[WORK IN PROGRESS, FEEDBACK IN THREAD WELCOMED]
LZX Industries Patchable Video Standard V1.0
The Patchable Video Standard proposes an electrical and interface specification for wide bandwidth analog computing instruments intended for creative and expressive applications. It is optimized for, but not limited to: (1) generating and processing analog RGB video graphics in SD resolutions, (2) affordable devices which are accessible to working artists and not just big studios, and (3) maximum patchability via a universal DC voltage range and high impedance connections.
Presented below are the guidelines we follow at LZX Industries when designing our products to communicate with each other. We openly invite and warmly welcome others interested in developing instruments which also speak the Patchable Video Standard.
- An input voltage of +/-1V corresponds to +/-100% of a parameter’s range.
- Some inputs respond only to positive voltages. This is OK. For example, the red channel input on a video encoder module responds to 0% to 100% red, but negative values are ignored. However, when subtracting one signal from another using the Passage module, negative output voltages are retained.
- Signal generators output unipolar voltages, with 0V to +1V corresponding to the generator’s minimum and maximum values. For example, the red channel output of Visual Cortex input decoder section will output 0V when there is no red in the input image, and 1V if there is a maximum amount of red in the image. As another example, Prismatic Ray’s outputs oscillate between 0V and 1V across their frequency period.
- Inputs must be tolerant of voltages up to +/-12V.
- Output voltages must not exceed +/-12V.
Impedance & Bandwidth
- Inputs are terminated to 0V through a 100K ohm resistor. Inputs are usually immediately buffered with a video bandwidth op amp to avoid lowering input impedance.
- Outputs are usually driven through a 470-499 ohm resistor. Output drivers must be capable of driving up to four separate inputs without signal loss greater than 5% or a corresponding loss in bandwidth. Usually outputs are individually buffered with a video bandwidth op amp part.
- Signal paths must be capable of processing signal bandwidths from DC to 10MHz. This is to ensure SD video signals are passed without degradation in edge sharpness. In practice, this means op amp ICs specially designed for video bandwidths are used throughout the entire signal path. Some modules may be designed to intentionally alter the image quality. For example, a low pass filter designed to blur parts of the image, or an intentionally lo-fi approach to edge keying. This is OK so long as user expectations are set accordingly.
- Inputs and outputs use 3.5mm TS phono jacks.
- Connections are made with 3.5mm TS phono cables less than two meters in length.
- Outputs may be split using passive mults, stacking cables and splitting hubs.
- External connection to video cameras and displays, MIDI interfaces, ILDA laser projectors, etc. shall follow established electrical standards for the device in question. Conversion interfaces between Patchable Video and other signal formats should perform all processing and conditioning required to account for valid output under all input conditions. For example, the composite video outputs on Visual Cortex follow all expectations for consumer video devices: connection via a yellow RCA jack, 75 ohms output impedance, driven at double gain, sync signals and color subcarrier embedded, etc. With the Cyclops module, the output is via a DB-24 port used by International Laser Display Association for laser projector interfaces. In both of these cases, there are signal clippers, conditioners, and any other processors required to ensure valid output under all patchable video input signal conditions.
- Devices interacting with a video signal’s timing must be capable of synchronization to a master timing reference.
- Video synchronization methods follow broadcast video expectations for genlock using black burst or composite sync signals passed over RCA or BNC connectors. For example, on our current EuroRack modules there is often RCA sync in / passthru jacks located on the rear of the module for daisy chained sync connections.
- A synchronization input must be capable of accepting any valid video source as the master reference input. For example, I could synchronize a Prismatic Ray module to the Sync Out from the rear of Visual Cortex, or to the composite video signal from my Sony Camcorder.
- As an RGB graphics system by design, synchronization circuits need not worry about chroma subcarrier lock in composite video, which is handled as a function of output video encoder modules. Composite video inputs are typically time base corrected and decoded to RGB before being passed out as Patchable Video Standard color channels.
Note: The 14-Pin Video Sync Distribution Cable format, used by discontinued Visionary series EuroRack modules and Cadet/Castle DIY series modules, is deprecated for the purpose of this standards document.