HDMI signals

TMDS

Transition Minimised Differential Signalling

TMDS is the layer underneath everything else on an HDMI cable. Its clock is also what the Diva listens to when it decides that a source has woken up.

TMDS stands for Transition Minimised Differential Signalling. It arrived with DVI in 1999 and HDMI inherited it unchanged in 2002, which is why an HDMI source and a DVI monitor can be joined with a passive adapter. Three differential pairs carry the picture, one for each colour component, and a fourth pair carries the clock they are all timed against.

Each pair takes eight bits of pixel data and sends them as a ten bit symbol. The encoder picks whichever of the available symbols has the fewest level changes, then inverts it if that helps keep the running count of ones and zeros near even. Fewer transitions means less radiated interference and a cleaner eye at the far end; the balanced average means the receiver can recover its own threshold and its own clock from the data itself. The two extra bits are the price of both.

That ten bits per pixel per channel is where every HDMI bandwidth figure comes from. Three channels at ten bits each, clocked at 600MHz, give 18Gbps, which is the ceiling the Diva works to and the reason a 4K60 4:4:4 link stops at 8-bit.

How the HDfury Diva uses the TMDS clock

AUTOSWITCH INPUTS in the CONFIG section is described in the manual as the TMDS and pixel clock autoswitch: when it is on, the unit switches to an active signal if the selected signal is lost. The detail that matters is what it watches. Most switches watch the +5V line, which a source can hold high while it is asleep, so an Apple TV 4K, an Xbox One X or a Shield looks permanently active and the switching logic never fires. Watching for a running TMDS clock instead tells you whether the source is genuinely sending pixels. The command is set autosw on, and AUTOSWITCH PRIORITY, set autoswprio on, decides whether a newly detected input takes over at once.

The clock is also reported to you. The incoming signal line on the OLED ends with the pixel clock and the HDCP revision, for example 4K50 4:2:2 BT2020 12b LLDV 594MHz 2.2, and the same line appears against each receiver in the file produced by EXPORT CONFIG.

Reading the clock figure

Because the clock is the real currency, it tells you what a format costs before you go looking for a cable fault. A 4:2:2 signal carries half the colour samples, so 4K50 4:2:2 at 12-bit runs at the same 594MHz a 4K50 4:4:4 8-bit signal would. Push the same picture to 4:4:4 at 12-bit and the clock has nowhere left to go. For 600MHz work the manual asks for cables certified or tested for 600MHz and 18Gbps, ideally between 6ft and 12ft, and notes that the Diva acts as an HDMI repeater, so it can often replace an amplifier you were already using.

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