Lisa Composite Video Characteristics

[sigma7]

Some oscilloscope waveforms and measurements (note that timing measurements collected via Logic Analyzer will be much more precise than those collected from this oscilloscope).

Removing the 75 Ohm load approximately doubles the voltages.

[stepleton]

Here is my attempt to recreate these driver circuits in the Falstad CircuitJS simulator: link

It would be a good idea to check to see whether I've copied the schematics faithfully. There is certainly a difference in levels apparent in both in the simulated oscilloscope. (Tip: mouse over the L1_RCA and L210_RCA labels to see which label is which in the simulated 'scope --- note that it may still be hard to tell if you have certain kinds of colourblindness. The L1_RCA has consistently higher voltages everywhere.)

[stepleton]

It's interesting to note slightly different circuits to the ones I've transcribed in some motherboard schematics: for the Lisa 1, some different values in this schematic as well as a 100R resistor between Q1 emitter and Q2 collector.

Meanwhile the Lisa 2/10 motherboard (note: link goes to a TIFF file) shows a single component U2 creating the video signal from CVOUT and VID; I presume this is the hybrid circuit that has been mentioned elsewhere?