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.
Application Notes (Continued)
using a series C-R network. This may be necessary in
applications which require high horizontal scan rates in com-
bination with normal (60 Hz–120 Hz) vertical scan rates.
VIDEO LINE SELECTOR
The circuit in Figure 3 puts out a singe 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 transi-
tions are counted using the borrow out of the desired number
of counters. The final borrow out pulse is used to turn on the
analog switch (CD4066BC) during the desired line. The fall-
ing edge of this signal also resets the start count latch,
thereby terminating the counting.
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
synchronization and decoding into R.G.B. components. For
frame memory storage applications, the odd/even field lever
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 time—the
rising edge of the first vertical serration in the sync wave-
form. This means that individual lines within the vertical
blanking period (or anywhere in the active scan line period)
can easily be extracted by counting the required number of
transitions in the composite sync waveform following the
start of the vertical output pulse.
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 buffer
is required to drive low impedance loads.
The vertical blanking interval is proving popular as a means
to transmit data which will not appear on a normal T.V.
receiver 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 signals,
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.
MULTIPLE CONTIGUOUS VIDEO LINE
SELECTOR WITH BLACK LEVEL RESTORATION
The circuit in Figure 4 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 kΩ, 10 µF) providing black level
restoration at the video output when the output selected
line(s) is not being gated through.
Since the vertical output pulse from the LM1881 coincides
with the leading edge of the first vertical serration, sixteen
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