should only enable
Gemstar slicing on lines where VBI data is expected.
GDECAD, Gemstar Decode Ancillary Data Format,
Address 0x4C [0]
The decoded data from Gemstar-compatible transmissions or
closed caption transmission is inserted into the horizontal
blanking period of the respective line of video. A potential
problem can arise if the retrieved data bytes have the value 0x00
or 0xFF. In an ITU-R BT.656-compatible data stream, these
values are reserved and used only to form a fixed preamble.
The GDECAD bit allows the data to be inserted into the
horizontal blanking period in two ways:
•
Insert all data straight into the data stream, even the reserved
values of 0x00 and 0xFF, if they occur. This may violate the
output data format specification ITU-R BT.1364.
•
Split all data into nibbles and insert the half-bytes over
twice the number of cycles in a 4-bit format.
0 (default)—The data is split into half-bytes and inserted.
1—The data is output straight in 8-bit format.
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 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] is updated. Register LB_LCT [7:0] reports
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, 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 letterbox detection ends 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 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 two-field delay in the reporting of line count parameters.
There is no letterbox detected bit. Read the LB_LCT [7:0] and
LB_LCB [7:0] bit values to conclude whether or not the
letterbox type of 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 96. LB_LCx [7:0] Access Information
1
Signal Name Address
LB_LCT [7:0] 0x9B
LB_LCM [7:0] 0x9C
LB_LCB [7:0] 0x9D
1
This register is a readback register; the default value does not apply.