Can I send a 640 × 480 signal to a CRT when the desktop menu will not offer it?

On a supported Apple-silicon connection, a low-level connection-mode switch can select an available 640 × 480 signal even when normal desktop selection will not use that size. The desktop can remain larger than the signal and become cropped, so this is an advanced workaround rather than a complete scaled-desktop mode.

Inspect the connection’s actual modes and save the current identifier before changing anything:

betterdisplaycli get -name="CRT Display" -connectionModeListAll
betterdisplaycli get -name="CRT Display" -connectionMode

If the desired timing is listed, use its identifier:

betterdisplaycli set -name="CRT Display" -connectionMode="MODE_ID"

Substitute your display name and an identifier from its current list. Keep a working screen or other way to restore the saved mode, since an unsupported timing can leave the target blank. The command selects a connection mode. It does not invent a timing or guarantee that the adapter and CRT accept it.

If the image is cropped, the content must fit inside the visible portion of the larger desktop. One approach is to run the content on a virtual screen and present its image in that region using a positioned PIP window, stream or a separate capture app. For a BetterDisplay stream, 1:1 Pixel Mapping and top-left alignment can help place pixels without fitting the image to the entire desktop. Compare the source’s actual rendering dimensions, especially in HiDPI, with the visible signal area. PIP and streaming require Pro and capture permission.

For the rendering background, see how macOS scaling, HiDPI and LoDPI work.

Do not copy arbitrary native-resolution overrides from another CRT setup: they change scaling assumptions and can make other modes appear cropped or boxed. See native-resolution editing and stream placement for those separate mechanisms. A successful signal switch alone does not establish correct game placement or pixel mapping.