Computer

Oscilloscope graphics using a amiga (bonus vectrex)

Last Updated or created 2022-05-16

Somewhere in 1992 i got hold of a Oscilloscope, probably borrowed from someone. I don’t know what happend to it. I got the idea to generate drawings on the scope, because it had two inputs with you could switch to x and y inputs.

Example oscilloscope

My friend Sepp got into it also, we both wrote some software to do funky stuff with this. I found some software today (20220516), and having bought a old skool scope 2 years ago …

So i found source machine code, no executables. Now i needed to get a assembler running again.

Sidenote: I recently fixed a Amiga 500 and got a disk switch installed on the even cia.

Disk df0 df1 switch print at the center of the image


Booting some old seka disks and starting MasterSeka again in a looong time.

ESC - open editor
r (read file)
v (directory)
a + enter + enter (no options assemble)
g (go running the program)
FIrst part of machine code .. at the bottom part are arrays of coordinates to draw things

Some programs on the disk: (some are made by Sepp, who is a far better coder than i am)

  • Funny triangles
  • Lissajous figures
  • Moving square
  • House
  • House with door
  • Draw with mouse

Lissajous figures are simple sine and cosine functions to get:

So how does this work, well a amiga has stereo outputs. These are controlled by two DAC outputs on the 8364 (Paula) chip. (DAC – Digital Analog Convertor) ( Paula has 4 DMA controlled DACs !! )

Looking at the schematics of the audio part, we see a lot going on concering audio filters. The tests i’ve done today (2022) are on a amiga with unmodified audio filters. (Low on my prio list)
So frequencies are not direct what you get directly converted from digital values. Besides that, syncronisation between left and right channel, even using DMA can be an issue.
(DMA – Direct Memory Access, this means that it can be controlled without using the CPU)

Running the house draw code:

A little house

Note: Due to different hardware not a good working example .. yet

More examples .. hard to capture a still image

I tried a few years after we did this, to modify a generic monitor to display things using two inputs, not using scanlines. But to no avail. Only flipping the screen and colors using relais (more on this later)

Bonus part: Above did remind me of a Vectrex, a game console which utilises same display technic. So no raster lines and pixels, but line drawing by controlling the beam.

Two games on a borrowed Vectrex – You can see the continuous line – faint to see, even harder IRL
Movie from 2017 .. Vectrex was made in 1982-1983