 Compatibility is almost full at BIOS level, but even at register level, a very high value of compatibility is reached.VGA is not directly compatible with the special IBM PCjr or HGC video modes, despite having sufficient resolution, color, refresh rate and memory capabilities; any emulation of these modes has to be performed in software instead.The intended standard value for the horizontal frequency of VGA is exactly double the value used in the NTSC-M video system, as this made it much easier to offer optional TV-out solutions or external VGA-to-TV converter boxes at the time of VGA's development, a technique proposed by Zia Shlaimoun; it is also at least nominally twice that of CGA, which itself used broadcast-frequency monitors, essentially tunerless televisions with more direct signal inputs.