Understanding Sync-on-Luma, Sync-on-Composite, and External Sync Strippers

The single biggest mistake in a retro gaming setup is assuming the sync signal is a solved problem. It is not. Sync on luma extracts timing from the luminance channel of S Video. Sync on composite pulls it from the full composite video signal. A sync stripper is a dedicated circuit that cleans either into a dedicated sync line called csync. Which one your console delivers natively determines whether your scaler sees a stable image or drops sync during a white flash.

Sync on luma is the timing signal embedded in the luminance component of an S Video signal. On a Super Nintendo, sync on luma lives on pin 7 of the multi out connector. The level is 0.3 V peak to peak unloaded into 75 ohms. That is a low level signal, and it is clean because it is separated from the colour information before it leaves the console.

Sync on composite is the same timing signal but it is carried inside the full composite video signal. On a Super Nintendo that is pin 9. The level is higher at 1.0 V peak to peak into 75 ohms. The problem is that the composite video signal contains colour subcarrier information that can bleed into the sync detection circuitry, causing jitter or complete dropout on bright screens.

Csync, sometimes written CSYNC, is a dedicated sync signal on its own wire. On a Super Nintendo that is pin 3, also 0.3 V peak to peak into 75 ohms. Because there is no video information mixed in, csync is the cleanest sync type and the one every scaler manufacturer recommends. The RetroRGB signal path specifications document this hierarchy clearly: csync over sync on luma over sync on composite, with composite video as the last resort.

The console sync type chart is not uniform. Nintendo, Sega, Sony and NEC each made different choices. Here is the native output per console, sourced from console service manuals and the Shmups Forum cable development threads.

Nintendo Consoles

Super Nintendo and Super Famicom: The NTSC SNES delivers sync on luma on pin 7 and sync on composite on pin 9. Csync on pin 3 is present but at TTL levels on early models and attenuated on later ones. A csync restoration mod adds a 330 to 470 ohm resistor and a 220 to 470 microfarad capacitor in series, tapping pin 3 of the S C PUN chip or pin 7 of the S RGB chip, to produce a clean 0.3 V csync.

Nintendo 64: NTSC units deliver sync on luma on pin 7. PAL units deliver sync on luma on pin 9. Neither delivers csync natively. You need a sync stripper or a mod to get a dedicated sync line.

GameCube: PAL GameCube analogue AV out puts sync on luma on pin 9 and sync on composite on pin 7. NTSC GameCube analogue AV out puts sync on luma on pin 9 and sync on composite on pin 7. Csync is on pin 3 on NTSC units. PAL units lack csync on the multi out entirely.

Sony, Sega And Others

PlayStation 1, 2 and 3: All three use the same AV multi out pinout. Sync on luma on pin 5, sync on composite on pin 6, csync on pin 10. The csync signal is present and usable at 0.3 V peak to peak into 75 ohms on most revisions. The PS1 csync restoration mod taps pin 152 of the GPU or pin 10 of the multi out, adding a 220 to 470 ohm resistor and a 220 microfarad capacitor.

Sega Saturn: Sync on luma on pin 1, sync on composite on pin 4, csync on pin 13. Csync is present on all revisions at 0.3 V peak to peak into 75 ohms. The Saturn is one of the few consoles where csync is usable without a mod. The csync restoration mod adds a 220 to 470 ohm series resistor and a 220 microfarad capacitor for safe 75 ohm termination.

Sega Mega Drive 2: Mini DIN connector. Sync on luma on pin 5, sync on composite on pin 4. Pin 5 doubles as csync when the console is configured for it, which requires a jumper inside the shell. Out of the box you get sync on luma.

PC Engine and TurboGrafx 16: DIN connector. Sync on luma on pin 5, sync on composite on pin 2. Pin 2 is also csync when configured. The stock signal is sync on composite.

Neo Geo AES: Sync on luma on pin 7, sync on composite on pin 3, csync on pin 10. Csync is present and usable at 0.3 V peak to peak.

Dreamcast: Sync on luma on pin 7, sync on composite on pin 8, csync on pin 6. Csync is present on most revisions.

Console	Sync On Luma Pin	Sync On Composite Pin	Csync Pin	Csync Present Natively
Super Nintendo	7	9	3	No, requires mod
Nintendo 64 NTSC	7	9	N/A	No
Nintendo 64 PAL	9	7	N/A	No
GameCube NTSC	9	7	3	Yes
GameCube PAL	9	7	N/A	No
PlayStation 1/2/3	5	6	10	Yes, on most revisions
Sega Saturn	1	4	13	Yes
Sega Mega Drive 2	5	4	5 (jumper)	No, requires jumper
PC Engine	5	2	2 (jumper)	No, requires jumper
Neo Geo AES	7	3	10	Yes
Dreamcast	7	8	6	Yes

Sync on composite noise is the most common cause of scaler instability. The composite video signal carries the 3.58 MHz NTSC or 4.43 MHz PAL colour subcarrier. When the scaler's sync separator circuit tries to extract the horizontal and vertical sync pulses from that signal, the colour subcarrier can be misinterpreted as a sync edge. The result is jitter on fine horizontal detail and, on full white frames, a complete sync dropout that turns the screen black for two to three seconds.

Csync versus sync on luma on a Super Nintendo is not a subtle difference. Sync on luma delivers 0.3 V peak to peak and is clean because the luminance signal has no colour subcarrier. Csync is even cleaner because it carries no video information at all. The electrical implication is that csync gives the scaler a timing reference that matches the original console's pixel clock exactly. Sync on composite introduces timing errors that vary with the image content.

The signal levels matter for scaler compatibility. The RetroTINK 5X Pro accepts sync on luma, sync on composite and csync across a 0.3 V to 2.0 V peak to peak range into 75 ohms. The OSSC accepts the same range but uses an ISL59885 sync separator chip that has a minimum input of 0.25 V peak to peak. That means a weak sync on luma signal at 0.3 V is right at the edge of reliable detection on the OSSC. The RetroTINK 5X Pro uses an LM1881 sync stripper, introduced in 1988 by Texas Instruments, which has a minimum input of 0.5 V peak to peak. That is a problem: 0.3 V sync on luma is below the LM1881's rated minimum. The RetroTINK 5X Pro works around this with a preamplifier stage, but not all scalers do.

A sync stripper is a small circuit board, often built around an LM1881, EL1883 or ISL59885 sync separator IC, that takes a composite video or sync on luma input and delivers a clean TTL level csync signal. You need one when your console does not deliver csync natively and your scaler cannot reliably lock to sync on composite or sync on luma.

The Three Main Sync Separator Chips

The LM1881 sync stripper IC requires a minimum input of 0.5 V peak to peak and a maximum of 2.0 V. Its csync output is 5.0 V peak to peak TTL at 10 ohms impedance with a propagation delay of 40 nanoseconds. Supply is 5 V DC at 10 mA. The EL1883, introduced in 2002 by Elantec, accepts inputs as low as 0.25 V peak to peak and delivers 3.3 V TTL at 10 ohms with a 30 nanosecond delay. The ISL59885, introduced in 2005 by Renesas, accepts 0.25 V to 2.0 V and delivers 3.3 V TTL at 10 ohms with a 25 nanosecond delay. The OSSC uses the ISL59885. The RetroTINK 5X Pro uses the LM1881.

The White Flash Problem

The failure case that forces you to buy a sync stripper is the SNES white flash. A Super Nintendo or Super Famicom running a game like Super Mario World draws a full white screen when entering a level. The sync on composite signal dips because the console's power supply cannot maintain the 1.0 V peak to peak level during a full white frame. If your scaler has tight sync tolerance, the signal drops below the threshold and the screen goes black for two to three seconds. A sync stripper isolates the sync extraction from the video amplitude, so the white flash does not affect it.

OSSC Compatibility

Sync stripper OSSC compatibility is straightforward. The OSSC has a built in ISL59885 sync stripper, but it only activates for certain input types. If you feed sync on composite into the OSSC's RGB SCART input, the internal stripper handles it. If you feed csync directly, the stripper is bypassed. The problem arises when your console delivers sync on composite on the csync pin, which some cables do. Then the OSSC sees the composite video waveform on the sync line and may fail to lock. The fix is an external sync stripper between the console and the OSSC that delivers clean csync.

The scaler's sync tolerance is measured by three parameters: the low pass filter cutoff frequency, the video threshold, and the horizontal PLL coast settings. The OSSC firmware 0.90 offers sync LPF options of Off, 2.5 MHz, 10 MHz and 35 MHz. The default video threshold is 125 mV with a range of 30 mV to 300 mV. The horizontal PLL pre coast and post coast each range from 0 to 3 lines. The RetroTINK 4K firmware 1.6.3 offers the same LPF options and threshold range.

If you are experiencing sync dropouts, adjust the sync LPF first. Set it to 2.5 MHz to filter out the colour subcarrier on sync on composite signals. Second, raise the video threshold so that low amplitude noise is not mistaken for a sync edge. Third, increase the H PLL coast values so the scaler holds its lock through the blanking interval during a resolution switch.

The signal levels in the console service manuals tell you whether a cable needs an attenuation resistor. A TTL csync signal at 5.0 V peak to peak is too high for a 75 ohm terminated scaler input. The SNES csync restoration mod delivers 5.0 V TTL, which must be attenuated to 0.3 V with a 330 to 470 ohm series resistor. Without that resistor, you risk damaging the scaler's input circuitry. The RetroRGB sync guide specifies that all csync lines should be 75 ohm terminated and at 0.3 V to 1.0 V peak to peak for consumer equipment.

The one thing that most often goes wrong is assuming that buying a csync cable solves everything. It does not. A csync cable from a third party manufacturer may route sync on composite to the csync pin, which is worse than sync on luma because it includes the colour subcarrier. The cable must be built to the RetroRGB specification, which means the csync pin carries a clean 0.3 V signal from the console's csync line or from a sync stripper. Without verifying the cable's internal wiring, you are guessing. The only way to know is to open the connector and check which wire goes to which pin, or to buy from a vendor like Insurrection Industries or HD Retrovision that publishes their wiring diagrams.