ADV7180WBSTZ datasheet(74/120 Pages) AD

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ADV7180

Data Sheet

Rev. G | Page 74 of 120

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].

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

Address

LB_LCT[7:0]

0x9B

LB_LCM[7:0]

0x9C

LB_LCB[7:0]

0x9D

LB_TH[4:0], Letterbox Threshold Control,

Address 0xDC[4:0]

Table 99. LB_TH Function

LB_TH[4:0]

Description

01100 (default)

Default threshold for detection of black lines

01101 to 10000

Increase threshold (need larger active video

content before identifying nonblack lines)

00000 to 01011

Decrease threshold (even small noise levels

can cause the detection of nonblack lines)

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.

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