LM1881
Video Sync Separator
General Description

Features

The LM1881 Video sync separator extracts timing information including composite and vertical sync, burst/back porchtiming, and odd/even field information from standard negative going sync NTSC, PAL* and SECAM video signals withamplitude from 0.5V to 2V p-p. The integrated circuit is also
capable of providing sync separation for non-standard,
faster horizontal rate video signals. The vertical output isproduced on the rising edge of the first serration in thevertical sync period. A default vertical output is produced
after a time delay if the rising edge mentioned above does
not occur within the externally set delay period, such as
might be the case for a non-standard video signal.

n AC coupled composite input signaln > 10 kΩ input resistancen < 10 mA power supply drain current
n Composite sync and vertical outputs
n Odd/even field output
n Burst gate/back porch output
n Horizontal scan rates to 150 kHz
n Edge triggered vertical output
n Default triggered vertical output for non-standard video
signal (video games-home computers)

Connection Diagram
LM1881N

00915001

Order Number LM1881M or LM1881N (0˚C to +70˚C)
See NS Package Number M08A or N08E

*PAL in this datasheet refers to European broadcast TV standard “Phase Alternating Line”, and not to Programmable Array Logic.

© 2006 National Semiconductor Corporation

DS009150

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LM1881 Video Sync Separator

May 2006

LM1881

Absolute Maximum Ratings (Note 1)

Storage Temperature Range

If Military/Aerospace specified devices are required,please contact the National Semiconductor Sales Office/Distributors for availability and specifications.

ESD Susceptibility (Note 3)

2 kV

ESD Susceptibility (Note 4)

200 V

Supply Voltage

Soldering Information

13.2V

Dual-In-Line Package (10 sec.)

3 VP-P (VCC = 5V)
6 VP-P (VCC ≥ 8V)

Input Voltage
Output Sink Currents; Pins, 1, 3, 5

5 mA

Output Sink Current; Pin 7

2 mA

Package Dissipation (Note 2)−65˚C to +150˚C

260˚C

Small Outline Package
Vapor Phase (60 sec.)

215˚C

Infrared (15 sec.)

220˚C

1100 mW

Electrical Characteristics LM1881
VCC = 5V; RSET = 680 kΩ; TA = 0˚C to +70˚C by correlation with 100% electrical testing at TA=25˚CParameter
Supply Current

Conditions
Outputs at
Logic 1Min

VCC = 5V
VCC = 12V

Typ (Note 5)

Max

Units

5.2
5.5

10
12

mA

DC Input Voltage

Pin 2

1.3

1.5

1.8

V

Input Threshold Voltage(Note 6)

55

70

85

mV

Input Discharge Current

Pin 2; VIN = 2V

6

11

16

Input Clamp Charge Current

Pin 2; VIN = 1V

0.2

0.8

RSET Pin Reference Voltage

Pin 6; (Note 7)

1.10

1.22

Composite Sync. & Vertical
Outputs

IOUT = 40 µA;
Logic 1

VCC = 5V
VCC = 12V

4.0
11.0

4.5

IOUT = 1.6 mA
Logic 1

VCC = 5V
VCC = 12V

2.4
10.0

3.6

Burst Gate & Odd/Even
Outputs

IOUT = 40 µA;
Logic 1

VCC = 5V
VCC = 12V

4.0
11.0

4.5

Composite Sync. Output

IOUT = −1.6 mA; Logic 0; Pin 1

0.2

0.8

V

Vertical Sync. Output

IOUT = −1.6 mA; Logic 0; Pin 3

0.2

0.8

V

Burst Gate Output

IOUT = −1.6 mA; Logic 0; Pin 5

0.2

0.8

V

Odd/Even Output

IOUT = −1.6 mA; Logic 0; Pin 7

0.2

0.8

V

230

300

µs

Vertical Sync Width

190

µA
mA

1.35

V
V
V
V

Burst Gate Width

2.7 kΩ from Pin 5 to VCC

2.5

4

4.7

µs

Vertical Default Time

(Note 8)

32

65

90

µs

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2

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. For guaranteed specifications and test conditions, see theElectrical Characteristics. The guaranteed specifications apply only for the test conditions listed.
Note 2: For operation in ambient temperatures above 25˚C, the device must be derated based on a 150˚C maximum junction temperature and a package thermal
resistance of 110˚C/W, junction to ambient.
Note 3: ESD susceptibility test uses the “human body model, 100 pF discharged through a 1.5 kΩ resistor”.
Note 4: Machine Model, 220 pF – 240 pF discharged through all pins.
Note 5: Typicals are at TJ = 25˚C and represent the most likely parametric norm.
Note 6: Relative difference between the input clamp voltage and the minimum input voltage which produces a horizontal output pulse.
Note 7: Careful attention should be made to prevent parasitic capacitance coupling from any output pin (Pins 1, 3, 5 and 7) to the RSET pin (Pin 6).
Note 8: Delay time between the start of vertical sync (at input) and the vertical output pulse.

Typical Performance Characteristics
RSET Value Selection
vs Vertical Serration
Pulse Separation

Vertical Default
Sync Delay Timevs RSET

00915007

00915008

Burst/Black Level
Gate Time vs RSET

Vertical Pulse
Width vs RSET

00915010
00915009

3

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LM1881

Electrical Characteristics LM1881 (Continued)

LM1881

Typical Performance Characteristics (Continued)
Vertical Pulse
Width vs Temperature

Supply Current vs
Supply Voltage

00915002

00915011

level of 0.5V (p-p), the clipping level is close to the halfway
point on the sync pulse amplitude (shown by the dashed line
on Figure 1(a). This threshold separation is independent of
the signal amplitude, therefore, for a 2V (p-p) input theclipping level occurs at 11% of the sync pulse amplitude. The
charging current for the input coupling capacitor is 0.8 mA,
Normally the signal source for the LM1881 is assumed to beclean and relatively noise-free, but some sources may haveexcessive video peaking, causing high frequency video andchroma components to extend below the black level reference. Some video discs keep the chroma burst pulse
present throughout the vertical blanking period so that the
burst actually appears on the sync tips for three line periods
instead of at black level. A clean composite sync signal canbe generated from these sources by filtering the input signal.
When the source impedance is low, typically 75Ω, a 620Ω
resistor in series with the source and a 510 pF capacitor to
ground will form a low pass filter with a corner frequency of
500 kHz. This bandwidth is more than sufficient to pass thesync pulse portion of the waveform; however, any subcarrier
content in the signal will be attenuated by almost 18 dB,
effectively taking it below the comparator threshold. Filtering
will also help if the source is contaminated with thermal
noise. The output waveforms will become delayed from between 40 ns to as much as 200 ns due to this filter. This
much delay will not usually be significant but it does contribute to the sync delay produced by any additional signal
processing. Since the original video may also undergo processing, the need for time delay correction will depend on
the total system, not just the sync stripper.

Application Notes
The LM1881 is designed to strip the synchronization signals
from composite video sources that are in, or similar to, the
N.T.S.C. format. Input signals with positive polarity video(increasing signal voltage signifies increasing scene brightness) from 0.5V (p-p) to 2V (p-p) can be accommodated.
The LM1881 operates from a single supply voltage between
5V DC and 12V DC. The only required external components
besides a power supply decoupling capacitor at pin 8 and a
set current decoupling capacitor at pin 6, are the composite
input coupling capacitor at pin 2 and one resistor at pin 6 thatsets internal current levels. The resistor on pin 6 (i.e. Rset)
allows the LM1881 to be adjusted for source signals with line
scan frequencies differing from 15.734 kHz. Four major sync
signals are available from the I/C; composite sync including
both horizontal and vertical scan timing information; a vertical sync pulse; a burst gate or back porch clamp pulse; andan odd/even output. The odd/even output level identifies
which video field of an interlaced video source is present at
the input. The outputs from the LM1881 can be used to
gen-lock video camera/VTR signals with graphics sources,
provide identification of video fields for memory storage,
recover suppressed or contaminated sync signals, and provide timing references for the extraction of coded or uncoded
data on specific video scan lines.
To better understand the LM1881 timing information and thetype of signals that are used, refer to Figure 1(a-e) whichshows a portion of the composite video signal from the endof one field through the beginning of the next field.
COMPOSITE SYNC OUTPUT
The composite sync output, Figure 1(b), is simply a reproduction of the signal waveform below the composite video
black level, with the video completely removed. This is obtained by clamping the video signal sync tips to 1.5V DC at
Pin 2 and using a comparator threshold set just above thisvoltage to strip the sync signal, which is then buffered out to
Pin 1. The threshold separation from the clamped sync tip isnominally 70 mV which means that for the minimum input
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VERTICAL SYNC OUTPUT
A vertical sync output is derived by internally integrating thecomposite sync waveform (Figure 2). To understand thegeneration of the vertical sync pulse, refer to the lower left
hand section Figure 2. Note that there are two comparatorsin the section. One comparator has an internally generated
voltage reference called V1 going to one of its inputs. The
other comparator has an internally generated voltage refer4

lets see how these two graphs relate to each other. The
Horizontal line is 64 µs long, or 32 µs for a horizontal halfline. Now round this off to 30 µs. In the “RSET Value Selection
vs Vertical Serration Pulse Separation” graph the minimum
resistor value for 30 µs serration pulse separation is about
550 kΩ. Going to the “Vertical Pulse Width vs RSET” graphone can see that 550 kΩ gives a vertical pulse width of about
180 µs, the total time for the vertical sync period of NTSC (3horizontal lines). A 550 kΩ will set the internal oscillator to afrequency such that eight cycles gives a time of 180 µs, justlong enough to prevent a double vertical sync pulse at thevertical sync output of the LM1881.

ence called V2 going to one of its inputs. Both comparators
have a common input at their noninverting input coming fromthe internal integrator. The internal integrator is used for
integrating the composite sync signal. This signal comesfrom the input side of the composite sync buffer and are
positive going sync pulses. The capacitor to the integrator isinternal to the LM1881. The capacitor charge current is set
by the value of the external resistor RSET. The output of theintegrator is going to be at a low voltage during the normal
horizontal lines because the integrator has a very short time
to charge the capacitor, which is during the horizontal sync
period. The equalization pulses will keep the output voltageof the integrator at about the same level, below the V1.
During the vertical sync period the narrow going positive
pulses shown in Figure 1 is called the serration pulse. The
wide negative portion of the vertical sync period is called thevertical sync pulse. At the start of the vertical sync period,
before the first Serration pulse occurs, the integrator now
charges the capacitor to a much higher voltage. At the firstserration pulse the integrator output should be between V1
and V2. This would give a high level at the output of thecomparator with V1 as one of its inputs. This high is clocked
into the “D” flip-flop by the falling edge of the serration pulse(remember the sync signal is inverted in this section of theLM1881). The “Q” output of the “D” flip-flop goes through the
OR gate, and sets the R/S flip-flop. The output of the R/S
flip-flop enables the internal oscillator and also clocks the
ODD/EVEN “D” flip-flop. The ODD/EVEN field pulse operation is covered in the next section. The output of the oscillator goes to a divide by 8 circuit, thus resetting the R/S
flip-flop after 8 cycles of the oscillator. The frequency of theoscillator is established by the internal capacitor going to theoscillator and the external RSET. The “Q” output of the R/S
flip-flop goes to pin 3 and is the actual vertical sync output of
the LM1881. By clocking the “D” flip-flop at the start of thefirst serration pulse means that the vertical sync output pulse
starts at this point in time and lasts for eight cycles of theinternal oscillator as shown in Figure 1.
How RSET affects the integrator and the internal oscillator isshown under the Typical Performance Characteristics. The
first graph is “RSET Value Selection vs Vertical Serration
Pulse Separation”. For this graph to be valid, the verticalsync pulse should last for at least 85% of the horizontal halfline (47% of a full horizontal line). A vertical sync pulse fromany standard should meet this requirement; both NTSC andPAL do meet this requirement (the serration pulse is theremainder of the period, 10% to 15% of the horizontal halfline). Remember this pulse is a positive pulse at the integrator but negative in Figure 1. This graph shows how long it
takes the integrator to charge its internal capacitor above V1.
With RSET too large the charging current of the integrator will
be too small to charge the capacitor above V1, thus there will
be no vertical synch output pulse. As mentioned above, RSET
also sets the frequency of the internal oscillator. If the oscillator runs too fast its eight cycles will be shorter than thevertical sync portion of the composite sync. Under this condition another vertical sync pulse can be generated on one of
the later serration pulse after the divide by 8 circuit resets the
R/S flip-flop. The first graph also shows the minimum RSET
necessary to prevent a double vertical pulse, assuming thatthe serration pulses last for only three full horizontal line
periods (six serration pulses for NTSC). The actual pulse
width of the vertical sync pulse is shown in the “Vertical
Pulse Width vs RSET” graph. Using NTSC as an example,

The LM1881 also generates a default vertical sync pulse
when the vertical sync period is unusually long and has noserration pulses. With a very long vertical sync time theintegrator has time to charge its internal capacitor above thevoltage level V2. Since there is no falling edge at the end ofa serration pulse to clock the “D” flip-flop, the only high signal
going to the OR gate is from the default comparator when
output of the integrator reaches V2. At this time the R/S
flip-flop is toggled by the default comparator, starting thevertical sync pulse at pin 3 of the LM1881. If the default
vertical sync period ends before the end of the input verticalsync period, then the falling edge of the vertical sync (positive pulse at the “D” flip-flop) will clock the high output fromthe comparator with V1 as a reference input. This will retrigger the oscillator, generating a second vertical sync output
pulse. The “Vertical Default Sync Delay Time vs RSET” graphshows the relationship between the RSET value and thedelay time from the start of the vertical sync period before
the default vertical sync pulse is generated. Using the NTSC
example again the smallest resistor for RSET is 500 kΩ. The
vertical default time delay is about 50 µs, much longer thanthe 30 µs serration pulse spacing.
A common question is how can one calculate the required
RSET with a video timing standard that has no serration
pulses during the vertical blanking. If the default vertical sync
is to be used this is a very easy task. Use the “Vertical
Default Sync Delay Time vs RSET” graph to select the necessary RSET to give the desired delay time for the verticalsync output signal. If a second pulse is undesirable, thencheck the “Vertical Pulse Width vs RSET” graph to make surethe vertical output pulse will extend beyond the end of the
input vertical sync period. In most systems the end of thevertical sync period may be very accurate. In this case thepreferred design may be to start the vertical sync pulse at theend of the vertical sync period, similar to starting the verticalsync pulse after the first serration pulse. A VGA standard is
to be used as an example to show how this is done. In thisstandard a horizontal line is 32 µs long. The vertical sync
period is two horizontal lines long, or 64 µs. The verticaldefault sync delay time must be longer than the verticalsync period of 64 µs. In this case RSET must be larger than
680 kΩ. RSET must still be small enough for the output of theintegrator to reach V1 before the end of the vertical period of
the input pulse. The first graph can be used to confirm that
RSET is small enough for the integrator. Instead of using thevertical serration pulse separation, use the actual pulse
width of the vertical sync period, or 64 µs in this example.
This graph is linear, meaning that a value as large as 2.7 MΩ
can be used for RSET (twice the value as the maximum at
30 µs). Due to leakage currents it is advisable to keep thevalue of RSET under 2.0 MΩ. In this example a value of 1.0
MΩ is selected, well above the minimum of 680 kΩ. With thisvalue for RSET the pulse width of the vertical sync output
pulse of the LM1881 is about 340 µs.

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LM1881

Application Notes (Continued)

LM1881

Application Notes (Continued)

00915003

FIGURE 1. (a) Composite Video; (b) Composite Sync; (c) Vertical Output Pulse;(d) Odd/Even Field Index; (e) Burst Gate/Back Porch Clamp

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6LM1881

Application Notes (Continued)

00915004*Components Optional, See Text

FIGURE 2.
ODD/EVEN FIELD PULSE
An unusual feature of LM1881 is an output level from Pin 7
that identifies the video field present at the input to theLM1881. This can be useful in frame memory storage applications or in extracting test signals that occur in alternate
fields. For a composite video signal that is interlaced, one of
the two fields that make up each video frame or picture musthave a half horizontal scan line period at the end of thevertical scan — i.e., at the bottom of the picture. This is called
the “odd field” or “even field”. The “even field” or “field 2” hasa complete horizontal scan line at the end of the field. An odd
field starts on the leading edge of the first equalizing pulse,
whereas the even field starts on the leading edge of thesecond equalizing pulse of the vertical retrace interval. Figure 1(a) shows the end of the even field and the start of the
odd field.
To detect the odd/even fields the LM1881 again integrates
the composite sync waveform (Figure 2). A capacitor ischarged during the period between sync pulses and discharged when the sync pulse is present. The period between
normal horizontal sync pulses is enough to allow the capacitor voltage to reach a threshold level of a comparator thatclears a flip-flop which is also being clocked by the sync
waveform. When the vertical interval is reached, the shorter

integration time between equalizing pulses prevents thisthreshold from being reached and the Q output of the flip-flopis 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 different
polarity on successive fields. Thus by comparing the Q
polarity with the vertical output pulse, an odd/even field index
is generated. Pin 7 remains low during the even field andhigh 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 µs long, is also the black level reference
for the subsequent video scan line. The LM1881 generates apulse at Pin 5 that can be used either to retrieve the chroma
burst from the composite video signal (thus providing asubcarrier 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
7

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LM1881

Application Notes (Continued)

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.

using a series C-R network. This may be necessary inapplications which require high horizontal scan rates in combination 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 theresult to the line select inputs. The falling edge of theLM1881’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 numberof counters. The final borrow out pulse is used to turn on theanalog 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 foreach field in an interlaced video system (television) or asingle 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; andthree counters will work for up to 2046 lines. An output buffer
is required to drive low impedance loads.

APPLICATIONSApart 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, theburst 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 leverallows identification of the appropriate field ensuring thecorrect read or write sequence. The vertical pulse output isparticularly useful since it begins at a precise time — therising edge of the first vertical serration in the sync waveform. This means that individual lines within the verticalblanking period (or anywhere in the active scan line period)
can easily be extracted by counting the required number oftransitions in the composite sync waveform following thestart 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 notused 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 numberof ways. Lines 17 and 18 are frequently used during studioprocessing to add and delete vertical interval test signals(VITS) while lines 14 through 18 and line 20 can be used forVideotex/Teletext data. Several institutions are proposing totransmit financial data on line 17 and cable systems use theavailable lines in the vertical interval to send decoding datafor descrambler terminals.

MULTIPLE CONTIGUOUS VIDEO LINE
SELECTOR WITH BLACK LEVEL RESTORATIONThe 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 number of contiguous output lines. The back porch pulse outputof 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 selectedline(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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8LM1881

Typical Applications

00915005

FIGURE 3. Video Line Selector

9

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LM1881

Typical Applications (Continued)

00915006

FIGURE 4. Multiple Contiguous Video Line Selector with Black Level Restorationwww.national.com

10LM1881

Physical Dimensions inches (millimeters) unless otherwise noted

Molded Small Outline Package (M)Order Number LM1881M (0˚C to +70˚C)
NS Package Number M08A

Molded Dual-In-Line Package (N)Order Number LM1881N (0˚C to +70˚C)
NS Package Number N08E

11

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LM1881 Video Sync Separator

Notes

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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