7.3 Video signals
Video on a patch jack, and the video ports beyond the patch field. VIDEO is a signal class like AUDIO and CV (7.2); its jacks are patch jacks and follow patch safety, and its interface is its own.
VIDEO
VIDEO carries video-rate signals on patch jacks: pictures, ramps, keys, and control. It is 0 to +1 V DC, range U1, with a ceiling of −0.1 to +1.2 V:
- A VIDEO input shall present 100 kΩ ±1 % to 0 V at the patch tip (SIG-009).
- A VIDEO output shall drive the patch tip through a 75 Ω ±1 % series resistor (SIG-010).
- A VIDEO input that reads a signal as high or low, such as a comparator, logic, or key input, shall switch at a threshold of +0.5 V: it reads above the threshold as high and below it as low (SIG-019).
+0.5 V is the midpoint of the VIDEO scale, so any VIDEO output drives any VIDEO logic input the same way, and a key or comparator splits a picture at mid-gray. EIS recommends that a VIDEO logic output give 0 V for low and +1 V for high, and a hysteresis of no more than 0.1 V, centered on the threshold, so that a slow ramp switches once and a fine picture is not coarsened. An input whose threshold is set by a control, such as a key level, meets SIG-019 at the control's marked center or default setting.
A 75 Ω source into a 100 kΩ load arrives at almost unity gain. The cable's capacitance and the input's set the bandwidth of the link (below), so EIS recommends an input capacitance of 30 pF or less. A control on the same 0 to +1 V scale is CV at U1, with the shared interface of 7.2; a VIDEO output drives it directly.
Video beyond the patch field
Video on a 75 Ω RCA, BNC, or SMA jack is not VIDEO:
- A video sync port on the rear is a COAX-IN, COAX-OUT, or COAX-THRU of 8.2, and lists the 8.3 formats it locks to or generates.
- A picture jack on the front, composite or component video, is listed in the port list by its Annex C class (VID-CVBS, VID-SVIDEO, VID-YPBPR, VID-RGBS, VID-RGSB, or VID-RGBHV) with the formats it carries. A digital port, such as SDI on a BNC jack, is listed by its Annex C class too.
A picture port has no rule of 8.2. EIS recommends that an analog picture port take the COAX-IN termination or the COAX-OUT back-termination of 8.2, so that it drives and loads a 75 Ω line the way a video sync port does.
Black, paths, and bandwidth
EIS recommends that a picture on VIDEO place black at 0 V and full scale (white, or a color channel at full saturation) at +1 V, so that a mixer or keyer can treat any two picture sources alike.
A VIDEO path is what lies between an input jack and an output jack of one module. A module shall publish, for each VIDEO output, the kind of path behind it and, for a linear or variable path, its −3 dB bandwidth measured as this clause states (SIG-016). The kind says whether a bandwidth describes the path at all:
| Kind | What the path does | Publishes |
|---|---|---|
| linear | A picture or control passes through gain, mixing, offset, inversion, or a buffer; the output follows the input in proportion | bandwidth (SIG-016); delay recommended |
| shaped | The output is a function of the input that is not proportional: a key, comparator, logic, clip, rectifier, or quantizer | rise time recommended; no bandwidth |
| variable | The response is set by a control, such as a filter or a slew | bandwidth at the widest setting (SIG-016), and the setting |
| generator | No VIDEO input feeds the output: an oscillator, ramp, shape, or decoder; a VIDEO input, if any, is a control | bandwidth of the output recommended |
In the port list:
- A path is linear when the module has a VIDEO input and the port list says nothing else, and a generator when the module has none.
- When a module has several inputs, the port list names the input a figure is measured from (
from). - Linear paths that share one figure publish it once, as a floor for them all (
video.bandwidth_mhz), with any output that differs listed beside it.
Bandwidth is the −3 dB frequency of the path relative to its response at 100 kHz, measured:
- from a 75 Ω source at the input jack;
- into a load of 100 kΩ in parallel with 30 pF at the output jack;
- with a 0.5 V peak-to-peak signal about +0.5 V;
- with the module's controls at the setting that gives the widest response, which the port list names when it is not the default.
The three measurements of Annex A, a stepped sine, a step, and a multiburst, give the figure; the stepped sine settles a disagreement.
EIS sets no minimum bandwidth: a 1 MHz filter and a 100 MHz buffer both conform, each with its figure beside it. The figure says what the path does, not what a reader may do with it. Compared with a format's picture bandwidth, a path at or above that figure carries the format at full sharpness, and a narrower path carries it softer.
EIS recommends writing a bandwidth, in a port list and on a panel, as its bandwidth mark: BW and the figure in megahertz, rounded down to a whole number, so that a 35.7 MHz path is BW35. The mark is a number, never a format name, and two port lists compare at a glance.
EIS recommends that a module also publish, in the same port list:
- the delay of each linear or variable path, in nanoseconds, from an edge at the input jack to the same edge at the output jack;
- the rise time of each shaped path, 10 % to 90 % at the output jack;
- the bandwidth of each generator output, into the same load, although it has no path to measure;
- the input capacitance of each VIDEO input, which with the output's 75 Ω and the cable sets the bandwidth of the link;
- the Annex A method its figures were measured by (
measured_by).
Delay matters when two copies of one picture reach a mixer by paths of different delay: they land side by side on the screen, offset by the difference, one HD sample per 13.5 ns at 74.25 MHz.
The patch link
A published bandwidth is the module's own; what arrives at the next module also depends on the cable. The output's 75 Ω, a patch cable of 100 pF/m, and a 30 pF input form one pole, whose −3 dB frequency falls as the cable grows:
| Cable | Link −3 dB, cable alone | With a 30 pF input |
|---|---|---|
| 150 mm | 141 MHz | 47 MHz |
| 300 mm | 71 MHz | 35 MHz |
| 500 mm | 42 MHz | 27 MHz |
| 1000 mm | 21 MHz | 16 MHz |
| 1500 mm | 14 MHz | 12 MHz |
| 3000 mm | 7 MHz | 6 MHz |
On a cable of ordinary length, above about 30 MHz the link, not the module, is the limit. A stacked cable or a passive multiple adds a second cable and a second input to the same pole.
EIS recommends a patch cable of 100 pF/m or less, and that a maker of patch cables publish the capacitance per metre. A patch cable is not an EIS product; the figure is the cable maker's to publish.
Timing pulses on patch jacks
A video module's timing pulses on patch jacks, horizontal, vertical, field, burst-gate, or composite video sync, are patched signals like any other (7.1):
- a 0 / +5 V field or frame pulse is GATE;
- a line-rate pulse, too fast for CLOCK's 1 ms states, is
unclassified, or VIDEO at 0 to +1 V; - a 0 to +1 V composite video sync waveform is VIDEO.
A module may reset a ramp, key, or trigger from such a pulse. Its own raster timing, the formats of 8.3, locks to an external video sync only at a video sync port on its rear (SIG-015, SIG-018). A module that derives its line and field timing from a patch jack locks to a patched signal and does not meet SIG-015; it lists that jack by its class and has no video sync port for it.