Quick answer: An analogue RGB feed has to be sampled, timed and scaled before an HDMI input will accept it. The box digitises the three colour channels, recovers the sync and writes a standard HDMI timing at a fixed refresh. It can only hold an output rate its own chipset supports, it needs audio on a separate input because RGB carries none, and it fails outright on sources that bury sync inside the green channel.
An old console, a workshop computer or a piece of lab equipment puts out red, green and blue on three BNC or VGA style pins, and the only display left has an HDMI socket. A plug with two different ends cannot help, because nothing in it samples a signal or creates a timing. What sits in the middle is a small computer, and its limits decide whether a picture arrives at all.
What the scaler has to do with an RGB signal
Three things happen in sequence, and any one of them can be the reason for a blank screen. The analogue levels are sampled to digital values, the sync is recovered so the box knows where each line begins, and a new HDMI timing is generated at a rate the display will lock to.
Digitising three channels
Each colour channel is sampled by an analogue to digital converter whose clock follows the source rather than a fixed crystal. A box sampling at the wrong moment picks up the blanking edge instead of the picture, which shows as a shifted image with a dark band down one side.
Building a timing HDMI will accept
HDMI sinks expect a small set of recognised timings with control packets attached. The box therefore rebuilds each frame at a rate it can generate, while the source keeps its own. When the two cannot be reconciled, the output is absent or visibly torn.
The output rate the box can hold
Output support is a short list burned into firmware, not a range. A unit that holds 1080p at 60 Hz may refuse the 75 Hz timing an old workstation sends, and a unit built for 720p will letterbox anything wider. Interlaced input adds a step, because fields have to be woven into frames before scaling.
| Incoming RGB timing | Work the box does | Output it can hold | Failure when it cannot |
|---|
| 640 by 480 at 60 Hz | Upscale four times | 1080p at 60 Hz | Soft edges, uneven pixel width |
| 720 by 480 interlaced | Weave fields, then scale | 1080p at 60 Hz | Comb teeth on moving edges |
| 1024 by 768 at 75 Hz | Rate convert to 60 Hz | 1080p at 60 Hz | Dropped frames, judder on pans |
| 1280 by 1024 at 60 Hz | Scale and pad to widescreen | 1080p with side bars | Stretched or cropped picture |
Audio has to arrive on its own input
RGB carries no audio, and no amount of scaling creates it. The box therefore has a second input, usually a 3.5 mm jack or a pair of RCA sockets, and it embeds whatever arrives there into the HDMI stream. Leave it empty and the display gets video with silence, which is normal rather than faulty.
The consequence is a second cable run and a lip sync question that did not exist before. Set one delay control once and leave the other alone, because two corrections stacked together produce audio that arrives early.
The sync on green trap
Some professional sources put sync pulses on the green channel instead of a separate pair of wires, which saves conductors. A box expecting separate sync sees a green channel with a raised black level and no timing information, so it either refuses the signal or produces a rolling, torn image.
What it looks like on screen
The picture may roll vertically, carry a green cast, or squeeze into the top half of the frame. None of that looks like a sync problem, which is why the box so often gets returned as faulty.
Test the box before you buy it
Check the source menu for a sync setting and switch it to separate sync if the option exists. If it does not, look for sync on green support in the box specification. Without that line in the listing, assume it is missing and plan a different converter.
Practical scenarios
- Arcade board into a modern panel: match the board line rate to a box that will hold 60 Hz, not one that only passes 1080p through.
- Workshop computer at 75 Hz: expect judder unless the box can rate convert, and prefer a fixed 60 Hz panel setting.
- Silent picture after connecting: the audio jack is empty, so run a lead from the source line output.
- Rolling image on professional gear: the source sends sync on green and the box is not built to read it.
FAQ
Does a simple adapter plug convert RGB to HDMI?
No. A plug with no electronics cannot sample analogue video or create an HDMI timing. Only a powered box with a scaler chip does that job.
Will the picture be pixel perfect?
Rarely. Scaling to a fixed output resamples the original grid, so sharp edges land between pixels. The result is usable, but never an exact copy of the source raster.
Can the HDMI output feed a capture card?
Usually yes, provided the card accepts the same fixed timing the box outputs. A card that only takes 1080p at 60 Hz is the simplest pairing.
Next step
Check cable against extender options if the display is far away, confirm class definitions in the HDMI buying guide, and read the optical cable guide before running more than a few metres.
See the matching cables and adapters
