Raster Scan in CRT display

Question

In raster scan when the beam reaches the right-hand side of the screen it undergoes a process known as horizontal flyback, in which its intensity is reduced and it is caused to "fly back" across the screen (the top-most mauve line). While the beam is flying back, it is also pulled a little way down the screen.

Because of the inductive inertia of the magnetic coils which deflect the electron beam, there is a delay named horizontal/vertical retrace time.

Why the beam does not scan even lines from right to left (like below image)?

Horizontal retrace(flyback) time can reduced. There is no need to deflect beam horizontally at end of each line. just a tiny vertical deflect needed.

Answer 1

The one-way scan principle directly mirrors the way magnetics in deflection circuitry work: first there is slow current ramp along with ray moving left-to-right while the deflection/flyback transformer is connected to the constant voltage supply, then "flyback" stage with higher, opposite-polarity voltage applied to the deflection coil, made by disconnecting flyback deflection transformer from constant voltage supply.

Both-ways deflection would need push-pull configuration of deflection drivers, and that probably was considered too complicated and too costly during the early days of TV standards emerging.

Another reason could be the fact that there would be problematic to match the position of picture on neighbouring scanlines -- unlike the single-way deflection circuitry, that goes exactly the same every scanline.

Answer 2

To expand on lvd's comment about matching neighbouring lines, the diagrams you have are a simplification; on a real classic CRT nothing is horizontal. A real complete field looks more like:

There's no coupling between horizontal and vertical motion. Each acts entirely independently. Left-to-right deflection is doing one thing while top-to-bottom does another. There is no communication between the two. Don't think of it like a line printer.

The image formed on the screen is solid because each scan has a certain height to it. So the rows join up. If left-to-right scanning were left-to-right-to-left-to-right as proposed then either: * the image wouldn't be solid. It'd be a zig zag of alternating diagonal lines that touched at the edges. Perceptually, it'd appear to be a lot darker in the middle; or * the image would be very heavily overdrawn. Still darker in the middle, and also indistinct at the edges.

I drew this myself, so be forgiving; the degree of diagonal slant is identical between the three options: