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First, distinguish the signals
“C-sync” can mean a sync-only waveform or, less precisely, composite video. They are not interchangeable:
- Composite video (CVBS) combines picture information, blanking, color burst and sync.
- Composite sync (C-sync) contains horizontal and vertical timing, including special pulses in the vertical interval.
- Separate H/V sync carries horizontal and vertical timing on distinct lines.
- Sync-on-green embeds sync in the green video channel.
The LM1881 is documented for negative-going composite-video inputs of 0.5–2 Vpp, rather than as a generic logic-level H/V converter. TI describes pin 1 as a reproduction of the part of the input below the video black level, with picture information removed. TI’s LM1881 datasheet is the reference for its input conditions and outputs.
What the LM1881 pins provide
| Pin | Function | Relevance to sync conversion |
|---|---|---|
| 1 | Composite sync output (CSOUT) | Contains horizontal and vertical timing; it is not dedicated H-sync. |
| 2 | Composite-video input | Documented input for the sync-separation function. |
| 3 | Vertical-sync output (VSOUT) | Provides a logic-level pulse identifying the vertical interval. |
| 4 | Ground | Supply return. |
| 5 | Burst/back-porch output | Optional timing output. |
| 6 | RSET | Sets internal timing current; the value may matter when line frequency differs from the usual range. |
| 7 | Odd/even field output | Optional field-identification output. |
| 8 | VCC | Single supply, 5–12 V recommended operating range. |
The datasheet gives a typical vertical-output pulse width of about 230 µs and a 190–300 µs range under its stated test conditions. Pin 3 is internally generated from the detected vertical interval; it is not a buffered copy of a separate V line, and its leading edge and width may not match what every monitor expects.
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- LM1881N is a video sync separator circuit extracting sync information from composite video signals
- Video processing systems television equipment and video signal conditioning applications
- Good noise immunity with sync tip clamping and noise immunity circuitry for stable operation
- Extracts vertical sync horizontal sync and burst flag from standard NTSC PAL or SECAM video
- Video processing equipment television monitors and video-based measurement systems
Why pin 1 is not H-sync
Ordinary C-sync contains horizontal pulses, but the vertical interval is different. Interlaced video uses equalizing pulses, serrations and half-line timing around vertical retrace. Pin 1 preserves composite-sync timing, including those features; it does not turn every line into a normal H-sync pulse. The LM1881 uses the vertical waveform structure to detect the interval and produce pin 3’s vertical pulse.
A receiver may accept C-sync on an input labeled H, but that is a property of that receiver—not proof that pin 1 is a standards-correct H-sync output. Check the display’s documentation and test the actual combination, particularly with interlaced sources.
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Can sync-only C-sync go into pin 2?
Possibly, but treat it as an application-specific experiment, not the LM1881’s clearest documented use. A sync-only input may produce useful vertical detection if it has negative-going polarity, suitable amplitude and pulse shape, and the expected vertical serrations. The datasheet’s normal input specification is for negative-going analog video at 0.5–2 Vpp; it does not guarantee operation with every C-sync waveform.
Do not connect a 5 V TTL C-sync signal directly on the assumption that it is equivalent to the specified analog-video input. The input is AC-coupled and clamped, but amplitude, polarity, DC restoration and source impedance still matter. Attenuation and coupling, a suitable front end, or a logic-compatible separator may be needed. A positive-going sync source also needs inversion or a different interface.
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Basic LM1881 wiring for composite video
For a conventional composite-video application, follow TI’s application circuit and the requirements for the exact source. The functional connections are:
- Pin 8 to a 5–12 V supply and pin 4 to ground.
- Pin 2 to AC-coupled composite video.
- Pin 6 to the timing resistor; 680 kΩ is a common datasheet test/example value, not a universal setting.
- Pin 1 for C-sync and pin 3 for V-sync; pins 5 and 7 are optional outputs.
- Supply decoupling at pin 8 and the specified decoupling associated with pin 6.
Use the datasheet schematic for capacitor values and connections rather than assuming one set fits every source impedance and video format. TI also describes an optional input low-pass filter of 620 Ω in series with the source and 510 pF to ground, with an approximately 500 kHz corner. It attenuates color-subcarrier content while passing sync, but adds roughly 40–200 ns of delay depending on conditions. It may be unnecessary for clean sync-only signals, can shift edges, and may be unsuitable for faster nonstandard scan rates.
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Check output levels, polarity and termination
The LM1881 outputs are logic-like and go low during active sync events. TI specifies low-level output values up to approximately 0.8 V under a 1.6 mA sink-current test condition; the high level depends on supply and load. Do not assume every output is universally TTL-compatible or suitable for every sync input.
- Confirm whether the receiver expects active-low or active-high sync and whether it accepts C-sync or requires separate H/V.
- Check the receiver’s input threshold and maximum voltage. A 3.3 V-only input must not be exposed blindly to a higher logic level.
- Determine whether the input is high-impedance logic or a 75-ohm analog termination. The two need different drive arrangements.
- Check whether the LM1881 output needs a pull-up or buffer, and whether the receiver already supplies one.
- Account for cable length and avoid adding a 75-ohm termination to a logic output unless the interface is designed to drive it.
Can external logic derive H-sync?
A logic stage can combine pin 1’s C-sync with pin 3’s vertical indication, but a simple mask such as H = C-sync AND NOT V-sync is not a universal conversion. It may suppress pulses during the vertical interval that a display expects, alter timing, or create edge errors because the two outputs have different propagation delays. Interlaced video and unusual scan timings make the result more demanding to validate.
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Possible approaches include a gate or comparator, a monostable, a counter or timing generator, programmable logic, or a microcontroller/FPGA timer. The right design depends on the source waveform and the receiving display’s requirements. Treat masking as a receiver-specific hobbyist experiment; verify pulse polarity, phase and behavior through the vertical interval with an oscilloscope and the actual display.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When to choose another separator
If the design genuinely needs separate H and V outputs, a device with an explicit H-sync output is the cleaner starting point. TI’s LMH1980 datasheet documents composite, horizontal and vertical sync outputs, plus burst/back-porch timing, odd/even field and HD-detect outputs. It supports analog CVBS, S-video/luma, component video and sync-on-green, across SD bi-level and HD tri-level sync formats. The specified input range is 0.5–2 Vpp; its supply range is 3.3–5 V and operating temperature range is −40°C to +85°C. TI describes its HSOUT as a dedicated negative-polarity horizontal-sync output. See also the LMH1980 product page for current product information.
The LMH1981 is another multi-format option with composite, horizontal and vertical outputs; consult its TI product page for device details. Neither part is a drop-in LM1881 replacement: package, pinout and external circuit requirements differ.
| Option | Best fit | Trade-off |
|---|---|---|
| LM1881 | Composite-video sync stripping when C-sync and V-sync are sufficient. | No dedicated H-sync output. |
| LMH1980 | Direct C/H/V extraction across a wider range of analog formats. | Different package and more involved design than a basic through-hole LM1881 circuit. |
| LMH1981 | Multi-format synchronization where its additional outputs or capabilities are useful. | More pins and complexity than a simple SD sync-separation job. |
| Discrete logic or programmable logic | Custom waveforms, unusual timing or exact H/V reconstruction. | Requires timing design and validation against the source and receiver. |
| Video processor or scaler | Sync conversion combined with resolution or timing conversion. | More system complexity and potentially added latency. |
Choose according to the input and display
- Choose the LM1881 when the source is ordinary analog composite video or luma-like video and the application needs C-sync and V-sync, or the destination accepts C-sync directly.
- Do not make it the default for TTL-level C-sync, strict RGBHV inputs, sync-on-green, HD or PC formats, unusual vertical timing, or systems that require reliable H pulses throughout the vertical interval.
- Prefer an LMH1980 or LMH1981 when dedicated H and V outputs or support for multiple analog formats is required, after checking the exact format and device documentation.
Troubleshoot a missing or unstable sync signal
- Identify the waveform: determine whether the source is CVBS, sync-only analog C-sync, TTL C-sync, sync-on-green or another format.
- Measure the input: check amplitude, polarity, pulse shape and source termination at pin 2; compare the signal with the LM1881’s documented input conditions.
- Check the circuit: verify supply and ground, pin-8 decoupling, input coupling, pin-6 timing components and the selected RSET value.
- Probe pin 1: confirm that C-sync is present and observe the vertical-interval waveform. Do not interpret it as dedicated H-sync.
- Probe pin 3: check whether a stable vertical pulse appears and whether its width and timing suit the receiver.
- Check the interface: confirm pull-ups, logic thresholds, polarity, cable loading and whether the receiver expects 75-ohm analog or high-impedance logic sync.
- Check timing compatibility: establish whether the source is interlaced or progressive and whether the display accepts its scan standard and V-pulse timing.
If pin 3 is absent or unstable, possible causes include missing expected vertical serrations, an incompatible sync-only waveform, unsuitable RSET timing, incorrect input amplitude or DC restoration, or excessive noise. If a monitor picture is unstable, investigate the signal type, polarity, level, termination and timing before changing the separator.
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