The main oscillator for the ASIC is 40MHz. A divide by 9 output at 4.444MHz is provided with a 5:4 mark/space ratio. It is possible to change the main crystal to 9 x 4.43619MHz = 39.90257 MHz, slowing the whole system by 0.25%. This may or may not upset the disk drives, but even if this is the case, a diskless unit could provide PAL subcarrier frequency locked to the master oscillator, thus improving the picture quality.
Note: The above applies only for the GX4000 with Composite video output (the GX4000 shares the same oscillator for the 4MHz CPU clock and 4.4MHz color clock) (not for 464+/6128+ with RGB output). As seen on [[[[ImageMedia:sp512topGX4000 PCB Top.jpg|this photo]], the GX4000 does actually have a 39.90 MHz oscillator.
===Locking of enhanced features===