Dot clock rates

The following frequencies are the dot clock rates of digital picture generators outputting 240p low-definition video signals that are compatible with NTSC monitors or otherwise at near-System M scan rates (15.7 kHz horizontal, 60 Hz vertical). The pixel aspect ratio that each rate produces, assuming the 480-line picture and 52.148 microsecond active scanline period of Rec. 601, is also given.

A few things may cause this ratio to differ in a particular environment:
 * Some coin-operated machines use a different active scanline period, and the arcade operator is expected to adjust the monitor's picture size to match a test pattern displayed in a particular game's service mode. An arcade-to-television adapter would produce the pixel aspect ratio listed below.
 * Some devices also support 480i (interlaced) video, where even scanlines are drawn in one field and odd scanlines are drawn in the other field. Interlaced video makes pixels half as tall as in 240p, doubling the pixel aspect ratio, and ratios assuming interlace are marked as such.
 * Sets of the 240p era were built for 4:3 display aspect ratio, as opposed to the 16:9 aspect ratio that became common between the late 1990s and mid-2000s; multiply the pixel aspect ratios by 4/3 if stretching the picture on a widescreen TV.

For comparison, here are some enhanced-definition (480p) rates, with a horizontal scan frequency close to 31.5 kHz: