8.2 Video on 75 Ω coaxial jacks (VID-75)

VID-75 is the 75 Ω coaxial interface of EIS: video between products on 75 Ω coaxial cable with RCA, BNC, or SMA jacks. It carries a row's video sync, black burst or bi-level or tri-level video sync of an 8.3 format, behind the panel from one source to every module that locks to it. A video sync distributor is a product, usually a bus board, with buffered COAX-THRU ports beside its COAX-IN, which fans the video sync out to several modules. The tables are in data/video_75.json.

A picture on a coaxial jack on the front, composite or component video, is not a video sync port: it is listed by its Annex C class (7.3) and follows no rule of this clause.

Video sync ports

A product that generates or locks to an external video sync takes or gives it at a video sync port on its rear (SIG-018), never at a patch jack (SIG-015, 7.1).

Every video sync port shall be a COAX-IN (75 Ω ±5 % to 0 V at the jack), a COAX-OUT (a 75 Ω ±5 % series source driven at twice the line level), or a COAX-THRU, buffered or looped, never a patch jack (V75-010). The tolerance is of the impedance at the jack. EIS recommends a ±1 % resistor for each, which leaves the rest of the tolerance to the buffer and the jack. Line level is defined below.

The difference a buyer needs most: a COAX-OUT can be the row's video sync, a COAX-IN follows it, and a COAX-THRU is neither a source nor a second input. The rules of each mode apply to a product that has a port of that mode:

• COAX-IN, video sync input. A product with a video sync input shall take the video sync at a COAX-IN on its rear, lock any video timing of its own to it, and list the formats it accepts (V75-006): the ones it locks to, or, on a distributor, the ones it passes.
• COAX-OUT, video sync output. A product with a video sync output shall generate video sync of a listed format itself and give it at a COAX-OUT on its rear (V75-007). A copy of the video sync it takes at its COAX-IN is a COAX-THRU, not a COAX-OUT.
• COAX-THRU, video sync through. A product shall have a video sync through only if it also has a video sync input (V75-008). Each through shall be a COAX-THRU, marked THRU, and its port list shall say which kind, buffered or looped (V75-008).

The two kinds of COAX-THRU:

• Buffered: a copy of the video sync at the COAX-IN, driven as a COAX-OUT into one COAX-IN load. Each THRU drives the next module's input, and a chain of about 18 keeps a tri-level edge inside its tolerance (Topology, V75-012).
• Looped: a jack bridged to the COAX-IN jack on one line. One cable run passes every module and only the last one terminates it, so the run takes no power and no buffer, but every module on it bridges the line. A product with a looped COAX-THRU shall give its COAX-IN a termination switch, marked TERM, that presents 75 Ω to 0 V across the line when on and no load when off, and shall lock the same with the switch on or off (V75-009). EIS recommends that a looped THRU with TERM off bridge the line at 10 kΩ or more in parallel with 20 pF or less.

Video sync ports are an optional interface (Part 3): a module, a bus board, or an enclosure may have them. The claim file lists each port with its mode and formats, and each COAX-IN, COAX-OUT, or COAX-THRU in that list brings in the rules of its mode. Distribution is a capability of whatever product has a COAX-IN and buffered COAX-THRU ports, not a product type: a distributing bus board claims EIS draft, bus board (6.5) (6.5).

Signal and line level

The video sync waveform follows the selected 8.3 format:

• black burst for the two interlaced formats;
• bi-level video sync for any standard-definition format;
• tri-level video sync for any high-definition format.

Line level, in this clause, is the level of the signal's own standard at a terminated input:

• black burst: the 1 V p-p composite scale, pulse tip −300 mV and white +700 mV (−286 mV and +714 mV for 525i59.94, per SMPTE ST 170);
• bi-level video sync alone: 300 mV p-p, pulse tip −300 mV (−286 mV for 525i59.94), blanking 0 V;
• tri-level video sync: ±300 mV about blanking (SMPTE ST 274 and ST 296).

A buffered distributor may AC-couple its buffered THRU outputs when its manual says so.

Jacks

Every jack in a VID-75 tree shall be an RCA, BNC, or SMA jack (V75-001). On every jack in a VID-75 tree, the center pin shall carry the video signal and the shell shall be 0 V (V75-002). A video sync port shall be marked beside it as an input (IN), an output (OUT), or a through (THRU), and a termination switch shall be marked TERM (V75-011). EIS recommends that each jack be insulated from the panel (7.1).

No jack inside the case is a BNC (SIG-017, 7.1). A video sync port on the rear of a module or on a bus board is therefore an RCA or an SMA, and a facility video sync arrives at a BNC on the enclosure's wall, wired inside to RCA or SMA.

The three jack types are electrically the same here. An SMA jack is a 50 Ω connector, a catalog BNC jack is as often 50 Ω as 75 Ω, and an RCA jack has no controlled impedance; but each is electrically short below 100 MHz, so the discontinuity does not matter and either BNC impedance serves. A product picks the type that suits its panel, and a cable may change type from one end to the other. A cable's SMA end is a 50 Ω assembly, and the same argument covers it.

Topology

A tree is the star of cables from one source to the COAX-IN of every module in it. The source is a COAX-OUT, or an external video sync at one COAX-IN; the branches are buffered COAX-THRU ports, or one looped run. Each COAX-IN on the tree is named in its module's port list (V75-006) and terminated as every COAX-IN is (V75-010).

• A COAX-OUT or a buffered COAX-THRU shall drive one COAX-IN load, and an output shall not be split passively to several terminated ports (V75-003). A second termination on the same output drops the level to two thirds, and a third to half.
• A COAX-OUT port or a buffered COAX-THRU port shall pass the video sync with a −3 dB bandwidth of at least 30 MHz into a 75 Ω load (V75-012). A tri-level video sync edge is 54 ns ± 20 ns (SMPTE ST 274 and ST 296), and a 30 MHz stage adds about 12 ns of rise in quadrature, so a 54 ns edge stays inside that tolerance through about 18 buffered THRUs.
• A looped run is one line: it is terminated once, at the module whose TERM is on, and every other module on it bridges it.

Cables

A VID-75 cable shall be a 75 Ω coaxial cable, with RCA, BNC, or SMA plugs, that connects center pin to center pin and shell to shell at both ends (V75-004).

Summary