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參數資料
| 型號： | LM1886N |
| 英文描述： | Color Encoder Circuit |
| 中文描述： | 顏色編碼器電路 |
| 文件頁數： | 7/10頁 |
| 文件大?。?/td> | 172K |
| 代理商： | LM1886N |
Application Notes
(Continued)
threshold from being reached and the Q output of the flip-
flop is toggled with each equalizing pulse. Since the half line
period at the end of the odd field will have the same effect
as an equalizing pulse period, the Q output will have a differ-
ent polarity on successive fields. Thus by comparing the Q
polarity with the vertical output pulse, an odd/even field in-
dex is generated. Pin 7 remains low during the even field
and high during the odd field.
BURST/BACKPORCH OUTPUT PULSE
In a composite video signal, the chroma burst is located on
the backporch of the horizontal blanking period. This period,
approximately 4.8
m
s long, is also the black level reference
for the subsequent video scan line. The LM1881 generates
a pulse at Pin 5 that can be used either to retrieve the chro-
ma burst from the composite video signal (thus providing a
subcarrier synchronizing signal) or as a clamp for the DC
restoration of the video waveform. This output is obtained
simply by charging an internal capacitor starting on the trail-
ing edge of the horizontal sync pulses. Simultaneously the
output of Pin 5 is pulled low and held until the capacitor
charge circuit times outD4
m
s later. A shorter output burst
gate pulse can be derived by differentiating the burst output
using a series C-R network. This may be necessary in appli-
cations which require high horizontal scan rates in combina-
tion with normal (60–120 Hz) vertical scan rates.
APPLICATIONS
Apart from extracting a composite sync signal free of video
information, the LM1881 outputs allow a number of interest-
ing applications to be developed. As mentioned above, the
burst gate/backporch clamp pulse allows DC restoration of
the original video waveform for display or remodulation on
an R.F. carrier, and retrieval of the color burst for color syn-
chronization and decoding into R.G.B. components. For
frame memory storage applications, the odd/even field lev-
el allows identification of the appropriate field ensuring the
correct read or write sequence. The vertical pulse output is
particularly useful since it begins at a precise timeDthe ris-
ing edge of the first vertical serration in the sync waveform.
This means that individual lines within the vertical blanking
period (or anywhere in the active scan line period) can easi-
ly be extracted by counting the required number of tran-
sitions in the composite sync waveform following the start of
the vertical output pulse.
The vertical blanking interval is proving popular as a means
to transmit data which will not appear on a normal T.V. re-
ceiver screen. Data can be inserted beginning with line 10
(the first horizontal scan line on which the color burst ap-
pears) through to line 21. Usually lines 10 through 13 are
not used which leaves lines 14 through 21 for inserting sig-
nals, which may be different from field to field. In the U.S.,
line 19 is normally reserved for a vertical interval reference
signal (VIRS) and line 21 is reserved for closed caption data
for the hearing impaired. The remaining lines are used in a
number of ways. Lines 17 and 18 are frequently used during
studio processing to add and delete vertical interval test
signals (VITS) while lines 14 through 18 and line 20 can be
used for Videotex/Teletext data. Several institutions are
proposing to transmit financial data on line 17 and cable
systems use the available lines in the vertical interval to
send decoding data for descrambler terminals.
Since the vertical output pulse from the LM1881 coincides
with the leading edge of the first vertical serration, sixteen
positive or negative transitions later will be the start of line
14 in either field. At this point simple counters can be used
to select the desired line(s) for insertion or deletion of data.
VIDEO LINE SELECTOR
The circuit inFigure 4 puts out a single video line according
to the binary coded information applied to line select bits
b0–b7. A line is selected by adding two to the desired line
number, converting to a binary equivalent and applying the
result to the line select inputs. The falling edge of the
LM1881’s vertical pulse is used to load the appropriate
number into the counters (MM74C193N) and to set a start
count latch using two NAND gates. Composite sync tran-
sitions are counted using the borrow out of the desired num-
ber of counters. The final borrow out pulse is used to turn on
the analog switch (CD4066BC) during the desired line. The
falling edge of this signal also resets the start count latch,
thereby terminating the counting.
The circuit, as shown, will provide a single line output for
each field in an interlaced video system (television) or a
single line output in each frame for a non-interlaced video
system (computer monitor). When a particular line in only
one field of an interlaced video signal is desired, the odd/
even field index output must be used instead of the vertical
output pulse (invert the field index output to select the odd
field). A single counter is needed for selecting lines 3 to 14;
two counters are needed for selecting lines 15 to 253; and
three counters will work for up to 2046 lines. An output buff-
er is required to drive low impedance loads.
MULTIPLE CONTIGUOUS VIDEO LINE
SELECTOR WITH BLACK LEVEL RESTORATION
The circuit inFigure 5 will select a number of adjoining lines
starting with the line selected as in the previous example.
Additional counters can be added as described previously
for either higher starting line numbers or an increased num-
ber of contiguous output lines. The back porch pulse output
of the LM1881 is used to gate the video input’s black level
through a low pass filter (10 k
X
, 10
m
F) providing black level
restoration at the video output when the output selected
line(s) is not being gated through.
7
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相關代理商/技術參數 |
參數描述 |
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| LM1886N/B+ | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Color Encoder Circuit |
| LM1889 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:LM1889 TV Video Modulator |
| LM1889N | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:LM1889 TV Video Modulator |
| LM1889N/B+ | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:RF Modulator |
| LM1893 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:LM1893/LM2893 Carrier-Current Transceiver |
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