Two ways a television picture falls apart.
The first is a signal losing its horizontal hold. Bands of the picture slide sideways and white noise crawls over the top, on and off, never quite settling.
The second is the tape look, and it is the more considered of the two. Rather than shifting every band by the same amount, it looks at how fast a noise field is changing as it runs down the frame, and only tears the picture where that change is steep. Everywhere else stays rock steady. That is what a worn tape actually does: it holds fine for most of the picture and then rips apart in one place, which is far more unsettling than a picture that wobbles all over.
The band spacing works out to roughly one per two rows of the render, so it tracks the resolution instead of being a fixed size, and it stays horizontal on any shape of canvas.