Clamp
A video amp with a failed clamp, for Resolume
What it does
An FFGL effect for Resolume Arena and Avenue. Video amplifiers were joined by capacitors, and a capacitor does not pass DC. The picture's black level is DC. It was put back every line by a clamp, a switch that pulled the signal to a reference in the back porch. When the clamp is weak or dead, the coupling capacitor is a high-pass filter on the whole raster.
Clamp places the host's frame as the active area of a real 625/50 or 525/59.94 field, with sync, back porch, picture, front porch, the blank lines and the half line, in real time, and runs that one RC through all of it, in time order.
Everything recognisable falls out. Black wanders with picture content, because the average over the time constant is forced to the reference: a bright scene pushes black down and a dark one lifts it. A bright moment darkens the next one, which recovers on the RC's curve. Lines tilt across their width when the time constant is near a line, and fields tilt top to bottom when it is near a field. A healthy clamp takes it all out, except the residual its own switch leaves.
Two separate mechanisms sit on top: the supply sags with average picture level and recovers on its own time constants, so the gain breathes with content, and an optional triode stage clips hard at cutoff and softly where grid current loads the source. Show Blanking draws the whole field as a raster.
- One RC coupling run through every line and blanking interval of a real field
- 625/50 and 525/59.94 timing, per channel or on luma alone
- Clamp Health from dead to perfect, and a Clamp Reference
- Supply sag with its own attack and recovery
- A triode stage with bias and drive
- Show Blanking: the whole field, sync to vertical blanking
- Universal macOS bundle and a Windows x64 DLL
Where it stands
v0.1.0, and honestly early. The amplifier is measured, not asserted, by an offline harness that drives the real plugin class headlessly at two rasters: after a burst of white, black falls back on exp(−t/τ) with every blanking interval counted, fitted τ 63.998 µs, 20.000 ms and 0.99999 s against 64 µs, 20 ms and 1 s; a flat field tilts 0.555906 across a line at τ of a line, as stated; black shifts with APL by the closed form of the steady state; the clamp leaves e^−n of the error behind each porch to six places; every pixel matches a serial double-precision run of the same timeline to within 2.3 float ULPs; and six host rates give one trajectory. Sixteen negative controls and a one-character mutation of the shipped shader are caught. All 13 swept controls change the picture. Render cost on macOS at the defaults is 0.82 ms/frame at 4K. It has not yet been loaded into Resolume on macOS. On Windows it has: a build of this source loads, registers and renders in Resolume Arena 7.27.1 on software rendering (win-lab, no GPU), with every parameter as declared; Sag Attack and Sag Recovery could not be shown moving there, because the gate's picture is a still whose level never changes. The triode stage and Show Blanking are looks, checked only for being alive. There is a user guide and a browser demo; no presets and no OpenFX port. Check it in your own rig before trusting it in a show.
Watch it running
Nothing loads from YouTube until you press play.Watch it there instead