An extender’s data sheet often says “supports 4K” without saying whether that means 4K30 or 4K60, full or reduced colour, or whether HDCP will pass. Settling this before you choose equipment saves troubleshooting on site.
1. Do the bandwidth sum first
HDMI carries the picture as TMDS. On every pixel clock each of the three data channels sends 10 bits (8b/10b encoding included), so the rate for 8-bit 4:4:4 is about 30 times the pixel clock. Common formats:
| Format | TMDS clock | Data rate (encoded) |
|---|---|---|
| 1080p60, 8-bit 4:4:4 | 148.5 MHz | about 4.46 Gbps |
| 4K30, 8-bit 4:4:4 | 297 MHz | about 8.91 Gbps |
| 4K60, 8-bit 4:2:0 | 297 MHz | about 8.91 Gbps |
| 4K60, 8-bit 4:4:4 | 594 MHz | about 17.82 Gbps |
| 4K60, 10-bit 4:4:4 | 742.5 MHz | about 22.28 Gbps, beyond HDMI 2.0 |
| HDMI 1.4 limit | 340 MHz | 10.2 Gbps |
| HDMI 2.0 limit | 600 MHz | 18 Gbps |
The 18 Gbps of HDMI 2.0 includes the encoding overhead, so the picture data it carries is about 14.4 Gbps. 4K60 at full colour needs 17.82 Gbps, only about 1% under the limit, so one link in the chain that stops at the 10.2 Gbps of HDMI 1.4 rules it out. HDR usually uses 10-bit video, and 4K60 at 10-bit 4:4:4 is already beyond HDMI 2.0, so it must drop to 4:2:2 or 4:2:0.
2. Chroma subsampling
An image can be described as brightness plus colour difference, and because the eye is less sensitive to colour, colour detail can be reduced to save bandwidth. In 4:4:4 every pixel carries full colour. In 4:2:2 colour is halved horizontally. In 4:2:0 it is halved both horizontally and vertically, and the total data is about half that of 4:4:4.
Film and photographs show little difference; a computer desktop does. Fine text, thin lines and coloured text show fringing or blur in 4:2:0, red text on a dark background most of all. For computer sources, keep 4:4:4 as the first choice and confirm it at acceptance with real screen content.
3. Why some extenders compress or fall back to 4:2:0
Traditional HDBaseT has a bandwidth of roughly 10 Gbps, which cannot hold the 17.82 Gbps of 4K60 at full colour, so many extenders drop the picture to 4:2:0 or compress it first. Newer specifications and some fibre products offer more, but check each data sheet. AV over IP and KVM over IP are the same: the pixel data of 4K60 at full colour is about 11.9 Gbps, more than 10 GbE can carry, so it has to be compressed.
Compression is not a fault in itself: light, low-latency, visually lossless compression shows no difference on most content. Fine text and fast-changing pictures are where vendors differ, so test with your own content.
4. EDID: telling the source what to send
EDID is the data in which a display tells the source which resolutions, colour formats and audio formats it supports, read over DDC. Matrices, extenders and splitters sit in the middle and handle EDID too. Three problems are common. Displays with different EDIDs leave the source able to choose only one, so it outputs at the lowest common capability. EDID briefly disappears during a switch, the source decides the display was unplugged, and windows move. And an EDID that declares multichannel or bitstream audio makes the computer send a format the room’s loudspeakers cannot handle, so there is no sound.
The remedy is to fix one EDID at each matrix input: 1080p60 with two-channel PCM where compatibility matters, and a 4K EDID for sources that need 4K. Document it and test it at acceptance.
5. HDCP: versions and the handshake
HDCP protects copyrighted content, and a source and a display authenticate each other with a handshake when they connect. The common versions are HDCP 1.4, 2.2 and 2.3. Protected 4K content normally requires HDCP 2.2 along the whole chain. If one device supports only HDCP 1.4, the source may lower the resolution, stop sending protected content or show a black screen.
Matrices and extenders act as repeaters. They authenticate downstream first and then report the downstream devices upstream, so every switch and every display power-up triggers a new handshake, and a few seconds of black after switching is common. More levels and devices make failure likelier, and the specification limits both. Over a network, check HDCP 2.x support: it limits round-trip time, so authentication fails if network delay is too large.
Use HDCP 2.2 equipment throughout, keep sources that must play protected content apart from ordinary presentation sources, avoid cascading, and accept 1080p output for older displays that support only HDCP 1.4.
6. A checklist before you commit
- List every signal path with its source, display, resolution, refresh rate and colour format (4:4:4 or 4:2:0, 8-bit or 10-bit), and whether HDR is involved.
- Check that each extender, matrix and cable genuinely supports that format, not just “4K”.
- Test with real sources, including fine text, coloured text and fast-moving pictures, not only a test pattern.
- Decide how EDID and HDCP will be handled and write it down.
- Leave margin on distance and cable quality: run shorter than the maximum on the data sheet.