United Retrotronics Model 2, Rasterized Oscilloscope Clock

Managed to successfully convert a 52 year old analog oscilloscope into a Raspberry Pi driven desktop clock. Proof of concept is complete!

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  Education

[bpoag]

I'll be refining the algorithm a little to draw the bitmaps boustrophedonically, just to shave a little more off the time the beam has to spend moving to a new position, and thus reducing flicker.

Net result: Bitmaps on an oscilloscope! :)

[jonshouse1]

nice. Any details ?

[bpoag]

In order to create pictures on an oscilloscope screen in XY mode, you need to have a DAC of some sort. There are popular DACs available like the MCP4725, but there's a far better (and faster) one right under our nose..the sound chip. That's all a sound chip really is, when you think about it. An extremely fast DAC.

I wrote some code that takes a 1-bit 256x256 image, and converts it into 8-bit stereo audio. The L and R channels of the resulting audio sample then control the position of the beam.

[bpoag]

Making the beam move back and forth to 256 different places along 256 different lines is very time consumuing. Even at 48 KHz audio sample resolution, the resulting image flickers very badly. To get past the flicker problem, my code will randomly skip X number of pixels when drawing, knowing that on the next pass, those pixels that were skipped will eventually be drawn on subsequent passes. The net result is a stable, but sparkly/grainy image that relies on human persistence-of-vision to work.

[Observ45er]

WOW! This seems incredably inefficient. However, I think I understand you are doing a bitmap to raster conversion in sort-of a bruit-force way, right? When a pixel is on, you stop and make a bright spot on your way along the raster scan, right? What is you did it more like a vector display (I do laser 'pure' vector graphics). Just jump the beam from one "on pixel" to the next "on pixel" and don't waste time going to off pixels. Does-zat make sense..?

[jonshouse1]

Note the missing samples, a good idea but don't you simply want to miss samples that are not within the sample time window rather than at random. You might do better to pick a slew rate that is real for the sample rate and simply drop samples that do not fit in the slew interval. That way a RC stuck on DAC would probably give you a sparkle free output - the compromise is getting the value of capacitor low enough to work but not so low it makes your lines curve.

[jonshouse1]

Ok, thanks. An audio chip is in fact a nothing special DAC, faster than an instrument DAC but in no way especially fast. If you plug in a cheap USB audio dongle then you should be able to get a real world 14 bits and 44.1 or even 96 Khz sample rate, its worth a go.

[Observ45er]

Son-of-a-gun, it is. Instead of sawtooth sweep, it's triangle...ok. Got it. For raster that's a good idea. So, your goal is a clock...analog face or digits, or are you after something completely different?

S

[Observ45er]

HUH? "boustrophedonically" izat a word?

[Observ45er]

Very nice indeed. I have been browsing all the Oscilloscope graphics videos. So, this is just X-Y and you have no intensity control, so you rely on the rapid beam movement to draw very light lines between "on" pixels? I've been doing Laser Graphics X-Y stuff {actual laser on the wall] since the 80's, so I understand X-Y , Vector and Raster and scopes.

Steve N