periodic noise is because the circuit somehow picks up the crt's ~100khz radiation (i.e. from whatever electronics that sends the electron beam from left to right). i know this because i still get it even if i cover up the photodiode
the electron gun can turn basically all the way off (decreasing the brightness and/or g2 voltage can easily give a dynamic range >100000. spacediver and i measured this a few years ago). but anyway that's not relevant because i get exactly the same noise when i cover the photodiode.
spacediver wrote:Also, from what I can tell from the code, the line only begins drawing from the left of the screen (I guess you'd call this HSync?). So doesn't that right there reduce the precision by a potential maximum 8 microseconds?
the love2d code just draws a static image, so it always starts from the left.
i'm not sure completely how graphics card works, but for a crt which goes from a full black image to a full white image, the first pixel to do so could be either in the middle of the screen or always at the very left. i can take a picture and try to see.
because the setup responds faster when the first pixel to light up is in the middle of the screen rather than at the top or bottom, theres maybe a few us of inaccuracy from that factor. looking at my oscilloscope plots, it looks like a maximum of a 2-3us difference