Diva features
Downscaling
4K to 1080p SDR on the TX1 output
In short
Converting a signal to a lower resolution, and often a lower dynamic range, so that equipment which cannot accept the original can display it.
TX1 exists for the equipment in your rack that never expected 4K or HDR. It takes whatever arrives and hands over 1080p SDR at the HDCP level that sink understands.
Downscaling means delivering a signal at a lower resolution than it arrived at, and in practice it usually involves more than resolution. Taking 4K to 1080p is arithmetic on pixels. Taking HDR to SDR is a change of transfer curve and of colour volume, which needs tone mapping: a decision about how to fold a thousand nits of highlight into a hundred without crushing everything above the knee.
The reason to do it in the middle of the chain rather than at the display is that some equipment cannot accept the original at all. A projector from before 4K, an AVR that predates HDCP 2.2, a video processor with a 1080p input: none of them fail politely when handed a 594MHz HDR signal, they show a black screen or an HDCP error.
Downscaling on the HDfury Diva
TX1 is the dedicated downscale output. It converts 4K HDR to 1080p SDR and encrypts to the HDCP level that particular sink expects, which is why the rule in the manual is that HDMI and HDCP 1.x devices must be connected to TX1. Unlike TX0 it is always automatic: the scaling style you choose with set scale auto applies to TX0, while TX1 scales UHD down to FHD whenever the connected sink requires it.
The tone mapping is not blind. PROFILE in the TOOLS section chooses the profile that suits your display, with Profile 2 the one most people settle on, and the command is set hdrprofile 2. Profile 0-OFF turns the HDR to SDR conversion off altogether. The setting applies only when the input is HDR10.
Two other things TX1 drives
The output is used by two features that have nothing to do with a second display, and both catch people out. The video responsive ambient light strip reads its picture from TX1, so if the television you are lighting sits on TX0 you have to route that same input to TX1 as well, replicating one input to two outputs. The input lag tester also draws its measurement square on TX1 only, which is why testing a display with a 4K HDR signal means looping TX1 back into a Diva input and routing that input to TX0.
Related terms
In the manual
The pages of the HDfury Diva manual where this comes up.