LM1881
SNLS384F –FEBRUARY 1995–REVISED MARCH 2013
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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 µs 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 chroma 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 trailing edge of the
horizontal sync pulses. Simultaneously the output of Pin 5 is pulled low and held until the capacitor charge circuit
times out—4 µs 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 applications which require high horizontal scan rates in
combination with normal (60 Hz–120 Hz) vertical scan rates.
APPLICATIONS
Apart from extracting a composite sync signal free of video information, the LM1881 outputs allow a number of
interesting 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 waveform. 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 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 appears) 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.
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 in Figure 10 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 transitions 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 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 buffer is required to drive low impedance loads.
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