HDMI signals

18Gbps and 600MHz

HDMI bandwidth, TMDS clock and what fits in it

Almost every format argument in an HDMI chain comes back to one number: how many bits per second the cable can carry. On the Diva that ceiling is 18Gbps, and knowing what fits inside it explains most of the settings.

HDMI carries picture and sound over three TMDS data channels, with a fourth pair carrying the clock those channels run against. The figure quoted in gigabits and the figure quoted in megahertz are the same fact stated twice: at a 600MHz clock each channel moves 6Gbps, and three of them together move 18Gbps.

That rate is not all picture. TMDS puts ten bits on the wire for every eight bits of data, so roughly a fifth of it pays for DC balance and clock recovery. What is left has to hold the resolution, the frame rate, the number of colour samples per pixel and the bits per sample, all at the same time. Raise one and something else has to give.

The format that sits immediately under the 18Gbps ceiling is 4K60 4:4:4 8-bit, which asks for a 594MHz pixel clock. 4K60 4:2:2 at 12-bit fits as well, because 4:2:2 sends colour information at half the horizontal resolution and occupies a fixed slot width regardless of bit depth. 4K60 4:4:4 at 10-bit or 12-bit does not fit, which is why 4K HDR usually travels as 4:2:2 or 4:2:0. The older 300MHz tier, around 9Gbps, holds 4K30 4:4:4 or 4K60 4:2:0 8-bit.

Where the HDfury Diva needs 18Gbps

All four Diva inputs accept true 4K60 4:4:4 at 600MHz and 18Gbps, and so does TX0. Those are the ports where cable quality decides whether the link works at all: any HDMI cable in the chain must support 18Gbps if an 18Gbps source or sink is connected to it. TX1 is the 1080p SDR downscale output and the dedicated HDMI audio output generates its own 720p or 1080p stream, so an ordinary high speed cable is enough on both of those.

The bandwidth tiers also run through the EDID tables. FIXED EDID presents a 4K60 4:2:0 8-bit 300MHz table, deliberately modest so that something appears in any setup, while the predefined banks separate 600MHz entries such as 4K60-444 600MHz HDR BT2020 All Sound from 300MHz ones such as 4K60-420 8-bit 300MHz All Sound. Choosing a table is choosing how much bandwidth your source is invited to use.

Reading the real clock

The incoming signal line on the OLED quotes the pixel clock alongside everything else, in the form 4K50 4:2:2 BT2020 12b LLDV 594MHz 2.2. Compare that with the IN and VIDEO rows in the INFO section of the web server and you can see whether the limit is coming from the source, from the EDID being presented, or from the sink. HDfury suggest cables between 6ft and 12ft for proper 600MHz pass-through, and because the Diva works as a repeater that amplifies and recovers the signal, a separate HDMI amplifier is rarely needed once it sits in the middle of a long run.

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