Examples of video output from circuits I’ve built.
EARLY VGA TESTS
Experimenting with video synthesis using composite video is challenging because the sync signal (which keeps the image stable on the screen) is encoded along with the image signal. If you try to process the image by e.g. passing it through filter circuits or feeding oscillators into it, the sync will get garbled and your video will look like scrambled Pay-Per-View cable TV. Working in VGA is a nice way to get around this. VGA has vertical and horizontal sync (Vsync and Hsync) on separate wires from the image signal, so you can tamper with the image without messing up the sync. The red, green and blue components of the image signal each are carried on their own wire, which allows fun colour mixing.
In this example, a composite-to-VGA converter box is providing the sync signals. The wires carrying sync are connected to an LCD monitor. Instead of the wires carrying the red, green and blue image signals from the converter, various electronic signals are being connected to the RGB pins on the monitor’s VGA input.
Here, the vertical lines are generated by a TS555 chip (a CMOS version of the 555 timer chip, similar to the standard varieties but with a higher maximum frequency). The Hsync wire from the VGA converter is connected to pin 2 (Trigger) of the TS555 through a signal diode. Whenever a new line is going to be drawn, the Hsync line spits out a pulse. This retriggers the TS555 so that its oscillation is always in phase with the display – in other words, its period of oscillation starts at the same point on every line. By contrast, if the 555 is not triggered, it will create spinning diagonal lines as it goes in and out of phase with the display.
The horizontal lines are being generated by an audio synth (a Korg Monotron) with its output plugged directly into the green channel of the monitor. Don’t forget to connect the grounds of all these devices together.
SINGIN’ IN THE ACID RAIN
In these examples, a VHS copy of Singin’ in the Rain is played into the composite input of the composite to VGA converter box. The RGB outputs of the converter are passed through some homebrew circuitry before being fed into the monitor.
The vertical lines that conform to the geometry of the original video are generated by a TS555. Here, one of the colour channels of the original video is connected to pin 2 (Trigger) of the TS555. Every time there’s an abrupt change in the voltage of the original video signal (e.g. a transition from a dark region to a light region; an edge), the TS555’s oscillation period is restarted. The result is semi-vertical lines that “echo” the forms of the original video, repeating off toward the right-hand side of the screen.
In addition to being appealing in its own right, the video-triggered TS555’s output can be fed into logic chips like the 4017 counter/divider, the outputs of which can be sent to the RGB inputs of a monitor. Intricate strobing geometric effects can be achieved in this way (to be demonstrated in an upcoming video).
The colour-changing effects are being achieved by feeding the various signals (original video and TS555 output) into potentiometers, and then into 2N3904 transistors set up in the emitter follower configuration. The transistors act as buffers, allowing the processed signals to be sent for further processing or display without overloading the preceding circuitry. The transistors are hooked up to the RGB inputs of the monitor, so the potentiometers form a simple buffered mixer. Twiddling the pots results in changing colours.