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 the oscilloscope).

Removing the 75 Ohm load approximately doubles the voltages.

Logic Analyzer Timing Measurements

ROM Version	H	3A
Pixel Shape	Tall	Square
Horizontal Pixels	720	608
Vertical Pixels	364	432
Using 20.37504 MHz Dot Clock
As measured at U6B on the CPU Board
VSync (U6B-5)
Low duration	267.764 us	212.796 us
Period	16.99948 ms	16.9614 ms
VSync low to HSync low	3.135 us	788 ns
HSync (U6B-16)
Low duration	28.265 us	22.764 us
High duration	15.710 us	14.927 us
Pulses per VSync period	379	450
CSync (U6B-12)
Low duration	40.040 us	34.548 us
High duration	3.931 us	3.143 us
Vid (U4D-8)
Front porch (last pixel to start of CSync)	352 ns	352 ns
Back porch (end of CSync to first pixel)	4.355 us	4.361 us

[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?