What an LED processor actually wants from your Mac

You have built a canvas that matches the wall exactly. It is 3840 × 1152, because that is what the panels add up to. You plug the Mac into the processor and get nothing, or a picture that is offset by a hundred pixels, or something that is nearly right and flickering.

The processor is not being difficult. It is a piece of hardware that accepts a specific set of input formats, and your beautiful canvas is not one of them.

Two resolutions, and they are not the same thing

This is the concept that untangles most LED problems on a Mac.

The panel resolution is what the wall is. Sixteen panels of 168 × 168 in a 8 × 2 layout is 1344 × 336. That number is decided by the hire company and physics, and it is almost never a standard video format.

The input resolution is what the processor accepts on its HDMI or SDI connector. That is a normal video mode — 1920 × 1080, 3840 × 2160, sometimes a custom EDID mode if the processor supports one.

Inside the processor, you tell it which region of the input maps to which panels. So the job is not “output 1344 × 336”. The job is “output a video mode the processor accepts, with your 1344 × 336 content in a known position inside it, and tell the processor where to find it.”

Get this the wrong way round and you will spend an hour trying to make macOS produce a resolution no display has ever reported.

The practical setup

The approach that works nearly everywhere:

1. Build your canvas at the panel resolution. 1344 × 336. This is what content is designed to, and what your mapping software treats as the wall.
2. Send a standard video mode to the processor. 1920 × 1080 at 60Hz is the safe default.
3. Place the canvas at a known origin inside that frame — top-left, 0,0 — pixel for pixel, with no scaling.
4. In the processor, set the input region to match. On Novastar this is the layer or input crop in NovaLCT or the Coex/H-series web UI; on Brompton it is the canvas mapping in Tessera.
5. Leave the rest of the 1080p frame black. It is carrier, not content.

Top-left at 0,0 is the convention worth adopting as a house rule. It is unambiguous, it is what most processors default to, and it means anyone can look at the confidence monitor and see immediately whether the geometry is right.

SpectraMap handles this as an output region: the canvas is the wall, and each output places a rectangle of it into a frame at a specific position and video mode, without resampling. That is the property that matters — one canvas pixel must land on one panel pixel, or the wall will look soft no matter how good the content is.

Scaling is the enemy

If the wall is 1344 pixels wide and anything in the chain resamples, you will see it. LED walls are unforgiving about it because the pixels are physically separate — a soft edge on an LCD reads as a soft edge, and on LED it reads as flicker and crawl on moving content.

The places scaling sneaks in:

• macOS display scaling. If the Mac thinks it is driving a Retina display, it renders at 2× and scales down. Set the display to a native, unscaled mode. In Displays, hold Option and click Scaled to see the real list.
• “Fit to screen” in the playback software. Any resize mode other than 1:1 or “actual size” is a resample.
• Overscan. Some HDMI paths apply a few percent overscan, which crops and scales. If your content is offset and slightly large, this is why.
• The processor’s own scaler. Most will happily scale an input region to fit the wall. Turn it off. If the processor is scaling, you have a resolution mismatch to fix upstream.

Frame rate, and why it matters more here

The panels have a refresh rate set in the processor, and the processor has an input frame rate. Those two need a sane relationship, and your Mac needs to match the input.

A 60Hz input into panels running at a multiple of 60 is clean. A 50Hz input into a 60Hz configuration produces judder that is subtle on a wall and obvious on camera. If the show is being filmed, the camera’s shutter and the panel’s refresh interact as well — that is where scan-line banding in the recording comes from, and it is fixed by talking to the LED tech about panel refresh and scan rate, not by anything on the Mac.

Decide the frame rate once, at the top of the job, and make every device agree. In Australia, if there is broadcast or a hire camera involved, that number is probably 50. If it is a corporate show with a Mac and a projector, it is probably 60. Do not let it be both.

Colour range: the grey-blacks problem

The single most common LED complaint that sounds like a content problem: blacks look grey and washed out, or highlights are crushed and clipping.

That is nearly always a full-range versus limited-range mismatch. Computers work in full range, 0–255. Video equipment traditionally works in limited range, 16–235. If the Mac sends full range and the processor expects limited, everything gets stretched: 0 becomes blacker than black and clips, 255 blows out. Send limited into a full-range expectation and you get grey blacks and no punch.

Both ends have a setting for this. Find them, make them agree, and write down which way round you set it. There is more on the whole colour chain in why the wall never matches your preview.

The physical path

Between the Mac and the processor there is a cable and usually a converter, and both have opinions.

• HDMI from a Mac is convenient and the source of most EDID weirdness. If a mode disappears or the display reconnects at the wrong resolution, suspect the HDMI handshake.
• SDI via a Blackmagic DeckLink or UltraStudio is far more predictable, because SDI modes are a short fixed list and there is no negotiation. If the wall is important, this is the better path. What matters when sending SDI out of a Mac covers the details.
• An HDMI-to-SDI converter is a cheap link that will silently change your colour range and occasionally your frame rate. If something is inexplicable, take the converter out of the chain and test.
• A display emulator or EDID minder is worth carrying. It stops the Mac rearranging its displays when a processor is power-cycled — which it will be, at the worst moment.

For the whole chain from canvas to panel rather than just this hand-off, see LED wall mapping on macOS with NDI and Syphon.

Sanity checks before you call the LED tech

1. Send a 1:1 pixel grid, not content. Errors that content hides show up instantly.
2. Send a full-range ramp and a 16–235 ramp; whichever looks correct tells you the range setting.
3. Confirm the Mac’s output mode is native and unscaled.
4. Confirm the processor is not scaling the input region.
5. Confirm frame rates match end to end.

Five checks, five minutes, and they resolve the overwhelming majority of “the wall looks wrong” calls before anyone has to open NovaLCT.