BNC is a type of connector that connects ENG or EFP cameras to any type of device, be it a live truck, a studio connector, etc. Cables were RG6 types, the same type of cable used for your antenna or cable TV, just with a different connector.
SDI stands for Serial Digital Interface. This was the first type of connection to make digital ENG or EFP cameras broadcast in a crystal connection without static. SDI uses BNC cables and connectors to attach said devices to another device be it a ENG van, a video monitor, a video router, etc.
Both of these types of connectors are used to tie gen-locks together, teleprompters, return monitors and other things you see in a studio production
Both of these were exclusive for broadcast (and film)
HDMI is the well known High Definition Media Interface, these devices are used to plug in consumer high def devices into an HD capable monitor. The cables vary by type, and what version of HDMI the devices have inside. There had been 3 versions of the standard. The types of cables can include Ethernet connectivity, for hard wired Internet that gets passed through a networking element on the connector and the circuitry inside the device of interest. HDMI displays can vary from a computer monitor to an HDTV to a cable box, to a BluRay to your digital camera. Unlike other connectors in the consumer world; HDMI carries sound and video on one cable.
What is HDMI to SDI converters?
Like USB cables, HDMI uses a lot of soldered connections. Unlike RG6 or RG22, where you can cut the sleeve and feed it into an antenna or SDI plug, you cannot splice HDMI. One instance, was I used to be at a facility where we had a super awesome conference room where there was an HDMI port at the surface level of a counter next to outlets that was fed through the drop ceiling back down to the other side of the room with 60″ HD screen. Something happened to the cable, and we had to hook directly to the TV creating a tripping hazard.
So if you are creating news, and say you’re a cord cutter, a cord never, or a cord-fencer (meaning you still have cable or you’re on the fence or just cutting back on the boxes), you could theoretically take your cable lines that you know are not being used, replace them with a BNC cable and use a BlackMagic HDMI to SDI converter on both sides of the drop and you can feed your Apple TV or another device in the house without having to unplug and relocate. I cannot verify this just yet, but one other use is for your ENG or EFP cameras to have central wiring points to plug in without having to deal with the bondage that is of HDMI. At least it’s going from the device to the converter and the converter is cable.