Boardroom, brand-new 4K display, equipment rack 20 meters away. The previous installer ran a bargain-bin HDMI cable with "4K" printed on the jacket, and the result is a genre classic: screen goes black for two seconds, picture comes back, white sparkles whenever someone moves the mouse. The vendor blames the TV, the client blames you, and the actual culprit is physics: a 4K60 signal weighs 14.26 Gbps, and passive copper HDMI starts giving up past 5 meters at that rate. The good news: the math that explains all of it fits on a napkin.
The math that saves installs
Over an HDMI or DisplayPort cable, video travels uncompressed: every pixel of every frame, all of its bits, sixty times a second. No codecs, no miracles — pure brute force, which is why the numbers get so big.
One detail throws people the first time: the signal carries more pixels than you see. Between lines and between frames travel "blank" pixels (the blanking interval), inherited from CRT tubes that needed time to swing the beam back to the start. A 1080p picture shows 1920×1080 but transmits 2200×1125; 4K shows 3840×2160 and transmits 4400×2250. Blanking isn't dead weight anymore — audio and metadata ride in it — so it counts. With that, the full formula:
- Bandwidth = total pixels per frame × frames per second × bits per pixel. At 8 bits per color with no subsampling (4:4:4), each pixel weighs 24 bits.
- 1080p60: 2200 × 1125 = 2,475,000 total pixels. × 60 frames = 148.5 million pixels per second (the famous 148.5 MHz pixel clock). × 24 bits = 3.564 Gbps. Any halfway-decent cable passes that — which is why "it worked at the house".
- 4K60, 8-bit: 4400 × 2250 = 9,900,000 pixels. × 60 = a 594 MHz pixel clock. × 24 bits = 14.256 Gbps. HDMI 2.0 carries 14.4 Gbps of actual data: it fits with about 1% to spare. Barely.
- 4K60, 10-bit, HDR: same pixel clock, but 30 bits per pixel: 594 MHz × 30 = 17.82 Gbps. That no longer fits in HDMI 2.0. You need HDMI 2.1, DisplayPort — or fewer color bits, which is the next chapter.
- Takeaway: quadrupling pixels (1080p → 4K) quadruples the Gbps, and HDR stacks another 25% on top. Before quoting cables and extenders, run the format through the HDMI/DP bandwidth calculator and you'll know exactly which ceiling you're fighting.
Chroma subsampling: the discount your eye never sees
Your retina is lopsided by design: it resolves changes in brightness with surgical precision and changes in hue only roughly. Professional video has exploited that asymmetry for decades: transmit luminance (Y) in full and dilute the color (chroma). The 4:x:x notation describes how many color samples survive in each 2×2 pixel block:
The trick is so good that nearly every movie and TV show you've ever watched is 4:2:0: Blu-ray, streaming, broadcast. On moving pictures, nobody notices. Where does it fall apart? On hard, static color edges: thin red text on a white background, the lines of a CAD drawing, the 12 px font of a spreadsheet. There, diluted color turns crisp edges into smudged fringes.
HDMI, DisplayPort and the fine print
Brochures advertise the raw number (18, 48, 80 Gbps), but part of those bits is packaging: line coding that keeps the signal stable on the wire. HDMI through 2.0 uses 8b/10b — 2 of every 10 bits are cardboard — so of the 18 raw Gbps, 14.4 carry data. HDMI 2.1 switched to 16b/18b (48 → 42.67) and DisplayPort 2.1 to 128b/132b, where almost everything is payload. Comparing brochures without subtracting the packaging is like comparing salaries without asking if they're gross or take-home.
| Version | Raw | Effective data | What fits (uncompressed) |
|---|---|---|---|
| HDMI 1.4 | 10.2 Gbps | 8.16 Gbps | 1080p60, 4K30 |
| HDMI 2.0 | 18 Gbps | 14.4 Gbps | 4K60 8b 4:4:4 (barely) |
| HDMI 2.1 | 48 Gbps | 42.67 Gbps | 4K120 10b, 8K60 with DSC |
| DP 1.4 | 32.4 Gbps | 25.92 Gbps | 4K60 10b with room to spare, 8K60 with DSC |
| DP 2.1 (UHBR20) | 80 Gbps | ≈ 77.6 Gbps | 8K60 10b (71.3 Gbps) uncompressed |
And about cables: at 1080p (3.5 Gbps) almost anything passes, which is why the bargain cable "works" in the quick test. At 4K60 you're asking for 6 Gbps per lane, and that's when the sparkles, dropouts and HDCP handshake failures show up. Buy cables with real certification — Premium High Speed (18 Gbps) or Ultra High Speed (48 Gbps), with the holographic label and scannable QR — and past 7 or 8 meters at 4K, forget passive copper: go active or optical (AOC), which reaches 30, 50 or 100 m. The word "4K" printed on the box is not a specification. It's decoration.
Past 10 meters: HDBaseT and AV over IP
When the distance exceeds what HDMI tolerates with dignity, you change vehicles. HDBaseT packs video, audio, control (RS-232 and IR), Ethernet and even power (PoH) into a single Cat6 twisted pair run: Class A reaches 100 m, Class B 70 m. The classic generations move the equivalent of HDMI 1.4 (10.2 Gbps raw), so 4K60 travels as 4:2:0 (7.13 Gbps: it fits); HDBaseT 3.0 raises that to 18 Gbps over Cat6A and passes 4K60 4:4:4 with no tricks. The operational beauty: the cable is ordinary UTP — pulled by the same tech who wires the network and certified with the same tools. Variants and reach are in the HDBaseT calculator.
AV over IP is the other philosophy: instead of a dedicated matrix, one encoder per source and one decoder per display, all hanging off a switch. On a 1 Gbps network it travels compressed (H.264/H.265, a few frames of latency — fine for signage, marginal for live presenting); on 10 Gbps (SDVoE, JPEG-XS) compression is so light and latency so low that it competes with the matrix. And it scales like the network it is: another display? Another switch port, not a new chassis.
Pocket rule: point-to-point up to 100 m → HDBaseT; big matrix, many floors or unpredictable growth → AV over IP. If you go IP, size the switch with multicast and proper IGMP snooping: twenty 900 Mbps streams don't route themselves.
Projection and walls: the screen gets calculated too
That leaves the other half of the project: how big the picture is and from where. For projectors the key is the throw ratio: distance divided by image width. A projector with a 1.5 TR filling a 2.4 m wide screen needs 1.5 × 2.4 = 3.6 m of distance — not a centimeter of faith involved. It works backwards too: a 0.8 short-throw hung 4.2 m away would paint a 5.25 m wide image, wider than the wall. That's why you pick the projector to match the room and not the week's special offer; the projection calculator solves it in both directions, lumens included.
And a video wall? Tile arithmetic, with one treacherous detail: the bezel. Take the classic 3×2 with 55" video-wall panels: an active area of 1210.6 × 681 mm per panel and 3.5 mm of combined bezel between neighbors. Total width: 3 × 1210.6 + 2 × 3.5 = 3638.8 mm ≈ 3.64 m. Height: 2 × 681 + 3.5 = 1365.5 mm ≈ 1.37 m. Combined resolution: 5760 × 2160 = 12.4 megapixels — 50% more pixels than 4K, and the wall controller has to feed every one of them. Pixel pitch lands at 0.63 mm: sharp even from a meter away.
Before promising a client a wall, run the panel grid through the video wall calculator: dimensions, resolution, pitch and minimum viewing distance come out on their own — and you find out in time that the "3×3 of 65-inch panels" doesn't fit on the 4-meter wall once the ceiling molding has its say.
The checklist before you quote
Every video project boils down to four questions: how many Gbps the signal weighs (calculate them), which link in the chain is the weakest, how many meters it has to travel, and what image size the room demands. Answer them with numbers before buying a single cable, and that display 20 meters away lights up on the first try — no sparkles, no black screens, no second site visit.