SECAM was pretty crazy because it required a delay line: a memory that would hold the previous scanline so it can be combined with the current one.

Without digital circuits, the delay line was a piece of glass. You’d convert the video signal to a sound wave, send it through the glass, and (hopefully) get it back exactly 64 us later so it aligns with the next scanline.

Here’s a picture: https://www.eevblog.com/forum/projects/glass-ultrasonic-dela...